June 2020 – The Garden Professors™

Summertime pruning

Summer is a time of bounty in the home garden. During June, July and August the majority of small fruits ripen on home orchard trees. Plant health care is important to consider in advance of summer bounty. Careful dormant season pruning, dormant sprays, mulching and care helped to produce a nice harvest. As the fruit comes off the tree, some summertime options are available. This is a time when some limited summer pruning can be done to manage the physiology and growth of many fruit trees. Even some citrus will benefit from careful summer pruning.

Summertime brings a harvest bounty for many home gardeners, and with it an opportunity to modify tree growth with pruning

One obvious reason to prune in summer is to repair broken and remove dead branches that may have occurred from excess fruit weight or other injuries. Breakage is common in peach, plum and apples if fruit loads are not thinned earlier in the season. Cut the broken branch from the stem it attaches to with an angled cut that leaves the branch collar intact. Do not cut branches flush with the stem they were attached to. Many years ago the myth of flush cuts for shade trees was found to permanently damage trees, but flush cuts are often still practiced on fruit bearing trees. Flush cuts allow decay organisms to enter trees leading to heart rot and other kinds of wood decay.

Sporophores or fungal fruiting bodies indicate the presence of wood decay in trees. Usually be the time sporophores are showing wood decay is extensive in the tree.

Another myth that persists in home fruit orchards is painting wounds with a ‘sealant’ or ‘protestant’. There is no reason to paint cuts. They do not limit the progress of decay or prevent decay from forming behind the paint. Pruning paints do not promote “healing” or callus formation to close the wound. There is some thinking that pruning paints may even accelerate the process of decay formation. So throw away the black tar, it has no practical purpose in support of pruning.

Pruning paints are a relic of past horticulture traditions. They have no place in modern arboriculture or pomology

While pruning paints are no longer used, paint has other functions that can be helpful. If a large branch was removed from a tree, sometimes the remaining branches may require protection from sunlight. Apples and other thin-barked trees (citrus, cherries, etc.) are very susceptible to sunburn. If branches that were previously shaded are suddenly exposed to high light levels, the bark can be destroyed leading to sunburn cankers and entry of disease-causing fungi such as Botryosphaeria spp. If repair pruning exposes a large gap in the canopy, it is appropriate to apply white wash or diluted white latex paint to exposed branches in order to protect them from bright sunlight. The most severe damage occurs on southern and west facing branches. Sunburn is one of the leading causes of abiotic damage and a predisposing factor for disease such as stem and branch cankers in apples.

Fireblight is another common disease on pears and apples and develops after bloom. Pruning out fireblight affected twigs helps to arrest disease progress. Finally, bacterial canker can be devastating to Prunus (plum, cherry, peach, nectarine and almond) in parts of the country with warm summer rains. Immediate removal of bacterial canker affected branches is necessary to prevent permanent damage to the tree. Tools used to remove cankered branches should be sanitized by flame (torch) or with disinfectants. Canker diseases are active in the warm summer growing season. Cankers can be caused by bacteria or fungi and should be dealt with as soon as symptoms are noticed. The earliest symptom of an active canker is slowed growth relative to other branches on the tree. Slowed growth results in smaller leaves and fruit and fewer leaves. Affected branches seem more open and just look “weaker” than their healthy counterparts. Slowed growth is often followed by wilt, leaf drop and eventually necrosis or death of the branch. It is best to remove diseased branches early before the organism spreads to the main stem. Since symptoms occur when leaves are on, summer pruning is the best approach to remove cankered branches. Regardless of where or when damage occurs, using correct pruning practices should be adhered to.

Cankers kill branches in fruit trees, they can be caused by either bacteria or fungi. Here *Botryosphaeria dothidea* has killed a branch in this apple.

Healthy growth on the tree above but thin, weak, small leaves on the tree below indicate a developing branch canker.

Pruning is used most widely on fruit trees to dwarf them so that fruit is produced at a height convenient for harvesting. Pruning creates two universal responses that apply to all woody plants:

I. Pruning is growth retarding. The part of a tree pruned will grow less than the unpruned part. Or, a pruned tree will grow less than an unpruned tree.

II. Pruning is a bud invigorating process. A pruned tree or branch will have more of its buds released to grow compared to the unpruned branch or tree where many buds remain in a dormant state.

The more a tree is pruned, the less its roots and stems will grow. Even though the more a tree is pruned the more latent or axillary buds will be released to grow, it will not be able to make up for the lost leaf potential of the unpruned tree. The pruned tree has reduced photosynthetic capacity, makes less energy and will grow less overall. The thing that is not very clear is how the timing of pruning affects the basic processes. In his review, Chandler makes clear that pruning in the dormant season will retard the growth of apples less than if they are pruned in the summer. Summer pruning also significantly reduces the growth of roots compared to dormant season pruning. Removing leaves in mid-summer or after all shoot growth has stopped (summer rest period), removes photosynthetic capacity and reduces stored energy in the tree, thus retarding growth overall. While buds may be invigorated and new summer growth may occur, this rarely makes up for the tissue lost and still results in growth reduction.

Summer pruning does not result in more fruitfulness the following year, and in apples does not increase the number of spurs formed for fruit formation. Summer pruning can open the canopy and allow branches to form lower down that are useful for easy harvest. The effect of summer pruning on next year’s fruit quality is uncertain. Summer pruning can accelerate the ultimate scaffolding or canopy shape for the mature tree.

Pruning citrus after harvest, during the warm season can affect fruit size in the following year. This may be due to fruit thinning as some citrus have green fruit formed by summer that ripen in winter or spring. Summer pruning removes fruit and remaining fruit can grow larger.

Summer pruning of fruit trees before fruit harvest increases light penetration into the tree and can increase color development of the fruit. Pruning must be done cautiously to avoid excess light penetration and sunburn to scaffold branches and resultant canker diseases. Summer reduction pruning is most often accomplished by pruning the ends of branches back to other branches or twigs. Removing about one half the current season’s wood (on a given branch) will achieve objectives usually without causing excessive light penetration into the canopy. Not every branch need be pruned but an even approach, removing branches consistently around the tree, will maintain form. No more than 15-20% of the canopy should be removed by summer pruning. On some vigorous growing trees such as Persian mulberry, pomegranate, or some peaches, heavier pruning doses can be used. Pears, apples, plums and cherries require less pruning and cuts should be made to preserve spurs and other fruit bearing wood. Some varieties of cherries can become ‘over spurred’ and thinning cuts to remove excess spur wood can sometimes be helpful to limit production and increase fruit quality in the next season.

References:

Chandler, W. H. 1923. Results of some experiments in pruning fruit trees. Cornell University Agriculture Experiment Station bulletin 415.

Ingels, C. and P. Geisel. 2014. Fruit and Nut Tree Pruning Guidelines for Arborists. University of California Agriculture and Natural Resources publication 8502. http://anrcatalog.ucanr.edu

Saure, M.C. 1987. Summer pruning effect in apple—a review. Scientia Horticulture 30: 253-282

Water Wise Gardening: Conserving and Irrigating Responsibly

While we can’t ever control or even predict the weather, in most places it is important to have a plan on how to deliver water to our home gardens during the hot, dry months of the summer. Aside from reducing water need through some good management practices, delivering water in an efficient and sustainable way is important when planning and planting our home gardens.

When there is scarcity, it is necessary to conserve. Several years I got to see scarcity in person on a sustainable agriculture tour of New Mexico. Farmers in New Mexico have only limited access to water from irrigation canals, to flood irrigate their fields, or even wells for drip irrigation.

This severe lack of water got me thinking about how much we take water for granted in our own gardens. We often apply as much as we want or need in an inefficient manner (using sprinklers, sprayers, etc.) because we think it will always be there when we turn on the tap.

Where I’m located in Nebraska we are also blessed to have water falling from the sky. Sometimes there’s too much, and at others there’s not enough. But that’s much better than in some places – I visited some parts of New Mexico on a farm tour where they get seven inches of rainfall in a normal year. Seven. Total.

Thinking about conserving what water we have means that we are good stewards and are ready for when issues do arise. And let’s face it, there are some times in the summer that are dry where water conservation will help reduce using water, which can also save money.

When we talk about conserving water, there are two ways to go about it. First, look for ways to reduce the need for water. Then, look at ways to reduce water waste and usage whenever you need to use water on your lawn, landscape or garden.

Reducing the need for water

During dry times, it can be necessary to provide water to the garden to keep it growing healthfully along. However, there are many ways to reduce water loss or increase the amount that stays in the soil around the plants.

Mulching not only reduces weeds, but also helps hold moisture in the soil. Having one to two inches of mulch on landscape beds can reduce evaporation from the soil and decrease the amount of water you need. Newly planted trees should be mulched for the first few years to help hold moisture in the root zone as well.

Mulching is also important in the vegetable garden. Using straw or shredded newspaper are simple ways to conserve moisture, beat weeds and even reduce diseases. Note that this is shredded newspaper used on top of the soil for a mulch, not whole sheets applied below another mulch or on top of the ground. That process is called “sheet mulching” and we typically don’t recommend it here at the GPs because it limits air movement into the soil and can disrupt the soil microbiome. Stick only to shredded newspaper as a top dressing. (See the bottom of the article for journal articles discussing paper and straw mulches).

Shredded newspaper in my tomato bed. There are 2ft woodchip mulch walkways between 4ft wide beds.

You can use woodchip mulch in the vegetable garden, but it can be difficult to manage when you are frequently planting, replanting, or harvesting crops. If you accidentally incorporate it into the soil, it can tie up nitrogen available to plants and cause deficiencies. As long as you are good at keeping it on the surface, it isn’t as much of an issue.

Large scale gardens or farms make use of black plastic as mulches to do much the same thing. Plastic mulches are typically beyond the scale needed for home vegetable gardens and have their own set of drawbacks such as limiting water and air movement, but for those struggling with difficult weeds or with issues limiting manual removal (disability, limited movement, etc) it may be explored for smaller scale production. There are now even biodegradable plastic and paper mulches available. Use of these does require drip irrigation beneath the mulch, as rain cannot penetrate to the root zone. With the issues associated with them, plastic mulches would be considered a last resort for all but the largest home vegetable gardens, and many of my GP colleagues recommend against them for all home garden situations – but they can have their very limited place in the home garden toolbox. And we definitely recommend against the use of plastics and landscape fabrics in ornamental beds and landscapes.

Choose plants that require less water. There are many plants available that have lower water requirements. Ornamental grasses, Liatris (blazing star), Kniphofia (red hot poker) and sunflowers come to mind. Most native plants are commonly thought to have lower water requirements, but this isn’t always the case and natives may not thrive in altered ecosystems (urban settings or even managed landscapes). Most bulbs also are water efficient and do not require extra watering, as are most culinary herbs.

Mowing less often in the hot and dry summer also can conserve water if you are one who waters the lawn. I’m not a big fan of watering lawns, since it is such a large water usage, but I know there are those who prefer to have their lawns lush and green at all times. Instead, when the summer gets hot and dry, leaving the grass on the taller side can help it stay green even without water. Many of the grasses we grow here are cool-season and go semi-dormant in the heat. Stopping mowing when the heat starts slows down growth and the need for water.

Irrigating Efficiently with Drip

When it comes to getting water to the garden, there are definitely more efficient ways to make it happen.

Unfortunately, the most common method — using sprinklers — is also the least efficient. It is hard to direct the water to the right place, and during periods of high heat evaporation takes up much more water than you think. But there are ways to get water to your thirsty plants without running up the water bill.

Drip irrigation is probably amongst the most efficient and sustainable ways to water your landscape or vegetable garden. This method allows you to apply water directly to plants in a controlled manner, rather than spraying an entire area with water. Also, since the water is applied directly to the ground rather than sprayed through the hot summer air, the water is much less likely to evaporate.

Drip irrigation tubing. Each drip opening emits on this version emits 1 gallon of water per hour.

There are a few different types of drip irrigation systems available. Probably the easiest to install is a drip tape system. This is a deflated tape that already has water-emitting slits cut into it. While each slit applies a precise amount of water over a given time period, the pre-determined regular placement of the slits makes this system better for plants grown in rows, like vegetables, rather than landscapes where plants are of differing sizes and spacing. And while it can be used for vegetable gardens, probably the easiest system for a landscape would be one where there are tubes you can cut to various lengths and insert controlled drip emitters at customized locations. Another use for this type of drip irrigation could be for containers on a porch or deck – you can easily run the tubing out of sight along a bannister or railing and direct individual emitters to individual containers.

It all sounds complicated, and larger systems can be, but there are small and simple kits you can easily find at many garden centers or online retailers available for home gardeners to install their own within a matter of hours. You will need to have some skill at reading directions to install them, but the process is pretty simple.

For information on setting up drip irrigation for your home garden, check out these great resources from Extension institutions across the country:

Drip Irrigation for Home Gardens

Building and Operating a Home Garden Irrigation System

DRIP: Watering the Home Garden

Soaker hoses are a similar concept to drip irrigation, but instead of small drips these hoses just emit water all along the hose. Still better than sprinklers, these hoses are quite a bit less efficient than drip, since you can’t direct the water exactly where you want it. They are also easy to apply too-much water to an area since they can emit large volumes. Installation is pretty simple, though, since you just lay the hose down where you want it.

One great benefit of both drip irrigation and soaker hoses is the application of automation. Using a timer can make it easy to keep the garden watered through the season. Timers can be as simple as a dial to manually run the irrigation for a specified time or fully automatic to run the irrigation for various lengths of time on different days of the week. Some more advanced timers also have rain sensors or soil probes to reduce or avoid running when rain makes watering unnecessary (if you don’t have a sensor, remember to stop automatic running until the soil has dried). And in today’s emerging technology, there are also timers or flow controls that can be automated or controlled from a phone app. The timer that I’m now using at home connects to my Wi-fi, and in addition to allowing me to control and observe the watering status from anywhere in the world, connects to local weather data to automatically set a “smart watering” schedule taking into account rainfall, temperature, wind speed, and other factors.

Another effective way of providing water to your garden is through water catchment. Water catchment is just a fancy way of saying that you use a rain barrel. Here you are collecting rain runoff to use in place of water from the tap. There are some ultra-low-flow drip irrigation systems that you can use with rain barrels (if they are raised high enough to get water pressure), but this use is usually for watering by hand. For larger gardens, the large IBC totes that hold 200 or more gallons can make good water catchment barrels. Just make sure that if you are using them (or any other barrel) for fruit or vegetable production that they are made of food-safe plastic and their previous contents were also food safe. (Check out our guide on Building a Rain Barrel)

Selected references:

Comparisons of shredded newspaper and wheat straw as crop mulches

Soil Temperature, Soil Moisture, Weed Control, and Tomato (Lycopersicon esculentum) Response to Mulching

Newspaper Mulches for Suppressing Weeds for Organic High-tunnel Cucumber Production

Tools, tips, and terrible traditions for raised beds – Part 3

*Young vegetables thrive in mulched, weed-free raised bed.*

Over the last couple of months I started a series on raised bed gardens, focusing on materials and preparation. In this final installment, I’ll focus on maintenance activities to avoid in your raised bed systems and remind you of three things you should always do.

Terrible traditions

We’ll start with some practices that damage soil structure and function (GP John Porter discussed this in much detail a few years ago). Tilling, once the mainstay of soil preparation for crops, is increasingly found to cause more damage than good. Grinding the soil into a material with the texture of coffee grounds might look pretty, but it’s devoid of the ped structure that allows water and gas to move through easily. It also increases microbial activity by bringing up microbial spores, which release carbon dioxide to the atmosphere as they digest whatever organic material is there. And tilling will increase the likelihood of erosion and compaction.

*Soil runoff from tilled, unprotected field. The same thing will happen in your garden. Photo from Wikimedia.*

This is the opposite of what gardeners should want: an optimal soil has natural structure which might look messy but allows for good drainage. It’s also more resistant to compaction and erosion, especially when it’s protected with mulch (more on this later).

Speaking of drainage, don’t be tempted to add gravel or some other coarse material at the bottom of the bed. The change in soil texture creates a perched water table, which makes for a soggy planting bed and optimal conditions for soil-borne diseases.

*Classic experiment that demonstrates water does not move easily through different soil textures.*

While we’re talking about unnecessary or harmful additions to your raised beds, let’s not forget the annual addition of compost or other rich organic material. This is a holdover from old agricultural practices and is not warranted unless you have an organic material deficiency. Without a soil test, you don’t have a clue about what your soil has or what it needs. The problems associated with routine amendments are discussed in this peer-reviewed fact sheet, and are exacerbated by the tillage that is often the means to incorporate compost. Likewise, don’t add fertilizers and pesticides unless you have conclusively identified nutrient deficiencies or pest issues.

*If your nutrients are off scale, don’t add any fertilizer!*

The last tradition I’d like to see shelved is growing cover crops. This practice originated in the management of agricultural fields, which were otherwise left bare after harvest. Outside of producing some kale or other winter vegetables, what’s the point of planting a cover crop in your garden, when you can protect the soil in other ways? Cover crops require water and nutrients, which eventually will need to be pulled or incorporated. You are forcing your soil system to continue to support plant growth and be subjected to disturbance with the planting and harvesting of the cover crop. Why not let the soil rest over the winter with a nice blanket of mulch? Give it a chance to regenerate its nutrient load while being protected from unnecessary disturbance.

*A great arborist chip mulch has leaves or needles as well as wood.*

Three tips

Two of these tips have been discussed many times in this forum, so I’ll direct you to longer discussions of soil testing and mulching. Mulching is not just important for protecting the soil bed after the growing season, but should be used on actively producing beds. A deep, coarse organic mulch will promote water and air movement, moderate soil temperatures, reduce weeds, and provide a slow feed of nutrients throughout the season. You’ll have to wait until your seeds are up to apply it, of course, but try to avoid bare soil as much as possible.

*Though you’ll need to leave the soil bare during seed germination, you can still protect unplanted areas of the bed with mulch.*

Soil testing is really crucial for any garden, but perhaps most important in vegetable gardens where harvesting may decrease key nutrients over time. It will also guide you in identifying potential heavy metal problems. The money you will save in not buying unnecessary fertilizers and other amendments will pay for many soil tests.

*There is so much great information in a soil test that will help you make decisions about what to add – and what to avoid.*

Sometimes you will need to add material to your existing beds if you are not using a natural soil. Soilless media (deceptively marketed as “potting soil” though no soil is to be found) will decompose and settle over time, leaving you with a sunken soil system. You will need to add more of the same sort of media to the beds, making sure you mix it in thoroughly to prevent a perched water table. (This is why you might consider using a natural soil and avoiding this annual chore – because a natural soil will not subside over time.)