Hot Enough For You?
written for the Davis Enterprise, July 25, 2002
"But it's a dry heat...."
Extreme sports....extreme skateboarding...extreme soda...."extreme" is already the overworked word of the decade. But here we do have Extreme Weather! In early June we had a North wind that lasted for three days, immediately following our first spate of 100+ temperatures. Lots of burned leaves, stressed lawns, and damaged seedlings ensued. Then two weeks ago  we had a heat wave: 109F on July 9, 111 on July 10, and then 3 days at 104.
While that is very hot, it is not uncommon for us to have triple digit temperatures off and on anytime between May and early October. Five days in a row is unusual, but the timing is not. If temperature records are going to be broken it'll be in early to mid-July. Typically a large high-pressure ridge develops over much of the western U.S.: a slowly revolving mass of air trapped in place and cooking during the longest days of the year, so the same murky air is getting hotter and hotter. Our delta breeze is blocked in the evening, so the air is still and hot and temperatures are uncomfortable well into the night. For a brief period we know what it is like to live in miserable places like, well, most of the rest of the country! The rarity of these weather events here makes them all the more memorable--three days later we topped out at 85F with a strong southwest breeze off the ocean.
Gardening during periods of extremely high temperatures isn't good for you or the plants. Above 90F most plants go into a resting condition. The leaf stomata partially close and the plant conserves energy by curling or drooping leaves. Make sure everything has a good soaking. Container plants will dry out very quickly and need careful watering. Set a hose on your larger trees and let it run at a slow volume for a couple of hours. Give your lawn an extra watering in the morning.
Our highest temperatures occur between 4 - 6 p.m. When I glanced across the street while we were watering our nursery at 4 p.m., temperature 108F, I saw a jogger go by--pounding the black pavement in full, unrelenting sun. I felt like grabbing a sheet, wrestling him to the ground, and calling a medic. C'mon folks--presumably you jog for your health! One advantage of gardening as exercise is that you can work at any pace and vary your job to suit your condition and the weather.
The effects of heat on plants vary from cosmetic to life-threatening, and symptoms may take several days to show up. Most are caused by inadequate soil moisture from a lack of deep watering. Burnt leaves, scorched fruit, blasted blossoms are short-term effects. At the other extreme, fungus that attack the tree at the crown are most active in hot weather. Shallow watering and mulch (or soil) piled up against the trunk make plants vulnerable to these diseases, which kill rapidly. There may be staining or oozing on the bark, and the leaves turn brown and drop quickly. Time for a requiem and a decent burial.
Brown spots can be caused by drought, disease, insects, dog urine, excess fertilizer--or just setting things on the grass in the sun. Even a frisbee left on the lawn on a hot sunny day can make a burnt spot! Take a sample to the nursery or the master gardeners for diagnosis. Lack of water will lead to a dull green appearance, areas that don't spring back up when you walk across them, and eventually to a dry, straw-like appearance.
Shallow watering, however frequent it may be, will not provide adequate moisture during a hot or windy period. The rule is to provide one inch of water (measure with a tuna can) two times each week. But water may not penetrate because of compacted soil, or may run off of a slope. Have your sprinkler timer run 2 - 3 times on a single day if necessary to get the full inch of water. You may need to augment your sprinkler watering with a hose on corners, slopes, or mounds.
will sulk, but the effects are temporary. Blossom-end rot of tomatoes, peppers, and squash leads to a mushy area on the developing fruit. This is technically caused by a calcium deficiency, but we aren't short of calcium in our hard water here! The plant can't take it up through its roots because the tiny root hairs are damaged by fluctuating temperatures or, most commonly, by erratic watering. Just pick the fruit off and throw them away; the next ones will likely be fine.
Tomato and pepper fruits exposed to the west sun scald on that side on a hot day. Young fruit stop growing and simply ripen prematurely at whatever size they have reached (peppers and eggplants especially). In either case the fruit is usable.
can sunburn on the exposed side. Pears and apples develop normally but look unsightly. Softer fruit such as peaches develop an undesirable texture if left to ripen fully, so pick them a little firm and use them in cooking.
don't open properly, or "bloom out" rapidly, when weather is extremely hot. A rose bud with color in the morning will be opened fully by the end of the day, and the petals of some varieties will burn on the edges. Rose foliage can sunburn, causing yellow-to-brown patches on the upper leaf surfaces. Roses like plenty of water--2 - 3 times per week--and the leaf edges of new growth scorch if the plant is dry. Dusty leaf surfaces are prone to spider mites, so a good washing of the foliage early in the day, followed by a good soaking, makes the plants happy. Very hot days are NOT good times to fertilize, nor should you spray pesticides when temperatures are above 90F. Roses will continue to bloom nicely when the Delta breeze predominates, and feeding in the late summer can make the fall bloom the best of the year.
Shrubs, Vines, and small Trees
with broad, evergreen leaves can show a particular type of damage: the upper surface of the leaf is sunburned, especially on the portion facing to the west. Dark-leaved shrubs and vines such as Pittosporum, privet, laurel, and Star jasmine are most affected, while plants with lighter leaves reflect the sun and protect themselves. Lack of water makes this problem worse. Although the damage is cosmetic, continued drought stress can lead to significant leaf drop. Do not prune evergreens during very hot weather! The interior leaves grew in the shade of the plant and will burn severely if they are suddenly exposed to direct sun.
Deciduous shrubs and young trees protect themselves from severe heat and drought by simply dropping all or most of their leaves. This is very common on newly planted Crape myrtles. It can be very disconcerting, but the plants will usually leaf out again when you give them a drink.
Trees from coastal climates or areas with high humidity and summer rainfall are most prone to damage during dry hot weather. The popular Coast redwood (Sequoia sempervirens) never tolerates drought. It is normal for older needles to turn brown and drop in the summer. If the trees are underwatered this normal browning will be greatly exacerbated and the needle drop will be considerable, stressing the tree.
Eastern trees such as birches, maples (especially Japanese maple), and both types of tulip trees (Magnolia soulangeana and Liriodendron tulipifera) have thin leaves that burn along the edges, and if they haven't been deep-watered the tops of the leaves will also scorch. Severely stressed trees will drop a significant percentage of their leaves. Birches, especially, may be weakened and then attacked by borers, stunting and eventually killing the tree. Most of these trees don't have time to put on new growth this summer, so they'll look burnt for the rest of the season. Deep watering is helpful, and fertilizing during cooler summer weather and again in the fall will make the tree healthier in the long run.
So, some plants are stressed by extreme heat, while others survive. But does anything really like hot temperatures? Sure! Crape myrtles, pomegranates, oleanders, annual Vinca, Portulaca and Verbena are just a few examples of plants that thrive in hot summer sun. All of these have bright-colored flowers that laugh off blazing sun--brilliant enough hues for us to enjoy them from our chaisse lounge across the yard...in the shade.
© 2008 Don Shor, Redwood Barn Nursery, Inc., 1607 Fifth Street, Davis, Ca 95616
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