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Healthy tomatoes grow in a garden. Photo courtesy Getty Images.
Do you have a gardening question? The UC Master Gardeners of Santa Clara County can help. Our volunteers are part of the University of California’s Cooperative Extension program, and we are trained to use research-based information to educate our fellow residents on home horticulture, pest management, and sustainable landscape practices specific to our area. 
Our monthly “Ask a Master Gardener” column will answer your questions and offer tips. Have a question? Email lifestyle@embarcaderopublishing.com

How can I tell if my soil is moist enough?

For trees or other landscaping, try pushing a soil probe or rod into the ground. When it stops going in, you’ll know how deeply the water has penetrated. Irrigate until the water reaches 12 inches deep for grass, 12 to 18 inches for shrubs and perennials, and 12 to 24 inches for trees.

For garden beds or pots, use a trowel or simply stick your finger a few inches into the soil to check whether it is moist or dry. Irrigate to keep the root zone moist.

With smaller pots, you can often tell whether they need water by lifting them. Watch out for pots that have become hydrophobic, meaning the water runs through them without being absorbed. They need to be watered very slowly or soaked in a larger container to rehydrate the soil. Check out our video on how to deal with hydrophobic soil.  

Remember that plant roots need air as well as water, so you want your soil to be moist, but not soggy.

August gardening tips

Safely preserve your tomato harvest

On the Peninsula, tomatoes typically begin ripening in late July, with peak harvests in August and September and the season continuing into October. If your garden is producing more tomatoes than you can eat fresh, canning is a great way to preserve the harvest.

A bumper crop is every gardener’s reward — and sometimes a challenge. Because tomatoes sit right at the edge of safe acidity for water-bath canning, it’s important to follow tested recipes. Their acidity can vary by variety, ripeness and growing conditions in ways you can’t measure at home.

UPCOMING GARDENING EVENTS

UC Master Gardener gardening events are taking place throughout the county this month. Here are some of them, with more listed on our events page.

To ensure safety, all tested recipes require one of the following acidifiers: bottled lemon juice, 5% acidity vinegar or citric acid. If using lemon juice, choose bottled rather than fresh because its acidity is standardized. 

Adding the right amount is simple and doesn’t noticeably affect the flavor.  Don’t skip acidification, even with recipes that worked fine in the past. Here are some tomato recipes to try.

Why your tomatoes have deep scars

Speaking of tomatoes, have you noticed misshapen fruit with deep folds, large scars or holes at the blossom end? This physiological disorder, called catfacing, is more common on large-fruited varieties such as beefsteak tomatoes.

Catfacing can be caused by weather conditions, such as cool, cloudy weather during bloom, which can cause blossoms to stick to the developing fruit. Heavy pruning may also increase the problem. The good news is that the fruit is perfectly safe to eat once the damaged portion is cut away.

Here’s more information on catfacing and other environmental stresses on tomatoes not caused by pests or disease.

Harvest and preserve herbs at their peak

If you plan to preserve your homegrown herbs, harvest them just before they flower, since flowering can sometimes make the leaves taste bitter. Harvest in mid-morning on a sunny day after the dew has evaporated, when essential oils are most concentrated. New leaves at the tip of the plant contain the strongest flavor.

Using clean scissors, snip stems just above a leaf or pair of leaves. Rinse herbs in cool water, shake off excess moisture and pat them dry. You’re now ready to dry them or freeze them— but save some to use fresh right now in your favorite summer recipes.  Here’s more on preserving herbs.

Protect plants from intense summer sun

While many plants thrive in the sun and warmth, excessive summer heat can stress them and push them into survival mode. Intense sun can stall fruit production, scorch mature leaves and wither tender seedlings.

Would You Like to Become a UC Master Gardener?

We are now gearing up to welcome and train the 2027 cohort of new  UC Master Gardeners of Santa Clara County. The training schedule includes weekly classes via Zoom on Thursday evenings from January through May 2027, plus eight hands-on workshops on Saturday mornings.

Interested in applying? Please attend one of the mandatory information sessions in early September, and submit your application by Sept. 22. Zoom interviews will take place in late October, and applicants will be notified of acceptance decisions by Nov. 6.

Fortunately, shade cloth offers an easy, economical and versatile way to protect your garden and reduce water loss. Available in various sizes, colors, materials and attachment styles, shade cloth can block anywhere from 20% to 90% of solar radiation.

The results speak for themselves: using a 40% shade cloth, one UC Master Gardener recorded a reduction in surface soil temperature of over 15 degrees!

Start planning your fall vegetable garden

We may be in the throes of summer, but if you’re interested in planting cool-season vegetables from seed, now is the time to start seedlings.

Broccoli, cauliflower, beets and fennel take about six weeks to reach transplant size. Greens such as lettuce, spinach, arugula, chard and kale are ready in about four weeks. For planting in mid- to late September, aim to start seeds in early to mid-August.

You can also direct-seed them into the garden if you have the space. The soil should still be warm enough for seeds to germinate, but be sure to provide afternoon shade and keep the soil consistently moist.

If you’re buying transplants from a nursery, you can wait until September. In Santa Clara County, many cool-season crops planted in September or October can be planted again in February and March, allowing you to harvest another crop before it’s time to plant summer vegetables.

One of the biggest advantages of cool-season vegetables is that they require less supplemental water because of cooler temperatures, shorter days and winter rain. They also tend to have fewer pest problems. Most cool-season vegetables grow best in temperatures between 55 and 75 degrees and in locations that receive six to eight hours of sun each day.

Check out our videos page for videos on cool season vegetables, including a 3-session course for more inspiration.

Would you like to ask a UC Master Gardener about a plant problem? Submit questions to our Help Desk online. Join our Zoom Plant Clinic Online on August 11 from 7:00 pm to 8:30 pm where you can directly ask gardening questions to a Master Gardener.

You can also subscribe to our monthly Tips and Events newsletter. San Mateo residents can also find gardening advice on the UC Master Gardeners of San Mateo and San Francisco website.

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