If your garden looks a little heat stressed recently, you are not alone. Although it may seem some of the advice is a little too late, you can apply these tips now and next year. Just when I looked up the OSU Extension website to research the topic, out came the August Gardening Calendar. These tips are from the calendar, while others arrived in my monthly "Fine Gardening" magazine:
Plants need to stay hydrated, just like us. Water more deeply than usual and more frequently. Where you may water for a half hour the rest of the year, you may need to increase to 45 minutes to ensure the water soaks at least six inches into the soil. Some plants need deep watering in early fall to develop bud growth for next spring.
Watch for signs of heat and water stress such as flagging leaves (wilted, limp or drooping) and drying edges.
Hanging baskets, especially, dry out in the heat. Water twice a day when needed. If you water in the evening, avoid the foliage. The potting mix stays moist longer for improved plant uptake. Water containers thoroughly until the water runs out the bottom.
A moisture meter can be inserted in deck planters and baskets to monitor soil moisture, because they are especially prone to drying out. Moisture meters help you accurately assess moisture levels. Excess watering can mimic the symptoms of water stress. Meters can be found at most garden centers.
Avoid applying fertilizer during the heat. Photosynthesis and plant growth is slowed in plants experiencing heat stress, so fertilizer can be wasted. It can also be harmful and lead to root burn. Once temperatures drop, return to normal fertilization schedule. Exceptions are cucumbers, summer squash and broccoli. Fertilizer helps maintain production while you continue harvesting.
Mulch can be added following a deep watering. Mulch helps conserve water in the soil, while moderating soil temperatures. If you apply an organic mulch such as compost, bark, and wood chips, the soil structure is improved and nutrients are released into the soil during decomposition.
Shade. Where possible, you may need to move planters into the shade in the afternoon to avoid the scorching summer sun. Shade cloth can also be hung over sensitive plants in the ground — or in a greenhouse — to reduce soil temperatures. Ensure the cloth does not touch the leaves to avoid leaf burn.
Avoid pruning the wilting leaves of your tomato plants now. It is tempting because they are unsightly, but they are providing shade for developing fruits and interior plant structures. Pruning can return once the heat wave is over.
Competition from weeds can also deprive your plants of the water and access to nutrients they need. Weeding in the early morning or evening is your best option to avoid the heat. Diligently weeding the area around your trees and shrubs will ensure their roots don’t have to share resources with these invaders.
Trees stressed by drought
Remember that we live in a desert. Very few plants will survive here without sufficient water. Juniper and cheat grass seem to be the exceptions! Tips below were found on numerous extension publications, including ones from Purdue and Michigan State:
Newly planted trees should be watered deeply and regularly. For vigorous growth, water trees daily for the first month and two to three times per week for the remainder of the establishment period. Be sure to water trees slowly to prevent runoff and ensure a deep soaking – moist to six inches deep.
Trees in shallow soils will be more susceptible to drought stress. Most soils in Central Oregon are very shallow and sit atop volcanic rock.
Even well-established trees can experience water stress. It may not be obvious immediately, but long-term drought stress will cause the trees to defoliate from the top and branch extremities, which may not be recovered in the following year. The tree or shrub slowly declines in size, leaving some dead branches, and concentrating leaves closer to the trunk of the plant.
Drought reduces carbohydrate production, significantly lowering energy reserves and production of defense chemicals in the tree. Trees in a weakened state from drought can them become more susceptible to pests, which can further weaken the tree, and even kill part or all of it. Numerous pine trees, weakened by stress are then attached by sequoia pitch moth, which causes it to exude copious amounts of pitch, and draining it’s water reserves.
You will first notice drought stress in the leaves:
Wilting: Leaves are drooping and curling.Leaf scorch: Leaves are discolored, usually browning from the outside. Pine needles will appear yellow.Defoliation: Leaves are dropping before the fall, even in conifers.Small Leaves: New leaves will form smaller than usual.Stem dieback: Stems die from the outer canopy inward due to a lack of sufficient nutrients.
If you begin to see signs of stress in your established trees and the ground under your trees is extremely dry, begin watering now. To test the dryness of the soil, use a spade or a long screwdriver in the ground at least six to eight inches and check for soil moisture. If dry, it is time to water. Slowly water the entire area under and around the canopy of the tree to the point where you have some water pooling and running off the surface.
Most established woody plants can recover when watered. Plants that appear to be dead, having dropped all or most of their leaves, might recover when watered. To determine if the plant is still alive, simply scrape the outer layer of a twig or the bark to see if a green layer exists, which indicates it is still alive. Do not remove this plant yet. Wait until the following year to see if it recovers. The Willamette Valley experienced a severe drought in the '90s. An entire forest of oak trees along the I-205 freeway dropped their leaves in early August. Arborists were concerned. Fortunately, they recovered the following year.
Excessive water stress can kill trees, especially very young trees, and those with less developed root systems. Current-year plantings and transplants will likely suffer high mortality rates. Long-term water stress may kill a tree or, more commonly, predispose it to a wide variety of ailments, some of which may not become visible for several years.
This is happening in Central Oregon now. Numerous examples can be viewed along the roadside, in people’s un-watered yards, along city streets and commercial properties. Casualties of the long-term drought.
Marilyn Clark is an Oregon State University Master Gardener™ Volunteer, having completed the program in 2017. A member of the Madras Garden Club since 2009, she has been gardening in Culver since 2004.
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