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I have a small succulent garden full of all sorts of different cacti and succulents. In the summers many of them would bake, a bit too much, so I planted a few small Eucalyptus along the west edge of the garden. Well, in no time, they became BIG eucalyptus... and now the drop a lot of leaf litter on the plants (which I try to 'blow' off with a blower as often as I can). At least they shade them well in the hot afternoon.
So far most seem OK, and definitely appreciate the shade. But I have heard so much about euc. leaves being toxic. Is that really true? Think succulents would care? If I had known how dirty these eucs. would be, I think I would have picked a different shade tree (though nothing good comes to mind).
I'm not sure. According to "Chemical Ecology," eucalyptus species are allelopathic, which means that they produce chemicals to inhibit the growth of other plants around them. Think walnut trees and juglone. It's been observed that euc. stands in the US have little or no undergrowth--no plants grow underneath them. However, in Australia there is a healthy understory, so apparently plants in Aust. have evolved to overcome any toxicity. Are any of your succulents from Australia? The bottom line is that most of the toxic or potentially toxic compounds are going to be released by rain, which interestingly enough can include fog that condenses on the trees and drips underneath. It sounds like your climate (if you can grow eucs, cacti and succulents that thrive outdoors) is quite hot and dry. I would make sure that the eucs are trimmed back so that they can't drip directly on the cacti when it rains/precipitates. I'd also make an effort to clean up the leaf litter from the ground before it rains, but as long as it stays dry, there doesn't sound like there's going to be a huge problem.
It also sounds like the eucalyptus have been there for a little while. Are the plants underneath them growing/behaving normally? Do they show signs of stress--growing more slowly, having a stunted appearance, smaller-than-normal root system, trouble off-setting or flowering? I'd also imagine that if the soil where your succulents are is well-draining, that if you do get a flush of toxins after a rain that you can give the soil a good soak and most will wash away. You may also be interested to note that allelopathic chemicals affect germination and the growth of seedlings most--it's an evolutionary trick to keep your own seedlings growing and to eliminate any competition.
Quite frankly, it sounds like an interesting problem, and I hope things work out well for you.
Hey, thanks for your answer... interesting things to think about. The Eucs are actually only 5 years old (but already about 30' tall). The plants for the most part below look fine (to me, but I have nothing to compare most to)... however very few, if any, are actually seedlings. So maybe the toxins won't affect larger plants. Hope that's true.
I don't think the Eucalyptus leaves are toxic, but they do take an awful long time to break down (years!). They form such a dense cover that nothing else can grow underneath them unless you remove all the leaves all the time. They are extremely labour intensive, and it is only getting worse as they are getting bigger. 30' for an Eucalyptus is still small! If I were you I would remove them while you still can and replace them with a deciduous tree. At least they only shed once a year, and usually with a kind of leaf that breaks down into compost within a reasonable time. An evergreen tree sheds all the time, all year round. And your succulents would appreciate increased light level and sun in winter.
Eucalyptus also takes up an incredible amount of water, depriving the rest of your garden. For this reason this tree is now here declared an alien invader and only allowed to grow in controlled commercial plantations and is being removed from water catchment areas.