Elinor Teague

Lawns are suffering under record-setting heat. (Photo: Pixabay)

Consecutive weeks of daytime temperatures exceeding 100 degrees and nighttime temperatures remaining in the high 70s have killed many plants and trees and weakened or stunted many others. Determining whether severely heat-stressed plants and trees are dead or dying and whether they have a chance to recover vigor and good health will be a challenge during the next few months. Branch dieback, premature leaf drop, dropping of immature fruits and nuts, dead leaves, blossom drop and flower desiccation as well as yellowed or brown lawn grasses are all signs of severe heat stress as well as drought stress. 

Determining the extent of the heat damage this next month will determine whether a plant or tree is salvageable. Dead leaves don't necessarily indicate that a plant has died. Remove dead leaves on flowering annuals and summer vegetables and scratch the stems to check for green tissue underneath the top layer of bark or stem tissue. Green tissue is still alive. Check leaf nodes for swelling which indicates that new leaves are forming at the node.

Keep the soil moist and delay removing plants which show signs of life. They may well recover and begin producing again in late August.

Protection from the scorching sun this month will speed recovery when temperatures cool in fall. Try to provide full shade this month for damaged plants or plants that do not recover easily from wilting. Move container plants into full shade. Place market umbrellas and shade structures where they can provide the most shade and leave them there. 

If major structural branches on mature landscape trees and fruit and nut trees are dead, the remaining branch scaffolding may not be strong enough to hold a crop or may create a hazardous imbalance. Branches that have lost their leaves or which still hold crispy brown leaves may look dead but still be alive. Remove any dead branches that can become projectiles during fall storms, but wait until deciduous trees are dormant in mid-winter to restructure branch scaffolding. Continue to deep irrigate trees and bushes into the fall months whenever a heat spike is predicted. 

Wait to fertilize all heat-stressed plants and trees until late August when nights will be longer and a little cooler. Deep irrigate before and after fertilization. It will be tempting to feed heat-stressed plants heavily to encourage rapid new growth but feeding at half the recommended rate for the next couple of months will encourage a slow and steady recovery. Apply low-nitrogen fertilizers on landscape plants and bushes and on summer vegetables. Give fruit and nut trees one feeding of a high-nitrogen fertilizer after harvest or in September if there was no crop left to harvest. 

Deep water citrus and other trees at least a day before a forecast heat spike. (Photo: Jeannette Warnert)

We need to deep irrigate more frequently than the previously recommended once a month and deep irrigation must begin at least one day before temps are predicted to be above 95 to 100 degrees for several days.

Check soil moisture levels underneath large landscape trees and deep irrigate whenever the top 5 to 6 inches of soil is dry. That could be several times a month from June to October. Younger trees and bushes planted within the last three years also may need several soakings a month. Soaker hoses are the most efficient and effective means of delivering deep slow irrigation at the outer edges of the canopy where the feeder roots lie but small oscillating sprinklers will also work. Drip emitters are not as effective. It will take at least 3 to 4 hours of slow irrigation for water to penetrate and soak the root zone. 

Insufficient water is the most common cause of poor crop set and premature fruit and nut drop. Fruit and nut trees need regular deep irrigation to maintain consistently moist soil from bloom until harvest time. Citrus trees need sufficient water in spring to set fruit and regular deep irrigation in summer through fall in order to hold and produce good-sized, juicy fruit.

During heat spikes, monitor soil moisture levels under citrus trees and deep irrigate whenever the top three to four inches of soil has dried. Plan on irrigating citrus every 3 to 4 days during heat spikes in July and August and once a week during the warm fall months.

Cool-climate redwood trees have been heavily planted in the hot and arid Central Valley to provide dense shade. Often in spaces too small for the mature tree. As a common practice, redwood trees are thinned and lower branches are removed. The heavy branch structure on redwoods creates a cooler microclimate in the interior of the canopy and thinning and removing branches on redwoods exposes the trunk and bark to the hot rays of the sun and high temperatures.  

In July, the inner needles on redwood trees will turn brown and fall. It's a normal process referred to as redwood dieback. The fallen needles form a thick mulch that will help cool the soil for these shallow-rooted trees and also slow evaporation. Redwoods will benefit from increased deep slow irrigation in summer.

Branch tip dieback is the earliest and most visible sign of drought-stress. Even one long-lasting heat spike without sufficient irrigation can cause tip die back on any type tree. Gusty winds now often accompany heat spikes. Dead branches can become hazardous projectiles and should be trimmed off when noticed, but corrective pruning to reshape branch scaffolding should wait until deciduous trees are dormant in late fall and winter.