Save the Tomatoes! How to Protect Your Raised Bed from a Heat Wave

“In August, it can really be a matter of just trying to keep your plants from expiring every minute of the day,” says Yvonne Savio, the former master gardener coordinator in Los Angeles County and popular blogger whose site, Gardening in LA, is a treasure trove of information no matter where you live.  

Of course, you don’t need to go to anyone’s website to know it’s hot, and that causes problems. For example, Savio says, it’s hit 95°F in Pasadena, where she lives, but 85°F is the top temperature for tomato plants to put out new blossoms. Translation: When the weather is in the 90s, no new tomatoes.  

Fortunately, Savio is here to tell us what to do when the temperature is climbing.  

We also got into what to plant for fall, which, thanks to climate change, is getting more complicated. After all, what the Farmer’s Almanac once predicted for September and October is often no longer true, with heat extending later into the year. Fortunately, Savio has a great hack, as you’ll see below.  

One other piece of advice for new plantings? Savio reminds us that the seedlings we buy have often been grown in select greenhouses with controlled climates. “They get in the real world and are like, ‘What’s going on? I’m outta here,’” she says, adding that beginning gardeners often blame themselves for killing the plant.   

Here, Savio’s tips on what to do about the heat that’s already descended, and what to do about potential heat and your fall plantings

So what do people need to know when it comes to watering their raised bed or vegetable garden in this heat?  

Photo by Reed Davis

You want your plants watered well enough at both the surface level and deeply enough so [it’s] reaching the bottom of their roots. For example, lettuce roots genetically will grow no deeper than 6 inches, squashes 1 foot, and tomatoes grow to 2 or 3 feet. To help them thrive instead of just survive, you need to get to where those roots are.  

You also want to have planted the plants together where they have similar water needs. Lavender and rosemary, they want the soil dry most of the time. Basil, cilantro, and parsley have vastly different needs and want it pretty wet.

What about mulch and compost?  

Certainly just making sure that you’ve got a lot of mulch and a lot of organic matter in the soil mix will help. If you’ve been growing something in your container garden from the spring to now, before you add more compost or mulch onto the bed, water it first. If you don’t, when you put another 2 or 3 inches of compost or mulch on top, you’re going to insulate the dryness. You want to insulate moist soil rather than insulate dry soil. Really, when it’s this hot, you’re just kind of waiting it out and trying to get as much harvest out of your plants as you can whether they’re tomatoes, squash, beans, or whatever they are. 

What about providing shade?  

When we get into the really hot weather of August and September, even the hot weather plants will benefit if you buy some spun-bonded polyester from the garden store and do a little shading. You can place it on the top of a trellis, perhaps, to give your plants some filtered light instead of blasting direct sun onto those plants during that 12 to 4 p.m. time period when it’s really bad.  

If you’re watering well, but your plants are looking droopy and even a little crispy and not recovering overnight, you can put cheesecloth over your plants, or use shade cloth. You don’t want the sun totally excluded, however. Your plants really do need the sun. You just want to protect them a little bit.  

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Do tomatoes need shade, too?  

If you put them in early in the season—say, in the early spring—they’re really pooping out by now. But those people who didn’t—especially those on the coast who plant their tomatoes later—they’re at the height of the season, so that bit of shade will help as well.  

What do you suggest in terms of thinking about fall planting?  

You should start gathering your materials for whatever you’re going to do next month. For example, if you’re going to be starting plants from seed you need to start them inside because it’s so hot outdoors, or some place in the shade.  

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For herbs especially, you can take cuttings and start them. So, trim them not only for culinary uses, but to also create more plants, because the warmth is going to get them. If you propagate them in water, you want to be sure to plant it in potting soil once you get about a quarter inch of those white roots or they will rot. To propagate them in pots, trim your herbs leaving three nodes with foliage on it and three nodes without leaves. Then cut right below the bottommost node. Bury your cutting into potting mix and keep it in the shade and keep it moist to root it.  

What else should we think about for September? 

September is that transition point when I recommend that people sow or put in the last seedlings of summer stuff, and plant the first of the cool season stuff. You never know what the weather is going to do. Lately, the heat has extended from the summer to the fall, but you never know. So, if you plant both the last of the warm stuff and the first of the cool stuff, you’ll be able to eat something wherever the weather goes. It’s always good to feel like you’re hedging your bets both ways. Some are going to work, some aren’t, but you’re guaranteed a little of each will work.

Any other tips?  

Keep in mind that it’s not the most enjoyable time to get into the garden. You may want to wait toward the end of the day to go out yourself so you’re not getting blasted.