The Garden Professors™ – Page 24 – Advancing the science of gardening and other stuff since 2009

Flammability of Landscape Plants–Why the lists are BAD!

California had the worst fires in the last two years of its existence as a state. Hundreds of thousands of acres of brush and forest burned. More importantly thousands lost their homes as fires moved across urban/rural interfaces to destroy communities. The entire town of Paradise, California was burned to the ground. Here in Ventura County, the Thomas Fire was the state’s largest fire by the time it was done, and hundreds lost homes. No other time in history have we been so focused on what will burn, why it will burn, and what we can do to have a “firewise” landscape.

In 2017 The Thomas Fire in Ojai, California was the largest brush fire in the history of California fire fighting. It was surpassed the following year by the Camp fire in Northern California.

Fire authorities around the world have advocated creating defensible spaces around homes that are clear of ignitable vegetation. Some authorities have mandated by law that mulch, pine needles and other debris be removed as a fire prevention measure near structures. There is a general recognition that any plant can burn. Even well irrigated plants will rapidly desiccate and become flammable in the face of strong wind and a heavy fuel load that is inflamed nearby.

Flammability of landscape around homes is dependent on several factors. Vegetation placement can obstruct or allow for fire fighters ability to reduce damage to a home. While it is natural to assume that avoiding flammable plants is a part of this process, there is no standard method for testing plant flammability. Many lists of firewise plants have unknown origin or are just guesses. Flammability can be assigned four dimensions: ignitability, sustainability, combustibility and consumability. These factors refer to time till ignition; time a material will burn; rapidity or intensity of burn and quantity of material that will burn. The components of combustion are influenced by moisture content, percentage of carbon, percentage of volatile compounds; surface area to volume ratio and other factors. The varied factors are usually not all studied at the same time and are not all equally important to plant flammability. Thus assessing flammability even within the context of a controlled study will only partially assess a material’s likelihood of burning under various conditions. Hence most of the lists are not that helpful.

While fresh wood chip mulches are consideder by some fire authorities to be a fire hazard, there is little published evidence of this and a single element like mulch can not be tied to flammbability of the landscape.

Behm et. al. showed that variations in flammability between plant species exists, and also that species within the same genus can vary widely in their flammable nature. Thus lists should not assume species in the same genus all have the same flammability. There is some thinking that flammability is an evolutionary trait that some plants exploit to their benefit, i.e. they are made to burn, such as the California Chaparral plant communities. Simply burning fuels in a laboratory setting does not take into account many of the factors associated with fuel burning intensities. Species differences notwithstanding, the amount of dead plant matter (dead twigs and leaves) vs. live matter, the arrangement of leaves, mulch and adjacent species all play a role in the flammability of the landscape itself which cannot be studied in a lab setting. Landscapes are “fuel bed complexes” with multiple elements that are not replicated in studies. For instance, small leaves from some shrubs ignite easily, but when burned as litter, develop low heat release rates because of poor ventilation.

Testing live plant materials alone is misleading because the flammability of an intact shrub is caused by the interaction of live matter with “necromatter”. Dead tissues are thermal catalysts which ignite live material. The ratio of necromatter to live matter influences flammability and is generally not well studied. Fire modelling also has a role in understanding what will burn. Both wind and slope increase the spread rate and the fireline intensity of burnable plants. Fire behavior characteristics on a given plant also are affected by both its physical and chemical characteristic — tissue mineral and water content have impacts on flammability. This bodes poorly for firesafe plant lists because lists do not consider plant physical or chemical attributes and if moisture levels are low it will burn regardless of its structure and geometry or its status on a list. Sometimes though a dense wall of well hydrated vegetation can save homes such as the avocado orchards that held back fire in the Thomas Fire in Montecito, Ca.

While lists don’t satisfy scientific rigor they are great for policy makers and homeowners who want to know what to plant. Unfortunately many lists are just compilations of other lists, none of which were based on research. Sometimes lists confuse one desirable characteristic with another, such as native plant lists that tout drought tolerance. Many drought tolerant plants are not fire resistant especially after a long dry period, indeed they often evolved to burn under such conditions.

For those that live in fire prone areas, fire resistant plant lists will always be an attractive or even required element of landscapes. Lists will not save a structure in the face of high winds and adequate fuel or embers. A defined defensible space around buildings, and maintenance of plantings that removes dead matter, maintains irrigation, and maintains proper distance from combustible surfaces will be more effective than choosing landscape plants from flammability lists.

References:
Fernandes, P.M. and M. G. Cruz. 2012. Plant flammability experiments offer limited insight into vegetation—fire dynamic interactions. New Phytologist 194: 606-609

Behm, A.L., M. L. Duryea, A.J. Long, and W.C. Zipperer. 2004. Flammability of native understory species in pine flatwood and hardwood hammock ecosystems and implications for the wildland –urban interface. International J. of Wildland fire 13: 355-365.

White, R.H. and W.C. Zipperer. 2010. Testing and classification of individual plants for fire behavior: plant selection for the wildland-urban interface. International J. of Wildland Fire 19:213-227.

A Cactus by Any Other Name: A Case of Mistaken Holiday Cactus Identity

Believe it or not, a cactus, of all things, is one of those plants that have come to represent the holidays and feature in the regular rotation of holiday houseplants. Then again, maybe it isn’t so strange amongst its peers that feature a flashy bulb-grown plant named for a horse’s head (the Latin name of amaryllis is Hippeastrum, literally meaning horse flower), a plant that has ugly flowers but brightly colored leaf bracts and leaks sticky and irritating latex when damaged, or some daffodil-like flowers that have musky odor so strong it makes some people nauseous. But…..I digress.

Back to the cactus. However you see it though, the cacti that make their debut at the holidays are suffering under a case of mistaken identity. What you typically buy as a Christmas cactus is not a Christmas cactus at all. It is actually a Thanksgiving cactus. Now this wouldn’t be such a big deal, except that there is such a thing as a “Christmas cactus” — but you won’t find one on store shelves. Nay, it is hard to even find one in garden catalogs. And this is sad, because the Christmas cactus is, I think, even more beautiful than the Thanksgiving cactus.

How did we end up ignoring the beautiful Christmas cactus in favor of its holiday cousin? It all comes down to timing and how we buy things for the holidays. It seems that, as the shopping and holiday seasons creep ever upward on the calendar, retailers have little love for a cactus that is actually programmed to bloom at Christmas. They need something that blooms earlier so that it can be on the store shelves as early as possible. (At this pace, breeders will need to develop and Independence Day cactus for the Christmas shopping season.)

Therefore, the Thanksgiving cactus has been rebranded as a ~~impostor~~ stand-in for the true Christmas cactus. We won’t even talk about the Easter cactus, which just totally feels left out of the family (and yes, there is such a thing and it is beautiful).

These cacti were in cultivation in Europe by 1818 and various different species were being hybridized, probably most notably by W. Buckley. The most notable hybrid, bred now named Schlumbergera ‘Buckleyi’ is considered to be the first actual “Christmas cactus” and associated S. x buckleyi hybrids are still grown as Christmas cacti. Cultivars and crosses of S. truncata are the Thanksgiving cacti that have been rebranded as Christmas cacti. They can be identified by their flattened stems (or cladodes or cladophylls) that have spiky, toothed edges and zygomorphic (now that’s a fancy word — it means that they have a two-sided, or bilateral, symmetry) flowers. Most of the Thanksgiving cacti that have these characteristics.

W. Fitch (drew), Swan (engraved) – Bot. Mag. 66. 3717, as *Epiphyllum russellianum* Source: Wikimedia commons

You’ll most commonly find them in pink colors, but you can now find them in yellowish colors. The flower shape often leads to its nickname: “Zygo cactus.”

S. x buckleyi are the true Christmas cacti and form what is called the Buckleyi group. Most of these have characteristics that come from the species S. russelliana, which was used in the early Buckley crosses. They can be identified by their rounded, less pointy cladodes and round, radially symmetrical flowers. They do have a similar growing form, but those in the know can tell the difference.

And for those following along at home, the Easter (or spring) cactus used to be considered part of the Schlumbergera genus (S. gaertneri) and then the Rhipsalidopsis genus, but now is classified as Hatiora gaertneri has radially symmetrical flowers but the cladodes are three dimensional rather than flat, elongated, and scalloped. They have a wide range of colors, such as red, pink, and even orange.

Source: https://www.extension.iastate.edu/linn/news/it-thanksgiving-christmas-or-easter-cactus

Holiday cactus care

It’s a cactus, so it should be easy to care for – I just water it sparingly and keep it dry, right? WRONG!

Whether you have a Thanksgiving or Christmas cactus (or an Easter one, for that matter), you take care of them the same way. Keys to their care come from their native habitat, which is not a desert but the cloud forests of costal south-east Brazil. The high-altitude costal areas where they’re from are cool, shaded, and relatively humid with the mists and moisture rich air. They are epiphytic or lithophytic – meaning that they grow on trees and in crevices with decaying plant material rather than in the soil. And while you don’t need to know this to grow them, the morphology of the flowers have developed to support the feeding of hummingbirds which act as their main pollinator.

*Schlumbergera x buckleyi*
Christmas cactus

Since we don’t grow them epiphytically, when we pot them we need to make sure that we provide a light substrate for them to grow and to get plenty of oxygen to the roots. Potting mixes should have a high ratio of peat or coir and even some bark or other coarse woody material. As for watering, you’ll want to keep the soil fairly moist, rather than dry. You’ll also want to let them dry slightly between watering, but don’t think that they like to live the life of dehydration — you do need to keep them watered.

One of the reasons that they bloom at very specific time of year has to do with light and, to a lesser extent, temperature. They are short-day (or rather long-night) plants, so they flower as days grow shorter (or longer, in the case of the Easter cactus) and nights grow longer. The Thanksgiving cactus will bloom with just a little shorter dark period than the Christmas cactus, which is why it blooms in late fall as opposed to the Christmas cactus that blooms closer to when days are the shortest around the solstice. They will also bloom better and longer if they have cooler temperatures, so keeping them in a cool area of the house is ideal. In high light situations the cladodes will turn red. Keeping them too dark, however, will limit growth and keep them from thriving.

Since they are short-day plants, the plants need a period of several weeks where the period of darkness at night is 12 hours or longer for their flowers to begin forming. This occurs naturally about mid-October, but you can delay flowering by using grow lights to lengthen the day (or keep in mind that bright indoor lights can also limit or reduce blooming). Also, don’t be alarmed if they bloom at odd times through the year. Since daylight coming into your windows can be altered by window treatments or films, the light levels can technically be “just right” for flowering at multiple times per year. In my old office the tint on the windows created the right conditions at least once or twice per year – one year I had a Halloween cactus and the next it was a Memorial Day cactus.

If your cactus does not flower, you need to move it to a spot where it gets at least 12 hours of relative darkness to initiate blooms (keep away from indoor light sources or windows near outdoor lights). Hopefully, you’ll have lots of colorful blooms for Christmas…..or whichever holiday your cactus celebrates.

Sources

Is it a Thanksgiving, Christmas, or Easter Cactus? https://www.extension.iastate.edu/linn/news/it-thanksgiving-christmas-or-easter-cactus

McMillan, A. J. S.; Horobin, J. F. (1995), Christmas Cacti: The Genus Schlumbergera and Its Hybrids (p/b ed.), Sherbourne, Dorset, UK: David Hunt

Soil or dirt? It’s really up to you

Dig up dirt. Treat like dirt. Dirt poor. Replace the word “dirt” with “soil” and you get phrases that make no sense. This is a roundabout way of explaining that “dirt” and “soil” are not the same things, either in idioms or in the garden. Yet many of us effectively turn our soils into dirt through poor garden practices.

For the purposes of this post, we’re going to use a single criterion to distinguish between soil and dirt: one is a living ecosystem and the other is not. A thriving soil ecosystem contains sufficient water, oxygen, and nutrients to support bacterial, fungal, plant, and animal life. Regardless of soil type, about half of the volume in a living soil should be pore space and the other half soil particles. Half of the pore space should be filled with water and the other half with air. When we make choices about activities that affect garden and landscape soils, we need to be proactive in preserving both the particle-pore balance as well as connectivity between the soil and the atmosphere.

All soils have pore spaces regardless of their texture.

The only way pore space can be reduced is through soil compaction. So don’t do it.

No driving. If equipment must be brought in, put down a thick layer of wood chips to protect the soil, or at least plywood.

Not really the way to do park renovations.

These amenity trees quickly became liabilities, thanks to soil compaction during construction. {Photo courtesy of Jim Flott)

No naked soil. Bare soils are compacted soils. Mulch!

No rototilling. It grinds your living soil into dirt. Disrupt the soil as little as possible when you plant.

No stomping, pressing, or otherwise compacting the soil during planting. Let water and gravity do that work for you.

Let’s just press that pore space out of existence.

Mud it in! Let water and gravity settle new transplants.

The only way soil and atmosphere connectivity can be disrupted is by covering the soil with low permeability materials. So don’t do it.

No soil layering. Don’t create abrupt layers of soils with different textures. It interferes with water and gas exchange.

Soil horizons change gradually in natural soils.

Poor surface drainage indicates a perched water table caused by abrupt changes in soil texture (photo courtesy of Rich Guggenheim).

No sheet mulches. I’m sure you’re tired of hearing me say that and I am tired of saying it. Sheet mulches have less permeability than chunky mulches. That means oxygen and water have more difficulty getting through. Period.

The less porous the material, the more slowly gas diffuses through it. Read more about it in our recent article

Chips are great. Why ruin them with cardboard underneath?

Landscape fabric is even worse than cardboard, but the weeds love it.

Do use lots of groundcovers, chunky mulches, and hardscape in areas where there’s considerable foot traffic. They all protect the soil and are important parts of a well-designed, sustainable landscape.

If you just can’t get enough about soil science for gardens and landscapes, do check out this new publication by Dr. Jim Downer and myself.

Fertilizer—Friend or Foe to disease causing organisms?

Gardeners that read this blog understand that minerals are absorbed mostly by plant roots as ions, and are essential for plant growth and development. Some minerals are required in parts per hundred, and are macro-nutrients while others are only required in parts per million or parts per billion, and are considered micronutrients. As long as enough of the 16 most essential minerals are available, plants grow and reproduce in a healthful way. When not enough of one of the essential elements are available, a deficiency occurs, and plants

Nutrient deficiency symptoms in new growth of Camphor tree

may present deficiency symptoms. Mineral nutrient deficiency symptoms are considered abiotic disorders. There are, however, cases where excess or deficiency of elements can be predisposing to disease caused by pathogens. Most some mineral elements do have a role in the development of disease caused by some pathogens but this is largely demonstrated in agriculture and often most home gardens do not suffer nutrient caused plant diseases.
Diseases can be either biotic with a living pathogen driving the disease or abiotic where a physiological condition is caused by the environment and host interactions. Mineral nutrients also are often implicated in abiotic disease.

Perhaps the most famous one is blossom end rot of tomato. This disorder is seen by gardeners around the country and is widely attributed to calcium deficiency. Expanding fruit are a tremendous “sink” for nutrients like calcium and it was thought that if not enough calcium was available in soil the disorder would occur. It is accepted that localized Ca deficiency (in fruit) may play a role in the initiation of blossom end rot, but there are many other factors that lead to the full blown condition, some of which are not fully understood. The fact that blossom end rot (BER) occurs in calcic soils in California underpins the complexity of this disorder. In many cases, simply adding calcium to soils does not correct the problem. Research in California suggest that the plant hormone abscisic acid (ABA) regulates water flow, the development of water conducting tissues, and calcium uptake in tomato. Researchers found that ABA treated tomatoes were cured of blossom end rot. For gardeners, making sure plants are fertilized, and avoiding varieties susceptible to BER is the best course of action.

Soil-borne pathogens are perhaps most affected by minerals dissolved in soil solution. Minerals can act in specific ways (specific ion effects) or as total ion effects (osmotic strength or concentration) having direct impact on pathogenic propagules or on the host itself. In a biological disease relationship there are several possibilities:
• Specific ions harm or favor the pathogen.
• Specific ions harm or support the host.
• Ionic strength changes the root environment making the host weak and susceptible.
• Ions change the pH of the soil solution making it more or less fit for a pathogen or the host.
• Ions change the soil physical environment making it more or less fit for a pathogen or the host.

Root rot of annual color plant is a common find in many garden centers

While it is often espoused that the well “fed” or fertilized plant is resistant to disease, it is rarely borne out in published research on ornamental plants. Keeping a good nutritional level in nursery stock will not necessarily protect plants from many of the virulent pathogens that are capable of causing disease. Excess fertilization may lead to luxury consumption by the fertilized plant and can produce succulent growth that will exacerbate of such diseases as powdery mildew. It is well known that seedling diseases caused (damping off) are more severe with increased medium salinity and it was later discovered that increased soil salinity also increases susceptibility of ornamental plants to Phytophthora root rot diseases. Phytophthora is the most common pathogen associated with rotted roots in most gardens.

Plant mineral nutrition supports plant health in two basic ways (1) formation of mechanical barriers (cell wall strengthening) and (2) synthesis of defense compounds that protect against pathogens. The role of specific elements and their compounds is complicated and unique to each disease/host system. Certainly deficiencies of molecules such as calcium and potassium can interrupt either defense mechanism and if nutrients are supplied enough to prevent deficiencies there is little role of nutrients in preventing disease.
Root rot is a disease of thousands of ornamental plants and a serious problem in many gardens. Root rots caused by Phytophthora spp. occur in a range of nutritional and pH ranges. Nitrogen has been shown to lessen root rots and this is likely due to conversion of nitrogen into ammonia gas in soil which acts as a fumigant. Many studies found no relationship of nitrogen source to root rot disease development. Calcium increases disease resistance to root rot in avocado and other plants. While it is understood that calcium has direct effects on plant membranes, root cell membrane leakage, cell wall thickness, and many other host factors, there are also direct effects on the pathogen in soil. Calcium ions reduce the production of disease spores and disrupt their ability to swim and find susceptible roots. When soils and soil less media are low in soluble calcium, when calcium is easily precipitated out of solution, or when the pH is high and limestone minerals decrease the availability of calcium, root rots will be able to infect. Increases of sodium ions in soils and soil less media can also increase Phytophthora caused diseases.

Some non-essential elements have become popular as disease suppressants. Research has shown Silicon increases resistance of plants to powdery mildew, root rots and to stress in general. Silicon is implicated in strengthening cell walls as well as in defense protein production in plants. But not all plants are capable of utilizing silicon, so its role in plant defense is limited to those species (mostly grasses) capable of metabolizing it. Silicon has been erroneously recommended for widespread disease prevention. Its actual utility is likely very narrow. Much more study is necessary to understand silicon’s role with ornamental plant-pathogen systems. Gardeners will find little use for silicon as a disease prevention too.

Plants extract minerals from container media and garden soils—the process is complicated; it is mediated by the substrate/soil, water chemistry, temperature and the applied minerals (fertilizers) as well as by plants. Gardeners should apply fertilizers that can supply a constant low level nutrient charge or rely on nutrients provided in mulches. Fertilizing decisions are best guided by having a low cost soil analysis by a University lab. Supplying extra soluble calcium may be helpful in managing root rots, especially where heavy rainfall is normal and soils may be highly leached. Preventing salt build up (by leaching irrigation) in high salinity soils (low rainfall places) and that can occur when media dries out, will also help plants avoid infection by root rot organisms. It is good to remember that fertilizers never cure diseases, but there may be a role in preventing disease when plants are nutrient deficient.
References
Baker K.F. 1957. The UC System Producing Healthy Container-Grown Plants. University of California Division of Agricultural Sciences Agricultural Experiment Station Publication #23.

Cherif M., Asselin A., Belanger R.R. 1994. Defense responses induced by soluble silicon in cucumber roots infected by Pythium spp. Phytopathology 84:236-242.

Datnoff, L.E., Elmer, W.H. and D. M. Huber eds. 2007. Mineral nutrition and plant disease. APS Press The American Phytopathological Society, St. Paul, MN. 278pp.

Downer A.J., Hodel D.R., Matthews D.M., Pittenger D.R. 2013. Effect of fertilizer nitrogen source on susceptibility of five species of field grown palms to Fusarium oxysporum f. sp. canariensis. Palms 57: 89-92.

Duvenhage J.A., Kotze J.M. 1991. The influence of calcium on saprophytic growth and pathogenicity of Phytopthora cinnamomi and on resistance of avocado to root rot. South African Avocado Growers Yearbook 14:13-14.

Faufeux F., Remus-Borei W., Menzies J.G., Belanger R.R. 2006. Silicon and plant disease resistance against pathogenic fungi. FEMS Microbiology Letters 249:1-6.

Kauss H., Seehaus K., Franke R., Gilbert S., Dietrich R.A., Kroger N.. 2003. Silica deposition by a strongly cationic proline-rich protein from systemically resistant cucumber plants. Plant J. 33:87-95.

Lee B.S., Zentmeyer GA. 1982. Influence of calcium nitrate and ammonium sulfate on Phytophthora root rot of Persea indica. Phytopathology 72:1558-1564.

Ma, J.F. 2011. Role of silicon in enhancing the resistance of plants to biotic and abiotic stresses. Soil Science and Plant Nutrition 50:11-18.

Macdonald J.D., Swiecki T.J., Blaker N.S., Shapiro J.D. 1984. Effects of salinity stress on the development of Phytophthora root rots. Cal Ag 38:23-24.

Messenger B.J., Menge J.A., Pond E. 2000. Effects of gypsum on zoospores and sporangia of Phytopthora cinnamomi. Plant Dis 84:617-621.

Powell C.W., Lindquist R.K. 1997. Ball Pest and Disease Manual (2nd ed). Ball Publishing Batavia Publishing. 426 pp.

Span T.M., Schumann A.W. 2010. Mineral nutrition contributes to plant disease and pest resistance. University of Florida Publication #HS1181. http://edis.ifas.ufl.edu.

Tonetto de Freitas, S., K.A. Shackel and E. J. Mitcham. 2011. Abscisic acid triggers whole-plant and fruit specific mechanisms to increase fruit calcium uptake and prevent blossom end rot development in tomato fruit. J. of Experimental Botany 62:2645-2656.

Daylor, M.D. and S. J. Locassio. 2004. Blossom-end rot: A calcium deficiency. J. of plant Nutrition 27: 123-139.

Zentmeyer G.A. 1963. Biological control of Phytophthora root rot of avocado with alfalfa meal. Phytopathology 53:1383-1387.

Hydroponics, Aquaponics, & Aeroponics, Part Deux

Last month I shared some basic info on the major techniques for growing plants without soil, namely hydroponics, aquaponics, and aeroponics. With such interest in these topics, I thought it would be good to dive a little further into the technologies used. I’ll provide a bit of basic information about each type of system used for production and provide some resources for further technical reading if you’re interested in learning more. For some simple diagrams of the systems, check out this link (we don’t know if we can “borrow” the images, so we didn’t copy them over).

DEEP WATER

“Deep” water may be a bit of a misnomer, as it usually brings to mind thoughts of mysterious sea creatures living in the dark depths of the ocean. Technically, the “deep” water can be just a few inches, as it is deep in reference to other methods. This is perhaps the simplest and least expensive of the systems and can be a great entry point for beginners.

For deep water culture, the nutrient solution is held in a large container with some sort of floating support holding the plants. The container is at least a few inches deep and holds a relatively high volume of water. There are some containers that are designed for deep water hydroponics, but repurposed containers will work as long as they are food safe (meaning that they do not leach or break down). Large plastic totes or even plastic buckets can be used. As for supporting structures for plants, Styrofoam is the most common. There are cell trays made of Styrofoam that are commonly used in production of small crops (or for growing transplants, which is a common use of this technique). Foam boards with holes to hold pots can also be used. Back when I was in grad school we developed hydroponic systems in my plant propagation class using foam insulation boards floating in large plastic totes.

One thing that you have to keep in mind for deep water culture is the need to incorporate oxygen into the system. We often talk about the issue of overwatering houseplants and how it can damage roots due to hypoxic, or low oxygen, situations. Imagine how roots growing only in water would create situations for poor root and plant growth. In all the other systems water flow helps incorporate oxygen into the water. In deep water, there is no moving water and therefore no air incorporation. The most common tool used for this, especially for small systems, is an aquarium air pump and air stones that help create bubbles in the system.

One benefit of this system is that it has a low level of risk when it comes to system failure. There are few moving parts to break down and loss of electric doesn’t result in roots drying out due to loss of water flow.

EBB & FLOW

Ebb and Flow troughs in an aquaponics system. Note the floating styrofoam rafts. (I did research in this system during my master’s program.)

These systems, also called flood and drain systems, are one step of complexity above the deep water systems by introducing water flow. Plants can either float as in deep water culture or be held in media that fills the container. While many containers can be used, the most common are longer channels that promote water flow from one end to the other. This system also introduces a reservoir of some sort that holds excess nutrient solution and a pump to deliver it to the container. The level of water in the container is controlled by a raised drain pipe where solution exits the system back to the reservoir.

The DIY system I build using gutter with the Rwandan students (mentioned in the first installment on hydroponics) is ebb and flow. The drain from the gutters is a few inches high within the channel, so the water raises those few inches before it drains out. Some producers use long channels the width of those floating cell trays to grow plants in a relay fashion, planting them on one end and move them along as new rafts are added until they are harvested on the other end.

This system is common not only in hydroponics, but aquaponics as well. Instead of a nutrient solution reservoir, the water from the tank(s) holding the aquatic stock (commonly fish, but could also be crustaceans like shrimp) is pumped into the plant channels and flows back into the system. Systems may be based on continuous flow into and out of the system, but most commonly a timer is used to have multiple periods of flow and rest mainly as a means to reduce power usage.

NUTRIENT FILM TECHNIQUE (NFT)

This system evolved one more step above ebb and flow by limiting the volume of water used in the system. Here, water is pumped from the solution reservoir to shallow channels where plants are held in pots or blocks of inert media such as rockwool. Roots are not submerged in water, but instead grow within a thin film of solution that flows almost continuously through the system. These channels have a slight slope where the end with the drain is a little bit lower than the end where the water enters. The slope can be adjusted slightly to affect the speed of the water through the system.

This system is becoming common in production of leafy greens and herbs because it uses a much smaller volume of water. But that small volume of water also presents a risk. If there is a power failure or a clog in the tubing that delivers water to the system the roots can very quickly dry out and crops die, especially in situations of high heat and light.

DRIP SYSTEMS

Perhaps one of the most commonly used systems across the world due to their simplicity, drip systems could be compared to a drip irrigation system used in the field. Drip emitters are used to supply nutrient solution to plants in containers containing an inert media such as peat, coir, perlite, or grow stones. The containers can be pots, buckets, or bags/blocks of the media and are most commonly placed on the floor of the greenhouse or growing location with gutters to collect the solution that flows through the containers. A common method is using long, narrow bags filled with coir or other media referred to as the slab method. Another common method, called the Dutch bucket method, uses buckets with drain holes in the bottom, commonly placed on a greenhouse floor. Water trickles down through the media and roots and leaves the system through the bottom of the container.

Systems vary in the collection of the used solution. Some may collect the solution that flows into the gutter and collect it in a reservoir to be reused, however some systems may allow the solution to flow out as waste. These differences depend on the needs of the producer, available resources, and local regulations.

One of the comments that we got on my first article was about people growing container plants could technically consider it a form of hydroponics. That might be a bit of a stretch, but you could technically consider growing container plants in soil-less media as drip or flow through hydroponics if you provide all of the nutrients through soluble fertilizers in the water.

WICK SYSTEMS

Typically used for small scale production, wick systems are one of the simple ways to grow plants without soil in terms of technology. In this system, a passive wick uptakes nutrient solution from a reservoir and pulls it into the media (usually absorbent itself). This wick could be a true wick, like a string made of absorbent material that inserts into an individual pot or it could be a mat made of absorbent material that pots or trays sit atop.

I’ve seen this commonly used perhaps not strictly in hydroponics, but for watering individual plants like African violets where yarn or twine is inserted into a drain hole in the pot and sits in water. Technically this could be hydroponics if the media doesn’t contribute nutrients to the plant and they are all contained in the water instead.

KRATKY METHOD

This is probably the simplest of the methods and is used primarily by small scale producers and home growers. It is similar to the deep water method in that there is no flowing water, but it is even simpler because there isn’t even an air bubbler. In this method, plants are grown in large containers or buckets and the structure that supports them is fixed to the top of the container rather than floating. As the growing solution is used up, the level of solution in the container decreases. This creates a zone where the roots are exposed to air, providing the oxygen that the roots need. The solution is kept at a level where at least the bottom portion of the roots are submerged in the nutrient solution.

AEROPONICS

Probably the most complex or technical system, aeroponics supplies water and nutrients to plants through a mist or aerosol emitted through pressurized nozzles. The roots hang in a chamber without media and are misted every few minutes with nutrient solution. The excess solution drops to the bottom of the chamber and is reused. This system uses very small amounts of water, which can be beneficial for growing in dry areas but also creates a potential risk if the system or power fails. Just like the NFT system, any prolonged period with out water will quickly result in plant damage or loss. Beside power loss, this systems is also prone to clogged emitters, since the pressurized nozzles rely on very tiny openings to pressurized the solution.

Keep in mind that several systems that are sold for home or small scale production that are labeled as aeroponic, such as AeroGarden and Tower Gardens, don’t technically use aeroponics to grow since the solution isn’t applied as a mist or aerosol. I would say they operate more like a vertical NFT system where water flows over the roots as it travels down the chamber.

RESOURCES

Hydroponic Greenhouse Production Resources – UMass Extension

Introduction to Hydroponics – Johnny’s Seed

All You Need to Know to Choose a Hydroponic System – Upstart Farmers

How to Start Growing with the Kratky Method – Upstart Farmers

Garden potions and notions to avoid

It’s Halloween and terrifying things abound – particularly at garden centers. Below you’ll find a pictorial approach to four frightful follies. Enjoy – and keep your garden safe!

Scary soaps. No. Not on your soil to aerate it. Not on your plants as part of some homemade devil’s brew. Soap stays in your house.

SOAP is not an acronym for Super Organic Agricultural Product

Petrifying phosphate. Not for flowers. Definitely not for transplanting. No matter how friendly and natural they look on the shelf, they are death to mycorrhizae and any aquatic system they wash into.

“Bone meal” can be anagrammed to make “le bemoan.”

Murderous mulches. No cardboard. No plastic. And definitely no astroturf. The word “smother” does not conjure up a pretty picture for living soils.

The chips are great. The cardboard….not so much.

Cardboard’s so resistant to water you can make ponds out of it!

Artificial lawn should only be used for indoor minigolf.

Zany zombies. These useless products live on in their science-free environments. Just…go…away.

Business is good even if efficacy isn’t.

Yes, because plants love stag’s bladders and cow mesenteries.

You can find posts on these products by using the “search” box on the left hand menu. Or you can consult your Ouija board using this handy mulch planchette.

Will my tree survive? Ouija board says “Goodbye.”

Trees and Turf a NO GO

It seems so simple to plant a tree. But to grow a tree is more difficult! In many parts of the United States there is enough water for trees and turfgrass, but it is often a bad idea to mix the two. You may have observed that sometimes young trees do not grow as well when planted in turfgrass. Certainly this is a generalized view and tree/turfgrass genetics are very different between their respective species. So it is natural to expect different outcomes when planting different species of trees in any landscape setting, turfgrass notwithstanding. Another factor to consider is time. The day we plant a tree is not the same time reference as ten years later. In ten years, the tree if it is successful, may have modified

A basic incompatibility: Eucalyptus growing in warm season turfgrass (kikuyugrass) resulted in excess surface moisture and crown rot of the eucalyptus killing it.

its environment significantly, making turfgrass cultivation more difficult. Most tree/turfgrass difficulties begin when the tree is young–as a newly planted tree. If it succeeds in growing a large canopy, difficulties will ensue for the turfgrass. Sometimes turfgrass cultural requirements (frequent irrigation) can predispose trees to root or root-collar diseases such as Phytophthora.

Trees and turfgrass have some similar and very different requirements from their respective landscape settings. Both trees and turfgrass require sunlight to photosynthesize and grow. Both would usually prefer full sunlight without shade. As trees grow they shade the turfgrass sward beneath their canopies. Turfgrass can lose density, and become a thinner sward that is more susceptible to diseases such as powdery mildew. Trees grow roots near the soil surface and as they become larger, some trees may even proliferate roots near the mowing height of turgrass and suffer repeated injury from mowers, also increasing the risk of pest invasion into the tree. Both trees and turfgrass need water and soil minerals to grow. While soil minerals are usually abundant enough for both, water is often limiting for one or the other in this landscape combination.

String Line Trimmer’s or weed whips will injure both young and older trees in the landscape. Image: Chicago Tribune.

The maintenance practices required for turfgrass often injure trees, especially young trees. Mowing near trees can injure the bark on the lower stem especially if the mower comes to close and actually scrapes the young stem. Since grass will grow longer where the mower can’t reach right near a tree stem there is a temptation to use a string line trimmer or weed whip to maintain the grass that has shot up around the tree stem. The repeated use of string line trimmers around trees removed young bark and can “girdle” the tree stem. While trees can survive these practices their growth rates are slowed considerably.

Constant injury from mowing equipment has injured this elm killing the tissue on a major root flare. This is now an entry point for decay and other fungi.

One approach to having trees growing with turfgrass is to remove a ring of turf away from the tree and replace it with mulch. This eliminates the need to maintain the turfgrass near the trees stem and root flare. Richard Harris and others (1977) found many years ago that a one foot circle removed around the stem of newly planted trees would increase their establishment rates compared to trees with turfgrass growing right near the stem. Whitcomb (1979) also recognized that turgrasses are competitors with newly planted shade trees. Whitcomb’s earlier research (1973) showed reduction in root density when trees were planted in a sward of Kentucky bluegrass.

A ring of mulch replacing turfgrass around this tree keeps turf maintenance equipment from injuring it.

As trees grow it is important to widen the ring around them giving more room for mulch and reducing the competing turfgrass underneath their expanding canopies. This is a general concept; some trees can live in turfgrass without problems as long as resources are not limiting. Riparian trees such as sycamore can grow well in swards of turfgrass, but other species such as Peruvian pepper (Schinus mole) tend to languish.

Trees are adapted to drop leaves, this is termed litterfall and it becomes part of their natural mulch. Litterfall tends to prevent annual plants such as grasses from developing. Fallen leaves, fruit and twigs are recycled by fungi providing nutrients back to the tree. Turfgrass cultivation interrupts this process and while trees obtain some of the nutrients supplied to turfgrass, as Whitcomb observed, turfgrasses are fierce competitors for nutrients so young trees are especially susceptible to nutrient deprivation in turfgrass swards. For the best results in your

This young tree has an expanding mulch area to help sustain it and reduce competition from turfgrass.

garden, it is best to maintain some distance between young trees and turfgrass. It is optimal if the mulched (no turf) area under a tree can expand to its dripline as it grows.

DIY Hydroponics: Going soil-less at home and abroad

It seems that as interest in gardening grows, especially among younger generations, interest in different techniques that home gardeners use and different plants they grow are also on the increase. You see the old standbys like straw bales and containers emerge. Terraria, succulents, and air plants are having their moment. And all kinds of technology driven indoor growing systems are all over the web, mostly hydroponic, but some aeroponic and aquaponic as well (we’ll talk about the difference in a bit – if you’re just here for that, skip the first 2/3 of the article).

I had been thinking about getting one of those new techno aeroponic growing systems as a demo for my office as a discussion starter for those interested in controlled environment growing whether on the homework commercial scale. There is a general interest and need for basic education for hydroponics and aquaponics in the area that I hope to build extension programming around, so having something at the office could provide some interest from walk-in and social media clients. I had dusted off a first generation AeroGarden that I found in the storage shelves in the office storage catacombs and set it up in my office. It is a far cry from the new models I saw in those online ads that are outside of my budget for “toys to show off at the office.” It doesn’t have nice LED lights or connect to my phone via Bluetooth like the fancy new models. Given its age, it produces more noise and heat than the lettuce and herbs I’ve tried to grow in it. Maybe I’ll be able to get one of the fancy models one day.

Then I remembered a book that an urban ag friend of mine had written on building DIY hydroponic systems from common building materials and resolved to not only build a system, but incorporate it into my programming somehow. The book, appropriately titled “DIY Hydroponic Gardens: How to Design and Build an Inexpensive System for Growing Plants in Water” by Tyler Baras shares plans for building a variety of types of hydroponic systems using basic building materials like gutters and lumber, drip irrigation tubing and fittings, and various other bits and bobs. Tyler had been a featured speaker for the West Virginia Urban Agriculture Conference that I started and hosted when I worked for WVU Extension, so the book was on my radar – I placed an order. (Note: I don’t get a kickback for sharing the book – just sharing a good resource that happens to be from a friend.)

Teaching Hydroponics to an Unlikely Audience

Image may contain: 3 people, including John Porter, people sitting, outdoor and nature — Learning traditional weaving methods using banana leaves. Banana leaf weaving is a common industry in rural Rwandan villages that allows women to provide modest incomes for their families.

As luck would have it, I had an opportunity to put the book, and my DIY hydroponic skills, to the test. Our university does quite a bit of work with and in Rwanda and in May I had the opportunity to travel to Rwanda as part of a study abroad program with my Ph.D. advisor. Rwanda is a very small country, just under the size of Massachussets, with a very big population by comparison – 12 million vs 7 million! Feeding that many people is a struggle, and even though Rwanda produces a lot of produce (and more lucrative export crops like coffee and tea), it still imports a lot of its fruits and vegetables. We were studying how innovation spreads in rural areas, and just before our trip I found a news article sharing that there would be an upcoming $8M USD ($8B RWF) investment in hydroponics in the country in order to increase production on the limited amount of land available.

In June I was scheduled to teach a group of Rwandan exchange students that are part of a sponsored program at the university, and remembering the planned investment in hydroponics I planned to add DIY hydroponics to the curriculum. This is fitting, since most small-scale operations would rely on finding what materials would be locally available. While the operations started by the investment would likely bring in “real” hydroponic systems, if small scale producers want to use the technology or if individuals want to build skills, they’re going to have to use what is at hand.

UNL CUSP Scholars students from Rwanda build a DIY Hydroponic System

Planting leafy greens and strawberries in the hydroponic system.

It was interesting teaching an audience who were interested in learning about the new technology, but have little experience or general knowledge on the subject. Even more interesting was the fact that many of the students had not used or even seen some of the basic power tools we used in building the system. I’m no shop teacher, but in the end the students not only learned a little bit about hydroponics and hydroponic systems, but also some skills using tools that they can apply in other applications.

Proudly showing off the team’s vertical hydroponic system.

Hydroponics, Aeroponics, & Aquaponics – Oh My!

Earlier I mentioned that there are differences between hydroponics, aeroponics, and aquaponics. In some ways, they use similar basic setups. All are based on soil-less growing using an inert media to support plants, supplying nutrients and water directly to the plant roots, and providing light to the plants using either natural sunlight or supplemental lighting. Differences come from the source of plant nutrients and from how they are delivered to the plant. I thought I’d take a few minutes to talk about the basics of each of the techniques so you can understand the differences just in case you want to buy or build your own system. If there’s interest, I hope to focus on hydroponics and controlled environment agriculture over my next few blog posts – tell me what you’re interested in learning.

Most people are familiar with the concept of hydroponics. This technique relies on roots being submerged in a nutrient-rich solution where the nutrient content is engineered from a variety of mineral sources. There are a variety of different systems (that will hopefully be the subject of an upcoming blog) where the root zone interacts directly with the solution. In some cases, roots are submerged in a large volume of solution while in others a film or shallow stream of water flows through the root zone. Systems where roots are submerged in the solution may simply be a large reservoir where the plants float on top where systems relying on flow may involve a pump. Movement of water adds another plant need -oxygen, which is required for respiration by the roots. In systems where there is no flow, air is often pumped in to provide oxygen.

Most flowing systems are recirculating, where the solution returns to a reservoir and is pumped back into a reservoir to be reused. While it may seem counterintuitive, these recirculating water based growing systems have been touted as production methods that conserve water. That’s why some of the leading hydroponic production and research comes from areas of the world where water is scarce. Less common are flow through systems where water and nutrients are not recaptured but discarded after initial use.

Aeroponic systems have much of the same basic setup but instead of the roots interfacing directly with water solution it is applied as a pressurized mist. These systems generally use a much smaller volume of water, but there are some drawbacks. Failure of the system, such as an electric outage or clogging of the nozzles that pressurize the mist (which is a common occurrence) can quickly result in plant failure since roots can dry out quickly. Several systems that are sold commercially that market themselves as aeroponic, such as the AeroGarden or Tower Gardens, are more similar to a flowing hydroponic system than a pressurized mist aeroponic system.

The plant growing structures of aquaponics are similar to those of hydroponics, with the addition of larger reservoirs to accommodate the addition of aquatic livestock such as fish (or sometimes crustaceans). The waste produced by the stock provide the nutrients needed by the plants rather than an engineered nutrient solution. These systems require having the technical knowledge to meet the needs of the aquatic stock and balancing those with the needs of the plants. The addition of the aquatic stock also introduces a microbiome of bacteria and fungi, many of which are required for animal health but can also introduce pathogens that can negatively affect human health.

Are you interested in learning more about these systems? What do you want learn about in hydroponic or other systems? Let me know in the comments and I’ll try to base some future articles on what our readers are interested in.

Late summer pruning: what happens, what won’t, and why

In the fall a gardener’s fancy lightly turns to thoughts of pruning (with apologies to Alfred, Lord Tennyson). In particular, people worry that pruning too late in the summer or early fall will stimulate plants to send out new growth, which is then damaged by freezing temperatures. Let’s dissect what actually happens when woody plants are pruned during this time.

First, we need to separate temperate trees and shrubs from tropical and subtropical species. For the most part, the latter don’t become winter dormant: pruning them at any time means you will have regrowth as long as there are sufficient resources. If planted in more temperate zones, they will continue to grow until they are killed by freeze damage. Instead, we’ll look at temperate species and how they are adapted to surviving winter conditions.

Tropical woody plants like this jade are not winter dormant species. Don’t leave them outside even if you think they are protected (lesson learned).

I wrote a couple of posts last year on cold hardiness (here and here), so I won’t repeat those discussions on how plants survive freezing. Instead, we’ll focus on the process of HOW plants enter winter dormancy and become cold hardy. It’s a two-step process that depends on two different environmental factors: one that never changes from year to year, and one that certainly can.

The first step to dormancy is initiated right after the summer solstice. Plants are exquisitely adapted to changes in the light-to-dark ratio, and days begin shortening after the summer solstice. The changes that occur are largely biochemical, but you can also see some changes in plants themselves. Many trees and shrubs slow their growth during this time so that fewer young leaves and shoots are produced. Instead, resources are put into the existing foliage, or flowers for summer bloomers. Excess resources are routed to woody parts of the plant for storage.

Light and dark ratios vary with latitude, but the seasonal changes are always the same time of year.

From a practical standpoint, this means that when you prune trees and shrubs where growth has stopped, you will NOT get regrowth. The vegetative buds below the pruning cut are dormant. The tricky thing is that the exact time when the switch is thrown varies by species and is affected by environmental conditions. Careful observation will allow you to estimate when the plants will no longer produce new growth.

Some temperate species naturally put on a spurt of late summer growth. The leaves on these new Japanese maple shoots generally die from cold damage, but the branches themselves survive.

The second step begins when night temperatures cool to near freezing, which is not a predictable date. Because many of the biochemical and physiological processes have already begun or are finished, the response to cold night temperatures is rapid and visible. Leaf colors change as the plant begins breaking down leaf materials for mobilization and storage elsewhere in preparation for winter dormancy.

This katsura has started the process of autumn leaf senescence.

This process, honed over millions of years, is unfortunately not infallible especially under abnormal environmental conditions. Two examples spring to mind:

High intensity street lights. If the normal light-to-dark ratio change is interrupted by significant levels of night light, the first step of dormancy is hijacked. You can see what happens in these previous blog posts here and here.

That street light in the middle has kept the nearby leaves green while those farther away are senescing.

Unseasonably cold weather. With climate change, we are seeing wild shifts in all sorts of weather patterns, including the date of the first hard freeze. Hard, early freezes are not the same as a light evening frost. You can see what happens here:

A hard freeze in early November fried the leaves on this hydrangea.

Given normal conditions, however, temperate trees and shrubs are well on their way to full winter dormancy by late summer and early fall. Pruning them is not going to induce new growth.

Professional Credentials and Gardening Expertise: Arborists

Arborists, Professional Credentials, and Designating Bodies

ISA Certified Arborist logo, courtesy of ISA.

The Merriam-Webster dictionary defines arboriculture as “the cultivation of trees and shrubs especially for ornamental purposes” (2019a) and arborists as the specialists that care for those trees (2019b). In the US there are two primary certifications for arborists. Arborists can become a Registered Consulting Arborist (RCA) through the American Society of Consulting Arborists (ASCA) (ASCA, 2019a) and/or become an International Society of Arboriculture (ISA) Certified Arborist (ISA, 2019a). The ISA also offers a range of associated certifications, including ISA Certified Arborist Utility Specialist, Arborist Municipal Specialist, Tree Worker Climber Specialist, Tree Worker Aerial Lift Specialist, Board Certified Master Arborist, and ISA Tree Risk Assessment Qualification. For the sake brevity, the focus of this post will be on the flagship programs from each organization, ASCA’s Registered Consulting Arborist and the ISA Certified Arborist credentials.

Relevance for Gardeners

Many gardeners who have trees are likely familiar with arborists, or at least with a local “tree guy”. While trees are beautiful, they can also suffer from disease, nutrient deficiencies, and other plant health issues. In addition, trees often require maintenance and pruning to ensure safety, avoid damage to houses or other property from falling branches, or to allow more light through the tree canopy to reach the ground and garden. Maintenance and pruning of trees requires experience and expertise to ensure that trees aren’t damaged in the process, and that pruning is done safely. When their services are needed, hiring an arborist with professional credentials can be an excellent way to ensure that trees are properly cared for and that nobody is hurt in the process.

In my personal experience, certified arborists in my community charge more per hour. However, the extra cost in exchange for ensuring the 60 year old pin oak that is the same age as my house lives on for another 60 years is worth it.

Other Considerations in Choosing an Arborist

Whether an arborist is certified or not, it is a good idea to check that the arborist is insured. That is because if they or one of their crew is injured on your property, you as the property owner may be liable for any injuries that occur while tending to the trees on your property.

Also, as with hiring any professional for home repairs or improvements, it’s important to check their references or get recommendations from colleagues and friends. You should also check whether your local government has any regulations on tree management within city limits. If so, be sure that who you hire has experience with following those regulations, and has all of the business registrations and approval from the local government.

It is also important what your needs are. If you need a large specimen tree pruned and the potential for tree injury or death would be devastating, then hiring someone with documented expertise is important. If you’re instead just trying to get the half-dead tree in the back yard chopped down and there’s no risk of falling a tree onto a power line or structure, then documented expertise might be less important (though insurance might be, see above).

In addition, if you are getting a large tree taken down, consider the value of the wood in your tree. Large hardwood trees may have significant value. Some tree guys will offer to take it down for free in exchange for the wood. While this may sound like a good deal, I know people who were shorted thousands of dollars from such transactions when considering the value of the wood and the per hour cost of an arborist.

Type of Credential: Professional Certificate

Both the RCA and ISA Certified Arborist credentials are considered professional certificates. This means that these are optional credentials. Arborists are not required by law to be certified. This means that an arborist can operate without certification. Arborists that are certified have had their credentials reviewed, and meet or abide by the criteria described below.

Education and Professional Experience Requirements

Registered Consulting Arborists must be a current ASCA member, and be a graduate of the ASCA’s Consulting Academy (ASCA, 2019b). No additional education or work experience is required.

ISA Certified Arborist must have three or more years of experience in arboriculture and/or a degree in the field of arboriculture, horticulture, landscape architecture, or forestry from an accredited institution of higher education (ISA, 2019b).

Qualifying Exams

Registered Consulting Arborists must pass an open-book exam as part of the ASCA’s Consulting Academy, which also includes other assignments before, during, and after the academy (ASCA, 2019c).

ISA Certified Arborists must pass a qualifying exam (ISA, 2018).

Code of Ethics

The ISA Certified Arborist program has a code of ethics that all certified arborists must abide by (ISA, 2019c). The RCA program does not have a code of ethics.

Continuing Education

Registered Consulting Arborists must complete 420 continuing education units (CEUs) to be eligible for the RCA credential (ASCA, 2019b). Their website does not specify if additional CEUs are required in order to maintain the RCA credential.

ISA Certified Arborists are required to complete 30 CEUs in a three-year period in order to maintain their certification (ISA, 2019d).

References

ASCA. 2019a. The RCA. American Society of Consulting Arborists. https://www.asca-consultants.org/page/RCA (accessed 25 September 2019).

ASCA. 2019b. Eligibility/Fees. American Society of Consulting Arborists. https://www.asca-consultants.org/page/EligibilityFeesRCAs (accessed 25 September 2019).

ASCA. 2019c. ASCA’s Consulting Academy. American Society of Consulting Arborists. https://www.asca-consultants.org/page/ConsultingAcademy (accessed 25 September 2019).

ISA. 2018. ISA Certified Arborist Application Guide. https://www.isa-arbor.com/Portals/0/Assets/PDF/Certification-Applications/cert-Application-Certified-Arborist.pdf.

ISA. 2019a. Types of Credentials. International Society of Arboriculture. https://www.isa-arbor.com/Credentials/Which-Credential-is-Right-for-You (accessed 25 September 2019).

ISA. 2019b. ISA Certified Arborist. International Society of Arboriculture. https://www.isa-arbor.com/Credentials/Types-of-Credentials/ISA-Certified-Arborist (accessed 25 September 2019).

ISA. 2019c. Code of Ethics. International Society of Arboriculture. https://www.isa-arbor.com/Credentials/ISA-Ethics-and-Integrity/Code-of-Ethics (accessed 25 September 2019).

ISA. 2019d. Maintaining Credentials. International Society of Arboriculture. https://www.isa-arbor.com/Credentials/Maintaining-Credentials (accessed 25 September 2019).

Merriam-Webster. 2019a. Definition of Arboriculture. Merriam-Webster. https://www.merriam-webster.com/dictionary/arboriculture (accessed 25 September 2019).

Merriam-Webster. 2019b. Definition of Arborist. Merriam-Webster. https://www.merriam-webster.com/dictionary/arborist (accessed 25 September 2019).