Gardener’s Corner

Monthly Article by Cate White, San Joaquin County Master Gardener

August 2020

Now that the hot days of August have arrived, it’s time to think about protecting yourself and your plants from over-exposure to the sun.

Just as some of us can tolerate sun and heat better than others, the same is true for plants.  And, just as we need to keep ourselves well hydrated, use sunscreen and wear hats for protection, plants need moisture, shelter and protection too.

Open Source

While all plants need sunlight to maintain healthy growth, their needs fall into three main categories.  Plants are generally labeled as needing full sun (6 hours of sun or more each day), part sun/shade (more or less equal amounts of shade and sun), and full shade (1 hour or less of direct sun).  Vegetables need at least 8 hours of sun per day, and fruit, nut and citrus trees need 6 to 8 hours. While roses enjoy full sun most of the day, they appreciate some shade in the late afternoon, especially in our hot valley climate.  Keep in mind that the morning sun is less intense than the afternoon sun, making eastern and northeastern exposures easier to tolerate for sensitive plants than hotter southern and western exposures.

Be aware of intense reflected sun from windows and walls, and confined spaces that concentrate heat such as entryways where the sun pours in.  Rock mulch is another source of reflected and concentrated heat, and since it absorbs heat during the day and reflects it back during the night, it can actually bake plants, cook their roots, and cause leaf-scorch and bark damage.  Lastly, remember that the angle of the sun changes a good deal over the course of the year, so the sun exposure a plant gets as the seasons change varies greatly.  It’s important to be aware of the year-round patterns of shade and sunlight in your yard before choosing a site for a specific plant.

Sunburned Camellia Leaves

Symptoms of sun damage can include leaf tip burn, leaf scorch, bud drop, sunscald on exposed fruits or vegetables, and wilting. Hydrangeas, Camellias and Japanese Maples can be especially prone to leaf burn.  Temporary wilting which occurs during very hot weather on broad-leaved plants such as Hydrangeas or squashes may be normal as long as the plants recover when the day cools into evening.  This sort of wilting occurs when a plant’s roots can’t take up water fast enough to keep its leaves hydrated during the hottest part of the day, so watering at this point is not helpful and can actually be harmful.  Trees may also suffer from sunburn or scorching of their bark, which can cause serious permanent damage.

Shade structures can be constructed for plants under stress using a trellis covered in shade cloth or an old umbrella.  If you observe wilting that lasts into the evening, increase watering times during hot spells.  To protect tree bark from sun-scorch, mix equal parts of flat white interior latex paint with water and apply to areas that get heavy sun exposure.  Your plants will appreciate the extra effort you make to protect them from the sun’s heat.

Sun Damaged Tree Bark

August Garden Checklist

  • Continue checking irrigation lines and valves to be sure they are functioning correctly.  Adjust timing according to weather conditions.
  • Be on the alert for mosquitoes.  Eliminate all standing water.  For birdbaths and ponds, control by adding Bacilus Thurengiensis israelensis or mosquito fish
  • Continue baiting for ants.
  • Check for aphids, whiteflies, caterpillars and spider mites.  Use a strong spray of water on whiteflies and aphids and mist the undersides of leaves for spider mites.  Neem oil and insecticidal soaps can also be used, but only during cool parts of the day and when bees are not present.  Spray Bt (bacillus thurengiensis) for caterpillars.
  • Use neem oil as described above for powdery mildew.
  • Put out traps to control yellow jackets.
  • If you have trouble with blossom end rot on your tomatoes, (dark leathery spots on the blossom end), be sure you are applying water evenly, keeping the soil uniformly moist, neither dry nor saturated.  Follow recommended fertilizer amounts. Some tomato varieties are more susceptible than others, but since this is not caused by a pathogen, there are no pesticide solutions.
  • Mow lawn down by no more that 1/3 when weather is hot
  • Deep water young trees every week or two in hottest weather

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An archive of Cate’s past articles may be found here.