Are Your Tomatoes Feeling the Heat?

It's hot. Tomatoes like heat, right? Actually, tomatoes like warm weather, between 65 and 85 degrees. When temperatures soar past 95, tomatoes stop growing.  In that kind of heat, their flowers fail to pollinate and instead they dry up and drop off, putting a pause on the production of new fruit. Tomatoes that have begun to color will halt at orange and fail to turn red.

We can't control the sun, but there are a few things we can do to help tomatoes stay productive in our hot valley summers.  In the first place, plant tomatoes in a spot that gets sun in the morning and filtered sun in the afternoon. If tomatoes are sitting in the blazing sun all day, create afternoon shade for them with a sheet of row cover material or shade cloth clipped to the west-facing side of the support structure on which the tomatoes are growing.  Arrange the cloth over the tops of the plants to protect from mid-day sun as well. Leave the east side of the plants free for morning sun and air circulation, and to allow pollinators to reach plants. Clothes pins or binder clips can be used to secure the row cover material or shade cloth and are easy to take off to allow access to harvest. Use of shade cloth can drop the ambient temperature by as much as 10 degrees and is often sufficient to keep tomatoes in that sweet spot of productivity.

If your tomato plant has good green leaf color, is flowering and has fruit developing, but its leaves are curling up, you are experiencing a common summer problem for tomatoes in our area. The culprit again is most likely heat. Tomato leaves transpire water from the undersides of their leaves. When a tomato plant can't take in as much water as it is losing, its leaves will curl up. This occurs when the soil is too dry, the temperature is too hot, or it's too windy. Check soil moisture by poking a finger an inch into the soil. If it's dry, it's time to water. Leaf curl is the tomato's way of reducing leaf surface area to reduce water loss. Curling up will not affect fruit production or the health of the plant.  

Upward leaf curl can also result from overwatering. Too much water can cause roots to rot. Rotten roots will not be able to pull in moisture and, though water is abundant, the tomato plant can be dying of thirst.  Again, a finger into the soil will tell you if the soil is too wet. The plant will not recover from root rot.  

There is one further thing to rule out before you rest in the tranquility that upward leaf curl requires no action on the gardener's part: pests. Check a few leaves to be sure the curl isn't harboring an insect or caterpillar cocoon. No caterpillar, no problem. Your tomatoes are fine in their self-protective upward curl.  

A two- to three-inch layer of mulch around tomatoes will help the soil retain moisture and stay cooler. Straw, wood chips, chopped leaves, and grass clippings are all fine mulches for this purpose. Mulch also helps keep moisture around tomato plants more even; this can keep the fruit from cracking and help to prevent blossom drop.

Ever wondered how to encourage sweeter tomatoes? Here temperature and sun are your friends, to a point. Ninety degrees and lots of sun will give you a sweeter tomato. At 100 degrees, fruits develop color on the outside, but stay green on the inside: not tasty. So when temperatures soar, pick tomatoes that have begun to color and allow them to ripen indoors. Remember: never refrigerate a tomato.

For the health and beauty of your tomatoes, give them a bit of protection in the heat of summer. Mulch and a bit of shade will help in maintaining happy plants that produce delicious tomatoes.

UC Master Gardeners of Butte County are part of the University of California Cooperative Extension (UCCE) system.  To learn more about us and our upcoming events, and for help with gardening in our area visit our website.  If you have a gardening question or problem, email the Hotline at mgbutte@ucanr.edu or leave a phone message on our Hotline at 530-552-5812. To speak to a Master Gardener about a gardening issue, or to drop by the MG office during Hotline hours, see the most current information on our Ask Us section of our website.