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September 25, 2021

How plants survive through the heat

What happens to a plant when the thermometer tops 100 degrees? Planning for more hot weather this summer is a requirement for a successful garden. Are there some plants that can survive tough times more easily?

Photosynthesis is one of the most remarkable biochemical processes on earth and allows plants to use sunlight to make food from water and carbon dioxide. But at temperatures about 104 degrees the enzymes that carry out photosynthesis lose their shape and functionality. A garden that provides optimum light and water but gets too hot will be less vigorous. Tomatoes, for example, will drop blossoms and not set fruit if temperatures are over 90 degrees. Plants that endure high heat can be stunted, weakened and attract pests and diseases even if water is available.

Plants do have natural systems that respond to heat problems. Some plants are better at this than others. Plants can cool themselves by pumping water out through the leaves for a kind of swamp cooler effect. They can also make “heat-shock” proteins which reduce problems from overheating. All these strategies do take resources away from a plant’s other needs like growth, flowering and fruiting. 

It’s no surprise that many California natives are adapted to high temperatures. Walking my old property the other day I noted which plants are regrowing after the fire, despite little winter rainfall. Every plant burned to the ground. The climate is much hotter up in Bonny Doon now that the tree cover is gone. The redwoods are trying to regrow but it’ll be decades before they provide shade again. Those plants that have regrown the best include California fuchsia (epilobium canum). They spread by underground rhizomes so it’s not surprising they survived. I didn’t see any bees or hummingbirds around but one of these years they’ll return. Another survivor is hummingbird sage (salvia spathacea). They are just starting to regrow and were not in bloom.

The Bees Bliss Sage, a low groundcover that can reach 6-8 ft wide is another plant that I used to enjoy. It has an extended bloom time from mid-spring to early fall with whorls of lavender-blue flower spikes. Bees, butterflies and hummingbirds all find it attractive. The jury’s out whether it’ll return or not. 

Another plant that can handle high temps is salvia clevelandii. Their blooming cycle of electric blue-purple flowers will last through the summer. This salvia survives without any supplemental irrigation but if you give it an occasional deep watering and wash off the foliage every so often it’s much happier.

Who doesn’t like color in their garden? Mimulus or Sticky Monkey Flower blooms are showy and the hummingbirds love them. The Jelly Bean series has added bright pink colors in addition to white, orange, red and yellow but the traditional aurantiacus types are the most tolerant of drought.

Other California native plants that can handle the heat with little water include eriogonum, manzanita, artemisia, California milkweed, ceanothus, mountain mahogany, bush poppy, bush lupine, native penstemon, monardella, mahonia nevinii, fremontodendron and holly-leafed cherry. 

Other non-native well-adapted plants that are known to be more tolerant of heat include butterfly bush, germander, rosemary, smoke tree, rudbeckia, coreopsis, lantana, plumbago, gaillardia, lilac, sedums, oregano and verbena.  

These plants can be the rock stars of your garden. Some natives that are able to survive with no irrigation after two years may look more attractive with a few deep waterings per summer. And don’t forget the wood chip mulch to encourage the soil microbes and keep the soil cool. 


Jan Nelson, a landscape designer and California-certified nursery professional, will answer questions about gardening in the Santa Cruz Mountains. Email her at [email protected], or visit jannelsonlandscapedesign.com.

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