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Gardeners often struggle to grow plants in containers. You may feel that you have a really black thumb at times when newly planted seedlings fall over dead or fail to thrive. The problem may not be disease or poor gardening acumen but rather your container media otherwise sold as “Potting Soil”. A trip to one of the big box stores or a larger retail nursery will offer gardeners many choices of bagged potting soils. They are marketed to give you the impression they will grow anything and everything. But do they?
Over the last couple of decades I have done comparative potting media trials where I plant small plugs (usually impatiens) three per six inch container. I go out and find every retail brand of potting mix I can find and plant them all up and then follow them for about two months. I’ve been thinking of revisiting the studies and seeing if anything has changed. I also want to test the assumption that you can’t predict the grow ability of a potting soil by reading the ingredient label as some research suggests. While there can always be a surprise with any given product, I think that from my many trials I can make some suggestions to improve the outcome of your gardening adventures in containers.

Growing media is not the same as soil. Since media are placed in containers, often plastic ones, they need to be very porous. Porosity of up to 50% is not uncommon in container media. The bulk of the media needs to hold water and minerals for plant growth. Usually an organic material that has a high cation exchange capacity is used. The darling of potting mixes has been Peat Moss. Since peat moss harvesting is damaging to the environment (see previous blog by Linda CS), many gardeners may want to avoid media with peat moss. Bulking agents that do not hold much water or nutrients are also added to “lighten” or aerate the medium. Horticultural perlite (expanded volcanic glass) is the most common. Sand is also sometimes used but it adds weight to the bag and is not preferred by manufacturers. Some media use bark or other wood products to provide greater porosity.
There are usually about 18 to 20 different media on the market at any given time and the results of growing plants in them is predictable. About 10 of the media will not grow anything very well, 5 give ok results and about 5 of the products will grow a nice plant. A lot of the reason for success or lack thereof is about nitrogen chemistry. If no fertilizer is added, the medium will likely not grow well. You can add your own fertilizer and make about  ½ of these poor growing media work. One quarter to one half a teaspoon (approximately 2gm) of ammonium sulfate usually peps up most media that are ok but lack nutrients.  This is an amount used in a standard height 6inch (15cm) diameter plastic container.  Larger containers and plants will require incrementally more fertilizer to achieve growth goals. 
Some media will not grow even when fertilized. This is because they may contain manures, or composts and manures that have added too much salt to the medium. Adding fertilizers to these products only makes them less growable. Sometimes these potting soils will improve with leaching but then fertilizer will need to be added back later to make up for what was leached away. Generally a salty potting mix is worth avoiding.
So how can you tell if you are getting a good or bad mix. You can start by reading the ingredient list. And you will need to decode that list to help you make some decisions. What manufacturers call things can be very misleading. Look for a medium that has fertilizer added and lists what kind of fertilizer was used. These media usually grow without help. Avoid media that use manures, they are not suitable container media ingredients.

Some potting soils claim they can grow plants bigger than others, some claim to be all organic and some claim to be friendly to the earth. This is all marketing. Look for a simple ingredient list that is fortified with a nutrient charge (fertilizer). Begin there. You may want to sieve the medium to remove large particles if you are growing seeds, add more bulking agent (bark, sand, perlite, pumice) for plants that need increased porosity such as orchids, bromeliads and cactus. Don’t be afraid to modify potting mixes to suit the needs you might have. If plants don’t grow, consider adding more nutrients. After growing for some time (months to years), many media will breakdown, and the plant will need to be repotted in a new medium.

Last week I helped to train Master Gardeners about pruning fruit trees. January and February are the months that we recommend fruit tree pruning in Southern California.  In colder climates, pruning may not occur until later when freezing temperatures are minimized and there is less chance of damage to new growth. While trees don’t “need” pruning to bear fruit, pruning practices can enhance fruit production, promote earlier fruiting bearing buds, and increase fruit quality if done in an informed way. In many respects, modern fruit trees have been bred for big fruit, and pruning might need to be done to prevent limb breakage, reduce the number of fruit and position it in the tree fore ease of harvest. Misinformed pruning can lead to disease or loss of bearing wood. “Fruit tree” is a broad category, but for this blog, I am referring to deciduous trees (not subtropicals such as citrus, avocado, mango etc.). Two main categories are common: Pome fruits such as apples and pears and Stone fruits such as cherries, plums, apricots, peaches, almonds and pluots.

The first thing to figure out when pruning any tree grown for fruit production is where the fruit will be formed. This requires examining and understanding buds, twigs and the age of growth that is produced. Second we need to understand the tree’s responses to pruning and how that will affect future fruit production. Finally an understanding of negative consequences of pruning is essential.

Why prune you ask if trees will produce without pruning? Pruning shapes a tree, and helps to create fruiting buds that are conveniently placed for harvest-this keeps fruit pickable with less time on ladders. Pruning gives an opportunity to remove fruiting buds thereby invigorating remaining buds and increasing size and quality of the fruit that will form with less fruit thinning later. Pruning also gives an opportunity to remove diseased, damaged, tangled or infested branches.  While various training styles can be used for structural pruning of young fruit trees the open vase or modefied central leader systems are preferred and descriptions of them can be found in extension leaflets.  For my own trees I usually do not prune them the first year after planting in order to encourage a stronger root system.  In the second and third years I pick scaffold branches or train branches on the central leader.

Fruit is produced on various aged twigs or branches depending on tree species. Peaches produce fruit on growth from the previous year or one year old wood. Since peaches grow vigorously fruiting wood ends up on the outside of a tree. Heading back (or heading) cuts (reducing last year’s branches by at least half their length) will remove ½ the fruit and stimulate buds lower in the tree that will make more fruiting wood. For this reason peaches are usually pruned “hard” to stimulate maximum amounts of fresh fruiting wood. Apples, Pears, Plums, Cherries and Apricots produce most of their fruit on small side branches called spurs. Apples and Pears may also produce fruit from the terminal bud.

Young trees often make many long whips and these are usually headed back (heading cuts remove the terminal bud) to stimulate spurs in the following years. Once the overall shape and size of the trees are set, less pruning is required as spurs may produce fruit over decades of time. As trees mature spurs build up so removing densely clustered spurs on mature trees with thinning cuts (removing an entire branch, spur or twig) will increase the size and quality of fruit formed on the remaining spurs.

Pruning is often used on newly planted trees to form the structure of the tree. When forming the branch structure do no indiscriminately head back every branch as this will stop the growth of the branch that is headed. New growth will only resume from buds that are released to grow. Think carefully about what you want to grow and what you want to slow-down in growth. Pruning is always a growth retarding practice. Branches are best spaced up and down and around a central leader. In other training systems for stone fruits one heading cut when the tree is just a whip will create an open vase shape where all the branches arise from a single point on the trunk. While this is considered a branch defect in shade trees, it is a convenient training system for fruit trees if you don’t let the tree get too large and manage the fruit loads that are produced. Trees trained to a modified open center where branches are spaced on a central leader have stronger branch attachments and can bear greater fruit.

As trees age and grow they require regular training with heading cuts to shorten vigorous branches of peaches or thinning cuts to remove whips, water sprouts or other unwanted branches.  Be careful not to over-prune especially in summer or sunburn can result.  When fruit sets in the spring or early summer it can be thinned by hand.  This form of pruning will increase size of the remaining fruit and quality.   Summer pruning is sometimes practiced on very vigorous trees to slow their growth and invigorate buds for the following spring.  Prune with care in the summer espeically on green barked trees like apple and pears to avoid sunburn.

[This blog post has been provided by Bec Wolfe-Thomas, an administrator for the Garden Professors blog group on Facebook.]

Woodpeckers (Picidae) frequently get a bad rap from gardeners. It’s often their impression that the birds irreparably damage trees, but this is untrue. Most woodpeckers are insect eaters; they can hear insects under the bark and in the wood of trees. They then target their drilling with uncanny precision to get their meal. This removal of insect pests, such as emerald ash borer, benefits the tree.

And what about the feeding holes left in the tree? This is an exciting bit of tree physiology! Trees are able to compartmentalize or isolate the wounds. After the woodpecker has made a hole to retrieve the insects within, the tree starts compartmentalizing the wound. How long it takes for a tree to compartmentalize a wound and close it depends on species and climate factors.

Below are photos of woodpecker holes in various states of compartmentalization, from freshly drilled to completely compartmentalized and closed holes. Woodpeckers help keep trees healthy by preventing large pest infestations. And while the small feeding holes might be an aesthetic concern to gardeners, they’re only temporary. They will eventually be compartmentalized and closed, and the tree will be healthier in the long run by having fewer pests.

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[Please note the larger holes excavated for nesting will compartmentalize but will not close over time.]

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.

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.

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.

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

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.

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.

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

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.

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.

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.

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

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.