Gardening here in the mid-Atlantic the past couple of summers has been more challenging than previous years because of our increased and prolonged heat ‘domes’ where we have hit the triple digits for multiple days in a row.  Excessive heat can affect all the plants in your garden adversely and can negatively impact growth, development, and overall productivity – actually stunting growth. Even photosynthesis can be impacted and cell damage can occur. Heat stressors that weaken a plant can also make a plant more vulnerable to pests and disease.

Fortunately this year, we have welcomed regular soaking rains, but last year there was a dearth of rain and I had to learn how to deal with unrelenting heat and sun in a drought.

The optimal temperature that most plants thrive at ranges from 68 to 86 F. If conditions are windy, that can further contribute to dehydration and heat stress. Young plants are much more impacted by heat stress than mature ones.

You can’t control the weather so dealing with heat has two major components – watering and shading. 

 

One of the biggest threats to your plants during the hottest months is lack of water, especially to newly planted plants that don’t have a mature root system.

Tip #1 Water in the Morning- 5AM-9AM

Watering in the morning accomplishes several things. First of all, the plants can better absorb the water when temperatures are cooler and the soil isn’t as saturated with heat, plus the plants can better cope with the approaching heat of the day. Another benefit is the water that hits the foliage will dry quickly, reducing the chance of fungal growth.

But sometimes I can’t get out quickly enough in the morning and settle for watering in the evening instead. It is still better than watering in the heat of the day when the heat and sun are at their peak and the water will evaporate instead of being absorbed into the soil and roots.

Tip #2- Water Smartly

Have you ever watered a plant and see the water roll right off the surface? When soil is compacted reducing the spaces between soil particles needed for water infiltration, the water will run off the surface because intense heat can cause organic compounds in the soil to become coated with waxy substances.  Also soils with low organic matter content have reduced water-holding capacity.

The most effective way to combat this problem, is to apply water in smaller quantities over a longer periods of time allowing the soil to absorb it more effectively. Practically, this means when I water and I see it run off, I apply a small quantity of water first at the base of the plant to prime the soil and then come back to that plant and water more completely and deeply. You are simply getting the surface ready to water so the water can infiltrate more efficiently.

Tip #3 – Water Deeply and Less Frequently

For watering, think long and slow. By watering deeply, you encourage the plant roots to develop deeper roots, which help them find nutrients and water during the long hot days. Plants lose water through their leaves (transpiration) and when the soil surface dries out, shallow roots become thirsty. You can see the effects of transpiration by wilting leaves.

Tip #4- Shading

As important as watering is, shading during a heat wave can greatly help a plant survive and thrive. Most people are surprised to hear that plants can get sunburnt!  I find that my peppers can get sun scald since the peppers are exposed to the direct sun beating down as their foliage is minimal. But tomatoes are usually much more leafy and that tends to shade the fruit. But I have had both – sun scalded peppers and tomatoes when the sun is really intense.  Both vegetables slow their production and become stressed when temperatures rise above 90 degrees F. Some plants such as corn will decrease the amount of their leaves facing the sun, by rolling into a cylinder.

Tip #5- Plant Intensively

Planting your garden tightly or intensively with little soil exposed keeps the soil surface moist and cool using the foliage shading the soil. Soil will dry out faster, if it isn’t covered with a canopy of foliage, herbs, and flowers.

Tip #6-Grow the Right Plants

There are some plants that won’t survive or thrive in the heat so it is important to make more informed decisions when purchasing a plant. Becoming more relevant as we are getting more hot weather, I have started to pay attention to this by reading the labeling that comes with the plant. Some labels are starting to include the heat zones for that particular plant. If in doubt, look up the heat tolerance of that particular plant.

Tropicals and succulents will grow fine in higher temperatures. Okra, eggplants, and New Zealand Spinach will thrive in a hot garden, but other veggies will stop producing.

Tomatoes that tolerate heat better and don’t drop their blossoms are Solar Flare, Summer Set, Heatmaster, Cherokee Purple, Black Krim, and Phoenix. Specifically bred or selected for their ability to set fruit, they continue to produce when temperatures are above 86 F. Interestingly, smaller tomatoes like cherry and grapes are less affected by high heat. Check the USDA or local Extension service for information on heat tolerances.

Heat Zone Map

The American Horticultural Society has developed a heat zone map to help gardeners understand heat tolerance. Most gardeners are familiar with the USDA plant hardiness zone map, but cold hardiness is only part of a plants tolerances. Divided into 12 zones, each heat zone indicates the average number of days in a year when the temperature rises above 86 F, which is the point where most plants start to experience physiological stress from heat.

I was interested in noting that my heat zone 6 area of Maryland experiences 45 to 60 days a year over 86F. That sounded low to me but released in 1997, the heat zone map hasn’t been updated and I would be interested to see what shifts have occurred, just like in the plant hardiness map.  Some plant labels in addition to listing the hardiness zone, list the heat zone as well.

For more information on the map, go to Heat Zones, Plant Health, and the AHS Heat Zone Map.