Chapter 48 Minimizing Sun Damage and Heat Stress

Most trees will grow faster, be less susceptible to disease, and be healthier overall if we adjust exposure seasonally to reduce environmental stress. We know intuitively that trees need to be watered more frequently when it gets hotter. What most of us don’t realize is that rising temperatures affect how trees respond to light too. During the summer, too much sun can damage trees just as much as insufficient water.

48.1 What Trees Need

Each tree species is genetically programmed to grow best under a particular range of light, water, and temperature. The range of conditions each species tolerates also changes over the course of a year. Species that we think of as more forgiving tend to tolerate a wider range of conditions. Many of the popular species used for bonsai fall into this group. Other species that we think of as fussy or demanding tend to require a narrower range of conditions, or are particularly susceptible to changes in light, water, or temperature.

Most guidebooks will describe the range of conditions in which a tree species grows best (full vs. part sun, wet vs. dry soil, etc.) assuming the tree is planted in the ground. The tolerated range of conditions will be much smaller when trees are grown above ground.

For example, if you insert a soil thermometer 12 inches into normally moist topsoil, the temperature in the root zone rarely exceeds 75oF between early March and late September. Contrast that with a tree in a nursery or training container, or final show pot; often the root temperature reaches 90-95oF. A certain amount of warmth in the root zone is beneficial in it stimulates root growth. However, several species that are used in bonsai prefer relatively cool roots. In a landscape this is accomplished by covering the root zone with a thick layer of bark mulch. So how can we achieve this with trees in small pots?

Depending on ambient temperature, the amount of light a tree needs to be its best can change over the course of the season too. Trees that are happy in full sun from October to May can be severely damaged by intense summer sun. Professional growers and horticulturalists have long used a variety of techniques to protect plants during summer, some of which can be adapted to bonsai tree care.

48.2 General Summer Classification Groups

I combed through several references to compile this list of species that can benefit from some protection during the summer. Later I give some options for modifying light levels and temperature in your growing area.

So we are all using the same terms, I have defined the exposure level for each group. Remember, these groups refer to the level of light during summer, when daytime temperatures consistently are higher than 80oF or so. For us here in Winston-Salem, this tends to be from mid- to late May, through the first half of September.

Some species are listed in two groups. If so, they can be maintained either way. Also, if you are unsure of the light levels in a particular location, keep a tree in a slightly more shaded location than it normally would prefer. If the nodes begin to stretch, move the tree to a slightly brighter spot. If it does not grow at all, the tree is likely to be dormant for the summer; do not move it to a brighter spot, as it will be heat stressed or sunburned.

48.2.1 Prefer to Be Shaded All Day

Some species simply dislike any full sun exposure. These should receive no more than a 1-2 hours of direct sun in the early morning. After 11 am, they really should stay in full shade.

  • Camellia (Camellia sinensis, C. sasanqua)
  • Azaleas (Rhododendron sp.)
  • Yew (Taxus sp.)

48.2.2 Best in Bright or Dappled Shade

Species that are adapted to the understory (beeches for example) or that originated from cooler climates than ours (like Japanese maples) often resent the heat. They might tolerate sun in the spring, but in summer prefer to be in a bright location where the sun filters through an overhead canopy of leaves. Any direct sunlight they receive should be in small patches, and only last for a few minutes. If you were to stand under the canopy of a large, isolated white oak tree in an open field, you would be in bright/dappled shade.

  • Trident maples (Acer buergerianum)
  • Japanese maples (Acer palmatum)
  • Hackberry (Celtis sp.)
  • Japanese cedar (Cryptomeria japonica)
  • Burning bush (Euonymus sp.)
  • Beeches (Fagus sp.) (Never let them overheat)
  • Ficus (Ficus sp_.)
  • Gingko (Gingko biloba_)
  • Crape myrtle (Lagerstroemia indica, L. sp.)
  • Orange jasmine (Murraya paniculata)
  • Heavenly bamboo (Nandina domestica)
  • Spruce (Picea sp.)
  • Flowering almond (Prunus mume)
  • Firethorn (Pyracantha sp.) likes full sun except in summer, then needs dappled shade.
  • Hemlocks (Tsuga sp.)
  • Elms (Ulmus sp.)
  • Wisteria (Wisteria sp.)

48.2.3 These Do Best With Some Mid-Day Shade

For many trees, it is the intensity of the sun during midday that is the problem. All of these species can tolerate full sun during the morning up to about 11 am, then again from about 4 pm onward. While the sun is overhead though, they like to be shaded.

  • Birches (Betula sp.)
  • Hornbeams (Carpinus sp.)
  • Quinces (Chaenomeles sp.)
  • Hinoki cypress (Chamaecyparis obtusa)
  • Japanese cedar (Cryptomeria japonica)
  • Winter jasmine (Jasminum nudiflorum)
  • Privets (Ligustrum sp.)
  • Myrtle (Myrtus communis)
  • Willows (Salix sp.)
  • Redwoods (Sequoiadendron sp.)

48.2.4 No Shade is Needed

Some species really love sun, and are not happy unless they get it. These you can leave in the full sun with little or no screening or protection.

  • Barberry (Berberis sp.) (Can adapt to shade)
  • Boxwoods (Buxus sp.)
  • Cotoneaster (Cotoneaster sp.)
  • Jades (Crassula sp.)
  • Honey locust (Gleditsia sp.)
  • Japanese holly (Ilex crenata) (Adapts to shade)
  • Junipers (Juniperus sp.)
  • Olive (Olea sp.)
  • Pines (Pinus_ sp.)

48.2.5 Species Preferring Cooler Roots

I already listed these species under their preferred light levels. However, they do best if the roots are kept somewhat cooler than usual.

  • Trident maples
  • Hornbeams
  • Celtis
  • Murraya
  • Elms
  • Wisteria

48.3 Changing Your Local Microclimate

I am fortunate in that I have a mix of bright shade, full shade, and mid-day shade all in one corner of my yard. All I need to do is move boxes and pots from place to place during the season. However, there are times when I have a tree planted in the ground that could use some shade. Also, I know not everyone is so lucky in terms of light. Fortunately there are a number of tools and tricks for adjusting light levels, and for keeping pots cool.

48.3.1 Shade on Demand

There are two strategies here: provide temporary shade for trees individually, or construct a permanent shaded areas. The simplest temporary solution is to stretch a layer of shade cloth, netting or some other material that is impervious to light directly over the area you wish to shade.

The top of the line option is professional shade cloth, which is a closely woven black, brown, or green mesh material. It is available in specific mesh sizes that create 20-70% light reduction. Keep things simple, and buy either 40% or 50% density. Be forewarned that shade cloth is expensive, but balance that with the fact it will last several years. It is available from greenhouse suppliers.

Burlap is a very inexpensive alternative that I use routinely. Rolls of burlap are available from Southern States, Lowes Hardware, and elsewhere. The mesh cuts approximately 50% of light (i.e., cuts full sun to bright shade), and can be doubled over to provide deeper shade. I drive 10-foot wooden tomato stakes about 2 feet into the ground, then stretch the burlap between them to form a shade tent against mid-day sun. The downsides of burlap are it sags when wet, and must be replaced after a year.

Between these two extremes are a variety of options for creating shade. Pretty much anything that is partly opaque and can be hung between posts can be used to create shade. Old bamboo windowshades, woven willow or bamboo fencing, plastic snow fence, bubble wrap, and even king-size bedsheets can be used to reduce the intensity of sunlight on trees, be they in the ground or on the bench.

If you are looking for a more permanent source of shade, you should build a simple post and frame lath-house.

  • Decide where you want to situate your shade house. Mark 4 corners of a square, 8 feet on a side.
  • Dig a hole 2 feet deep and 8-12 inches round at each corner
  • Place an 8-foot 4x4” wooden post in each hole, and LOOSELY fill the bottom 2-3 inches of the hole with soil. You may need to add some large stones to hold the posts upright while you attach the frame at the top.
  • Using exterior grade screws and pressure-treated, 8-foot 2x4s, connect the tops of the 4 posts.
  • Check that the frame is square and level at the top, then fill the holes around the posts with soil and pack it tightly.
  • Place 2 sheets of plastic or exterior grade wood lattice on top of the frame. Alternatively, use 2, 4x8-foot corrugated white fiberglass panels.
  • To create more shade, anchor additional lattice panels on the sunny side of the lath house.

48.3.2 Keeping Your Trees and Pots Cool

For trees in boxes or nursery pots, this trick of professional growers creates an environment that shades the pots directly from the sun, and also encourages evaporative cooling.

  • Pick a location where they will get their optimum level of sun.
  • Roll out enough landscape fabric to make an area big enough to hold all the trees you want to protect. (Do not use plastic; water must be able to pass through.)
  • Place the boxes and pots directly on the landscape fabric. Leave approximately 3 inches of space between individual containers.
  • Pile fresh (not decomposing) pine bark mulch in and around the containers and boxes, right up to the tops. Be sure to pile mulch at least 3 inches around the pots on the outside of the group. If the mulch wants to slide away from around the containers, you may need to place a short fence of wood scraps around the outer edge of the mulch pile.

Each time you water the trees, thoroughly soak the mulch between the boxes and pots too. The mulch will block the sun from directly heating the pots, while evaporation from the mulch keeps the roots cool.

If fresh pine bark is not available, you can use plain straw, but plan to change it after a month or so. Do not use old decomposing mulch, as this will tend to heat up and encourage your boxes to rot.

For individual trees that have been established in pots, you can keep the roots cool in much the same way using sand instead. This technique is akin to the sand table that some people use to protect small mame specimens during the summer.

  • Find a plastic wash pan or cement tub about twice the size and depth of the pot.
  • Punch several drain holes in the bottom
  • Pour in 1-2 inches of white or pale colored sand.
  • Place the tree and pot on top. Pour additional sand around the pot, burying it to the lip of the pot.
  • Each time you water the tree, thoroughly soak the sand. As water evaporates from the sand, it will cool the pot and roots of the tree.
  • In fall, simply lift the pot and rinse off any adhering sand.

Mame and shohin sized trees can suffer in our summer heat. To keep these small pots cooler, punch drainage holes in a large dishpan, sweater box or other plastic tray, then fill it with 3-4 inches of sand. Bury all the pots to their rims in the sand, then water both the pots and the sand. The sand insulates the roots and pots from temperature extremes, cools them by evaporation, and provides extra moisture from below.

Two dwarf hinoki cypresses being planted in a sandbox. One pot has already been sunk in sand. The other tree still sits atop the sand. Original photo by Dan Johnson.

Both hinoki cypresses have been sunk in the sand. This extra protection lets me put the pots in a sunnier location so they can fill out gaps in the foliage. Original photo by Dan Johnson.

There is one final strategy for keeping trees and pots cool: misting with tap water, which is considerably cooler than the summer air. There are two options. Misting fans are exterior-grade electrical fans on a stand that connect to a standard garden hose, and spray a fine cooling mist. These are very refreshing for people, and widely used for summer sports. Greenhouses often cool plants by attaching special misting heads to a garden hose then directing the mist onto the plants. Having described them, I hesitate to recommend either of these methods long-term for a number of reasons. There always is a risk that a fan will turn over and damage trees, or short out. Also, the breeze from a fan can damage leaves that are too close. A misting head for a hose can be useful for a small area but it is hard to keep a large number of trees cool this way. Misting can leave hard water deposits, and raises humidity, making fungal problems more likely. In sunny locations, water droplets on leaves also can act as lenses to cause leaf scorch. If you have hard water, evaporation will leave mineral deposits. Despite these concerns, in certain situations a misting fan or nozzle may be your best solution for short-term or emergency cooling. Fans and nozzles are available from Lowes and Home Depot.