Angela O'Callaghan explains why some plants play nicer than others.

When we're planning a garden for the spring (or any season, for that matter), one of the things we really should consider, but seldom do, is how well plants do with each other. We're pretty familiar with the concept that plants have evolved resistance to certain diseases or insect predation, but this is a little different.

Now, I'm not talking about how you place them in your planting beds. You definitely have to make sure that one's not crowding out another, or shading out another. You know you don't want to put a pumpkin plant, with its huge leaves, next to something that'd be overwhelmed, like basil. No, this is still another thing.

Several years ago, there was a series of gardening books with titles such as 'Roses love garlic' or 'Tomatoes love carrots' (or something to that effect). I didn't read the books, but they certainly point out a scientifically proven fact - some plants produce chemicals that have genuine effects on other plants.

In the case of those books, the effects were positive. The benefits can be the result of lots of different factors - sometimes the roots of one plant will secrete a chemical, which could be a hormone, or a sugar, or heaven only knows what) - that stimulates a neighbor to grow. Some of them can give seeds a push to germinate, and others might promote flowering. That's the background of the practice known as 'companion planting'. But not all of these interactions are positive. In fact, when people first started looking closely at them, they were noticing some very negative events.

The name of this phenomenon, for those of us who must know such things, is 'allelopathy'. Most of the plants that are allelopathic seem to produce something in the soil. I started looking up references on it, and found many more than I'd expected. Apparently it's been one of the hot topics in horticulture for the past few years.

Now, people have been growing plants for, oh, about 10,000 years. Wouldn't you think that somebody, way back when, would have noticed these kinds of interactions? Well the fact is that people did notice them, and describe them. Like the classic Black Walnut- there are many plants that cannot grow well if they're at all close to this tree. The roots secrete a chemical that's toxic to other plants. The ancient Greeks knew about this one. But for a long time, scientists were dubious about the whole concept of allelopathy. It's no wonder, really. It's often not just a case of one root, one chemical, one effect. Nothing's usually as simple as the black walnut.

Everything's more complicated than I'd like. What are some of these interesting plants? Well, they're certainly not all that uncommon. Eucalyptus, which I love, might have some toxicity to certain members of the grass family. Then again, some grasses are toxic to certain trees.

For instance, I found a scientific paper that was published last February. Researchers reported that fresh needles from Aleppo pine can interfere with the growth of Bermuda grass. At last, maybe there's a chance I can get rid of the Bermuda that insists on poking its runners through my rock mulch! But even more bizarre, I found another article from a few years ago, where researchers in Oklahoma found that Bermuda might limit the growth of pecan trees!

There're a couple of common weeds called 'knapweeds' that are major problems in the wild parts of Nevada. These plants not only out-compete our native plants for nutrients and water, but it turns out they produce chemicals that affect native plant root growth! Talk about adding insult to injury. But then again, there's some evidence that this plant might inhibit the growth of its own seedlings, the same way that creosote does. Now, there's a real effort to determine how the compound in knapweed could be used to limit other weeds.

There's lots of research going on. After all, if one plant can keep another one from growing, wouldn't that be a terrific alternative to commercial herbicides? In fact, that's one of the driving forces in the research. Trying to get the plants we want to crowd out, or even kill, the plants we don't. Gotta love it.

Quite a bit of the research is still in the 'Well it seems' category. In a publication I got from Florida Cooperative Extension, there's a statement that lantana can suppress milkweed, and even chaste tree can retard the growth of certain grasses.

Well, I think that's enough complication for now. The point is that some plants produce chemicals that interact with other plants. We usually only notice when they interfere with each other. And sometimes we don't even notice, then.

For KNPR's Desert Bloom, this is Dr. Angela O'Callaghan of the University of Nevada Cooperative Extension.

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