Following my earlier E Letter this morning, I've received questions that I'm attempting to address here. If you have further questions, keep me posted.
Is there a difference in how broadleaf and coniferous plants respond to drought? Yes, conifers will give you very little warning. Conifers include not only the pines, but all the junipers, arbor vitae (Emerald, Green Giant, Hogan), leyland cypress, Cryptomeria (Japanese cedar), Chamaecyparis (false cypress, Hinoki), etc. – all those needle bearing plants. We have some Atlantic white cedars (Chamaecyparis thyoides) at home that are about 6-7 feet tall and in their second summer. We’re still watering them about every week or two. Broadleaved plants, whether evergreen or deciduous, generally give you some warnings. Leaves curl, leaves wilt, leaves turn brown at the edges, leaves turn yellow and fall off. All of those can be signs that a plant is responding to water stress (or something).
Most conifers give very little evidence of drought stress. In fact if you see one of those plants going “off color,” it’s probably too late. If it’s dropping needles in the interior, it may (or may not) be too late. The only way I know to check on those plants is to pull out the built-in water sensor next to your thumb and probe the root zone. OK with the soil in its current condition, you may need something sturdier than your finger. And remember that water applied in the root zone will slowly diffuse into the surrounding soil. The surrounding soil may even absorb water away from planted root balls.
Broadleaf trees and shrubs do give some of the warning signs mentioned above – wilting, leaf curl, brown margins, yellowing, falling leaves. These events may be caused by other factors, but they are also techniques to reduce water loss. Plants that are exhibiting one or more of these symptoms merit your attention. It is also noteworthy that leaf drop is an active process of the plant. The leaf is abscised or cut off by the formation of an abscission layer at the point where the leaf joins the stem. If a plant declines or dies rapidly, formation of the abscission layer may not occur. In that case leaves will turn brown and crisp but remain on the tree. In most cases, that portion of a tree or shrub retaining brown leaves is dead. (Exceptions include winter leaf retention in the lower canopy by trees such as beech or some oaks.)
If you want to inspect that plant more carefully, you can start at the end of some stems. Live twigs will bend before breaking; dead ones snap off readily. If you scrape the bark with a fingernail or penknife, live twigs or stems have a green layer beneath the bark. Dead stems will have only brown. Remember that a portion of a plant may die while other parts remain viable.
What if the tree is shaded? Location (exposure) is everything for plants. Exposure to anything such as direct sun or wind that can increase water loss can make a bad situation worse. A plant in full sun will lose water more rapidly than the same plant in shade. Of course if the shade is caused by trees overhead, those trees may be competing adversely for water also. There are many variables. It often becomes a case of comparing the efficiency of the roots at extracting water with the efficiency of the leaves at transpiring water. Some plants will lose water to the atmosphere more rapidly than the roots can absorb water (Japanese maple for example). But in the “cool” of the evening roots may be able to replace that water. Plants that wilt in the afternoon may recover by morning. They may also need more water.
What can you do for a large tree or shrub that is in extreme stress? First, acknowledge that it may take hundreds or thousands of gallons of water to do any good. A little bit of water may just be wasted. If you make the commitment to supply that water, think trickle. Use a hose to slowly trickle water on the surface around the drip line of the plant. Move the hose occasionally all the way around the drip line. To be effective, you’ll need to wet the soil at least 6 inches down. If the water begins to run off, move the hose. You may need to repeat and go around the drip line more than once. As you move the hose, probe the soil to see how deep the water is moving. It may take all day to do a good job.
Watering a large tree is challenging. However, it’s much harder to replace that tree than it is to replace a lawn or flowerbed. On the other hand, if the tree was “saved” from construction and already not in good shape, it may not be worth further effort. Use your water where you feel you get the most bang for your buck.
alcooke