Let's Talk About Salt
I've seen a lot of posts on various garden blogs about natural herbicides, and they all fail to understand some simple things about how plants deal with salt.
First of all, plants see the soil in terms of tension. They don't know what wetness is: when they look for water in the soil the only way they have of seeing it is through tension, and the lower the amount of water, the higher the tension. They need some tension, but not too much: plants have evolved to be sensitive to a very specific range of tensions (what that range is depends on the plant and where it evolved, of course).
Salts (not just table salt, but the range of chemicals that are known as salts which includes fertilizers) make the soil have a higher tension than it would at the same moisture content but a lower salinity. So when the soil's salinity is high, there might be plenty of water in the soil but the plant's roots can't absorb it, and it dies (or wilts dramatically), despite what might feel like plenty-damp soil to you.
(Do you dig around and squeeze your soil? You should. Not only is it fun but it can tell you how much moisture is in your soil.)
Salts tend to build up in irrigated soils (if you have houseplants, that white crust that forms on the top is salt). Almost all water has some salts in it, and in places where the municipal water comes from wells it's likely to be higher. And if you spray or drip them into your garden via your irrigation system, those salts are going to hang around: they get there via irrigation water, but don't get to come along when the plant uses the water, or when the water evaporates. That means that if you have an irrigation system in your garden, you may well be building up a salt problem, even if you don't fertilize.
While that sounds fairly dire, it is relatively easy to get rid of salts. For plants in containers you soak them or water them with lots and lots of warm water to dissolve and rinse away the salts. Outside you just water the heck out of your patch of land and the salts will be dissolved and will dissipate into the water table, where they will be so greatly diluted as to cause no problems, unless you're applying massive amounts of fertilizer (in which case shame on you). You don't have to do this daily: flooding the garden every three to five years is perfectly fine, though if you live in an area like the Bay Area, with lots and lots of rain all winter, that may not even be necessary. You can test for salts in the soil through cooperative extension or wherever you prefer to have soil tests done if you want to know whether you need to be concerned about this.
Almost any plant is very susceptible to salts when it is germinating or very small. The size of the plant makes it more sensitive to water deprivation, and the delicacy of its leaves means that if you spray any kind of salt solution onto it -- be it dissolved table salt, chlorine bleach, or vinegar -- the plant is basically a goner. It bears repeating that salts can also come from fertilizers, so you have to be careful about spraying the leaves of new seedlings with Miracle Gro or compost tea. If you have a water softener, that water is salty, too.
You can use this to your advantage by spraying the emerging seedlings of weeds with salt solutions (presuming you can tell them from the plants you want). But salts are not a cure-all for your garden weeds, even if you spray every tiny sprig of weed that comes up (please don't). For plants that spread through runners or come from bulbs, salts sprayed on the sprouts don't have the same effect. The plant has a lot of energy to rely on in the mother plant or in the bulb, and while you may burn the leaves, it is still alive, and it will be back. You're hardly even hurting it, because the tiny sprig you sprayed is so small in relation to the mother plant.
That's where Roundup comes in.
Roundup's active ingredient glyphosate is an enzyme inhibitor. It is absorbed into the plant and locks up the plant's ability to store food for itself. It works only when sprayed directly on the leaves of the plant. When sprayed on soil, the same enzymes that lock up a plant's ability to store food bind to the soil -- which means that it can't be dissolved out and it also is unable to act on plants any more; glyphosate in soil is essentially inert. It's not something to be used indiscriminately, but if the weeds you are dealing with are relying on large underground stores of energy or a much larger mother plant, glyphosate is one way to kill them. Sprayed on a tiny sprig, it is absorbed into the mother plant and causes it harm, even if it might not kill it. Vinegar, which has been recommended to me three times now in the last few weeks, is completely and utterly ineffective against that sort of weed, and when you spray it on your garden you just raise the salinity of your soil, making it more stressful for the plants you want to keep.
This is why I am not afraid of using Roundup in the garden, and why I am very skeptical of "traditional" "natural" remedies. In this case, the "natural" remedy is more toxic to the plants I want to keep than the evil, corporate chemical solution. At the very beginning of our dry season, the last thing I want to be doing is raising the salinity of the soil.
As a side note, the other effective way of dealing with such weeds is to dig them out, sieve their roots out of the soil very carefully, and either burn or throw away the plants (in other words, don't compost them). That takes a lot of effort and may not be possible (for example, if you have Bermuda grass growing in your new rose hedge). Apart from bulb plants like oxalis, hand weeding works best, and even for oxalis hand weeding plus the sieve is pretty much foolproof.
For weed management, most landscapers will spray herbicide if the weed is under two inches tall, and if it is larger they hand-pull or otherwise mechanically remove the plant from the ground. I think that's a good guideline, if only because it prevents lots of large, keeled-over dead weeds lying around while you wait for the root to die. Of course, the earlier you get to the weeds, the better; Bermuda grass has less time to get to be a problem if you are killing it back every time it makes a foray into your garden; when you have to eradicate an established mother plant, you are looking at years of work with spraying and manual removal. So weed early and weed often, and understand how the chemicals (synthetic or natural) that you use in your garden work.
posted by ayse on 05/25/06