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How to save your plants during a heat wave

Soil moisture is key to survival in triple-digit temperatures

Wet tomato on vine with blue cage
Spraying tomato plants with water early in the
morning helps fight spider mites and aphids while
refreshing the leaves and raising the humidity
around the plants. The runoff helps the plants, too.
(Photos: Kathy Morrison)

As the weather forecasters like to say: We’re having a heat wave!

Triple-digit days dominate our Sacramento forecast at least through Saturday. In its Excessive Heat advisory, the National Weather Service warns, “Drink plenty of water; don’t wait until you’re thirsty.”

That same advice applies to your garden. Plants with roots in soil that stays evenly moist – not soggy – have the best chance of surviving, even thriving during this heat wave.

The secret is to keep the soil from completely drying out – not on top, but down 4 to 6 inches in the root zone. Use a moisture meter to check. Or use a long-handled screwdriver; if you can’t shove it in 6 inches, you need to irrigate.

Potted plants in containers are especially at risk. The rootball contracts as it dries out, leaving gaps around the sides of the pot. When you pour water on a severely dry potted plant, the water tends to run out the sides without ever penetrating the rootball. In such cases, you may need to submerge the pot in water, let soak for a few minutes, then drain. Don’t let the roots sit in standing water; they’ll suffocate.

Wilting in the heat of the afternoon is normal for many plants, especially those with big thin leaves such as hydrangeas or squash. But if plants are wilted in the morning, water immediately.

Burlap or shade cloth can help prevent sunburn
on tomatoes or other summer vegetables.

Are there brown spots on your peppers or bleached-out areas on tomatoes? For plants showing signs of sunburn, erect temporary shade. This can be as simple as draping some cloth (burlap, sheeting, etc.) over a trellis or tomato cage.

Spider mites in particular are loving this heat; this itty-bitty arachnid thrives in hot and dusty conditions. If you see wispy webs covering leaves and stems of shrubs and other plants, knock them down with a strong stream of water from the hose.

This summer shower destroys their webs and washes the mites and dirt off the plants. It also disrupts what attracted the mites in the first place – dry, dusty leaves.

Wash mites off plants early in the morning, so the plant’s foliage has all day to dry. Fungal diseases usually aren’t a problem right now; powdery mildew can’t stand anything above 90 degrees.

This same approach also works against aphids. Showering plants is not wasting water; the runoff will go to the plant’s roots. Most plants also absorb moisture via their foliage.

Avoid spraying neem oil or other treatments (often recommended for pest control). The oil coats the plant’s foliage and increases its sensitivity to extreme heat. Instead of helping the plant, the oil “cooks” its tender leaves.

And put off any fertilization until weather cools back to normal. Feeding increases a plant’s water needs. During this heat, it just adds to plant stress.

For more on heat stress and plants, check out this advice from the UC Integrated Pest Management Program: .


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Dig In: Garden Checklist

For week of March 19:

Spring will start a bit soggy, but there’s still plenty to do between showers:

* Fertilize roses, annual flowers and berries as spring growth begins to appear.

* Watch out for aphids. Wash off plants with strong blast from the hose.

* Pull weeds now! Don’t let them get started. Take a hoe and whack them as soon as they sprout.

* Prepare summer vegetable beds. Spade in compost and other amendments.

* Prune and fertilize spring-flowering shrubs after bloom.

* Feed camellias at the end of their bloom cycle. Pick up browned and fallen flowers to fight blossom blight.

* Feed citrus trees as they start to blossom.

* Cut back and fertilize perennial herbs to encourage new growth.

* Seed and renovate the lawn (if you still have one). Feed cool-season grasses such as bent, blue, rye and fescue with a slow-release fertilizer. Check the irrigation system and perform maintenance. Make sure sprinkler heads are turned toward the lawn, not the sidewalk.

* In the vegetable garden, transplant lettuce and kale.

* Seed chard and beets directly into the ground.

* Plant summer bulbs, including gladiolus, tuberous begonias and callas. Also plant dahlia tubers.

* Shop for perennials. Many varieties are available in local nurseries and at plant events. They can be transplanted now while the weather remains relatively cool.

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