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Master gardeners answer your summer gardening questions

Find out how to get rid of brown spots on your tomatoes and best gardening practices for July
Blossom-end rot on tomatoes starts as a small spot where the blossom once grew. Photo courtesy Getty Images.

Why are there large dark leathery spots on the bottoms of some of my tomatoes?

We suspect your tomatoes have blossom-end rot, an unfortunate but common problem most often affecting tomatoes, but also peppers and squash. It starts as a small spot where the blossom once grew, hence the name. As it enlarges and turns brown, it becomes a well-defined sunken spot. Some varieties are more susceptible than others.

Blossom-end rot is not caused by a disease or an insect. It’s primarily a result of calcium problems in the growing tomato. Calcium in the soil gets transported by water through the growing plant. But when the plant is growing quickly in the spring, the calcium may get used up by the leaves before it can reach the fruits. And if the weather is really hot and the soil is dry, less water moves through the plant, so even less calcium reaches the tomatoes.

Cloudy or cool weather also can cause less water movement because the plant doesn’t need as much moisture to be pulled from the ground. Planting tomatoes early, when the weather and soil are cold (below 60 degrees), means less water needs to be moved through the plant, thus making blossom-end rot more likely.

In general, soils in Santa Clara County contain sufficient calcium, so it’s rarely necessary to add more. As the summer season progresses, you may see less blossom-end rot. That’s because plant growth naturally slows down, allowing better delivery of the existing calcium to the fruits. Less fluctuation in weather patterns and regular irrigation scheduling also help.

Here are a few suggestions to minimize blossom-end rot:

• Keep soil conditions consistent -- water regularly and deeply. Don’t allow soil to completely dry out especially when tomatoes are growing rapidly.

• Use organic mulch along plant rows to moderate soil temperatures and evaporation, which will limit soil moisture fluctuations.

• When high temperatures are expected, reduce plant stress by irrigating early in the morning or evening.

• Don’t overfertilize (especially with nitrogen) because that causes more calcium-hungry foliage to grow.

• Even though blossom-end rot is unsightly, you can still eat your tomato by simply

cutting off the damaged area. Warm tomatoes just off the vine are one of the true pleasures of summer –- so don’t let a few ugly spots spoil your harvest!

Do you have any July gardening tips?

During the summer months, it's not uncommon for us to receive various inquiries related to keeping gardens healthy when the region experiences hot and dry weather. Here are some best practices for July:

• Mow lawns so that no more than one-third of the length of the grass blade is cut in each mowing. Leave clippings on the lawn to provide nitrogen for the lawn.

• Hand pollinate squash by brushing the pollen from a male flower onto a female flower.

• Be careful pruning plants that receive full sun because newly exposed branches are susceptible to sunscald.

• Add 3-4 inches of mulch to decrease moisture loss and regulate soil temperature.

• Plan for ornamental additions to your garden but hold off on other planting until fall.

• Continue deadheading summer flowers.

Want to speak with a master gardener about a plant problem? UC Master Gardeners of Santa Clara County offers Plant Clinic Online, a monthly clinic held via Zoom that puts residents in touch with experts who can help diagnose their plant problems. Upcoming clinics are scheduled for 10 a.m., July 8 and 7 p.m. on Aug. 8, Sept. 12 and Oct. 10. Find more garden events and classes here.

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