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I made a terrible mistake the other day. I left one of my potted plants outside on my back porch. It was exposed to the sun all day. To the bright southwestern light. I don't know if it'll ever forgive me, if it even survives. While looking at those poor, fried leaves with black spots and edges, I started thinking about the delicate relationship that plants have evolved with the sun. Of course, everyone knows that plants use light to take carbon dioxide and water and create sugar. That's the process called photosynthesis, and it's pretty much the source of all life on earth.
What we rarely think about, until we see those scorched leaves, is the problem that plants encounter when there's too much sunlight. Here in the desert, we rightly think about water - the fact that we often don't get any, and that too often, soils have such bad drainage that landscape plants wind up with their roots drowning in waterlogged holes- but there is, of course, more to the desert than a lack of water.
When we plan on going outside, what do we do? We put on hats and sunscreen, right? If I didn't, I'd look like a boiled lobster!
Well, we need to remember that plants can experience something resembling a sunburn process, too. This sunburn is not actually due to the high temperatures we get here in the Mojave, although the heat certainly does contribute to many of the problems plants can face here. We get something like 300 cloudless or nearly cloudless days every year. Lots of the plants we've imported into our landscapes evolved in locations where there's considerably less light. Some of them actually developed ways to catch light - all kinds of strategies like the big floppy leaves you see on a caladium, for instance; but many leaf characteristics evolved as a means to protect leaves against light.
Things like fuzzy leaves - the fuzz provides shade to the surface. Or think about leaves that have color - the deep purplish red in a red-leaf plum tree, or the vivid colors on a little coleus plant - that red color shelters the green pigment that permits the plant to photosynthesize.
From traveling around, you know that many desert plants have oddly shaped leaves, if they have any - creosote bush has many tiny, thick leaves, and cacti sacrificed their leaves entirely - so these desert natives don't suffer much from sunburn, at least in the wild. But the non-native plants we've brought in - their defenses evolved to protect leaves when they were growing in places where the sun was not anywhere nearly as intense as it is here. Some of them have been able to adapt - you can find some fruit tree varieties that do very well, as well as a number of rose cultivars - but not all plants from other locales. You can't just give many of them a hefty watering and expect them to thrive in the full Mojave sun.
For example, I have a Japanese privet that's proven to be a real fussbudget. Fortunately, it's in a pot, because right now it's sitting on the patio completely under cover. Everyplace else I've tried, it's developed hard burned patches on its leaves - neither attractive nor healthy.
Very few - in fact, virtually none - of the vegetables we grow in home gardens originated around here, and they don't have much in the line of resources to escape damage from excess light. Tomatoes, for example. (You can always use tomatoes as an example because everybody wants to grow them, but similar injury occurs on peppers, squash, many of the fruiting vegetables.) When the light gets too intense, you can see pale patches on the fruit, or even the whole fruit can become hard and bleached. When this happens, the plant can show other signs of weakness and become more vulnerable to diseases and to insect pests.
If your plants are showing sunburn problems already, you'll need to protect them from excess light. If you can't simply pick up the pot and move it to another part of the yard, you'll need to block some of the sun. Especially if you're worried about fruit. First, make sure that the plants have good foliage cover. Then try to cut down about 30% of the light that hits the fruit. There's an item you can use called 'shade cloth' - it's literally that - and you can buy it at most of the nurseries or home stores around town.
The best thing to do is avoid the problem - put a plant in its best location in the first place: shade-loving in the shade and desert-adapted plants in the sun. Then put on protection when it's necessary.
For KNPR's Desert Bloom, this is Dr. Angela O'Callaghan of the University of Nevada Cooperative Extension.