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Protecting Tomatoes during Hot Summer Days

Sun Damaged Tomato - UCANR
Sun Damaged Tomato - UCANR
We all know that tomatoes love warm sunny days, ideally daytime temperatures between 65 and 85 degrees. However, when summer temperatures rise above 95, problems with your plants can arise. Above 95 degrees, tomato plants will stop growing, their flowers are not able to pollinate and fruit cannot fully ripen. At high temperatures, fruit can actually suffer sunburn and plant stress can lead to increased susceptibility to pests and diseases.

Below is a list of suggestions to help your plants survive during those “Dog Days of Summer”:

1. Choose tomato varieties that are suited to your particular climate. Tomato varieties that tend to perform better in warmer areas include:

  • Big Beef
  • Black Krim
  • Box Car Willie
  • Cherokee Purple
  • Mortgage Lifter
  • Super Sweet 100

2. If you live in an area that tends to have extremely warm summer days, if possible, consider planting tomatoes where they can receive full morning sun and filtered shade in the afternoon. This works best with smaller tomatoes, as larger tomatoes need a minimum of 6 to 8 hours of sun daily.

Tomatoes with ShadeCloth- UCMGCC
Tomatoes with ShadeCloth- UCMGCC
3. If you can’t find afternoon shade, create some by using shade cloth over your plants. Shade cloth comes in many thicknesses - be sure the cloth you’re using doesn’t block more than 50% of the sunlight. It isn’t necessary to cover your entire bed; the important part is to provide shade during the hottest hours of the day. Attaching shade cloth to a frame using clothes pins or clips along the southwest side of your beds will protect your plants while still keeping them easy to harvest. Using shade cloth during the hottest time of day can reduce ambient temperatures by 10 degrees. Be sure to remove the shade cloth when temperatures drop back down below 95 degrees so the plants can get their needed time in the sun.

4. Mulch, mulch, mulch. Adding 2-3 inches of organic mulch, such as rice straw, wood chips, dried grass clippings or other organic material will help reduce soil temperature and slow down moisture evaporation. Studies have shown that adequate mulching can reduce soil moisture loss by up to 70%.

5. Don’t be shy with the water. As temperatures rise above 90 degrees your plant is going to shift into survival mode. Using a moisture meter or, even better, your finger, probe 2–3 inches down into your soil to check for moisture. If the soil is dry you will need to water it. During the hottest days, you may wish to check multiple times. Keeping the soil moist can also help reduce fruit cracking and can reduce blossom drop. When temperatures are high, tomato plants may look like they are wilting, and it’s tempting to immediately add water. However, wilting is the tomato plant’s survival technique; the plant will perk back up when the sun goes down. Using your moisture meter first will keep you from overwatering.

6. Pick your fruit early. Tomatoes growing in warmer temperatures (up to 90 degrees) will have more color and a sweeter flavor, however, at 100 degrees, the fruit develops color on the outside but fails to ripen inside, leaving you with a poor tasting tomato. During these periods, pick your tomatoes early and allow them to fully ripen indoors, just remember not to put them in your refrigerator.

Healthy Tomatoes_UCANR
Healthy Tomatoes_UCANR
So, when summer heat does arrive, give your plants a little protection, and you can enjoy a season of delicious fruit from your garden.

Article by Mike Corby, UC Master Gardener of Contra Costa County

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