If you're weary of purchasing fertilizer and mulch for your tomatoes every season, here's a way to get off that treadmill. Instead of buying, grow all that you'll need of both, during the off-season.
The technique is very simple. In late summer or early fall, you make a bed or beds for your tomatoes. Then sow the very hardy annual legume called hairy vetch, Vicia villosa. In spring you kill the vetch by simply cutting it close to the ground, and then lay it in place on the beds. Then set the tomato transplants right through the mulch.
That's it. Nothing to buy, and no need to dig, hoe, or weed. Do your regular staking, pruning and watering, and t he tomatoes will thrive until it's time to replant the vetch in fall, and yields will be as good as those you get with the method you are now using, and likely even better.
High yields, much less tilling and digging, and no need for compost, manure and other nitrogen fertilizers: If that sounds to you like a major breakthrough, you are not alone. This system has performed so well in USDA tests in Beltsville, Maryland, that researchers and farmers all over the country are trying it and expanding it to other crops.
Hairy vetch is hardy enough to grow in the far north (USDA Zones 5 and colder), the season length in those zones is too short to permit easy exploitation of the technique. In general, the hairy vetch-tomato rotation described here most readily adapts to gardens in zone 6 and warmer.
This planting system evolved out of a search for an alternative to plastic mulch, which worked like magic on tomatoes. As a matter of fact, over the last 40 years, plastic mulch has radically altered vegetable growing, both for farmers and many gardeners. But plastic brings problems. It is expensive to buy and difficult to dispose of in a responsible way. Commercial growers have begun to complain loudly, especially about the disposal problem.
A few years ago, Dr. Aref Abdul-Baki and his colleague Dr. John Teasdale, horticulturists with the Agricultural Research Service in Beltsville, began thinking about the plastic-mulch-and-tomatoes problem and how to solve it. The most obvious benefit of plastic is the soil heating that advances harvest by one to two weeks. But plastic mulch also increases total yields, and it does that by keeping the ground moist and blocking weeds. The researchers realized that any organic mulch could perform these two functions just as well as plastic does, and probably better.
Good vegetable gardeners already know that. We mulch our tomatoes with leaves, grass clippings, hay, straw, compost or whatever, and the results are great. But Abdul-Baki and Teasdale were thinking more about farmers, who would want to know exactly which mulch is ideal, where to get it cheap and how to put it on fast. If the researchers were to suggest the farmers use shredded leaves, straw or compost, they would probably reply, Just what we need! More work and more stuff to buy! We'll keep the plastic and its early yields, and deal with the other problems as best we can."
So the researchers began to look for mulches that farmers could grow themselves in the off-season, so mulch-growing wouldn't tie up valuable ground. And they pretty quickly settled on the idea of a winter-hardy annual, something that was easy to kill in spring and could maybe even be grown right where the vegetables would be planted. The method worked well from the start, and the best plant, in most regions, turned out to be hairy vetch.
The tomato-vetch trial plots, using no nitrogen fertilizer and without any weeding, have consistently outyielded plastic-and-fertilizer plots by about 25 percent, and fertilized bare soil by 100 percent.
Both systems provide plenty of nitrogen for a tomato crop, and both mulches keep the soil moist and eliminate weeds. But the vetch mulch, combined with less tilling, steadily improves the soil structure and the number of microbes, worms and other soil organisms at work. The thick mulch also keeps the soil evenly moist and cool, so roots can explore it thoroughly in their search for nutrients. The healthier root systems are better able to make use of available water and minerals so yields are much higher.
Vetch produce plenty of nitrogen for tomatoes, between 150 and 200 pounds per acre. About half of that is available to the tomato crop the first year; the other half remains bound up in organic matter and is used by the crops that follow. The nitrogen in decaying organic matter becomes available slowly and steadily through the season, more when it's warm and less when it's cool, down to about 50° F.
Adding chemical fertilizers, on the other hand, doesn't improve the soil. Once rain or irrigation leaches them away, they're gone for good. With the plastic-mulch test plots (and on the bare-ground plots, too), nitrogen is applied once a week through drip lines buried two inches deep. That's a more efficient way to apply fertilizer than spreading it before planting, but even so, nutrients are delivered only in the zone that is wetted by the emitters. With the vetch-mulch system, the organic matter supplying minerals to the roots becomes part of the topsoil of the entire bed.
Vetch needs to grow for about a month before the very hard frosts (about 22° F) shut down everything in the garden, often around the same time that most deciduous trees are losing their leaves. That means you should plant the vetch one to three weeks before the very first fall frost. You want the plants to get at least four inches tall before they stop growing for the season, though in warmer regions the vetch can keep growing all winter.
Abdul-Baki and Teasdale say that the success of the system depends on using a legume that grows well in your climate to give you the most nitrogen and the biomass you need for weed control and soil fertility. In most places that will mean hairy vetch, which is the most widely adapted winter-hardy legume. It grows quickly in fall, overwinters even in the Far North, and then starts regrowing quickly in the spring. But wherever it's quite warm in the fall when you want to plant (as in parts of zones 9 and 10), other legumes like crimson clover or subterranean clover may do as well or better. Consult your extension agent to confirm which winter legumes have proven themselves in your locale.
After you prepare a seed bed in your future tomato plot, plant the vetch, either by broadcasting or in shallow furrows at the rate of an ounce of seed per 10 square feet. The researchers plant in rows, seven of them across the four-foot-wide bed, or about six inches apart.
The vetch begins to grow again in earnest early the following spring. By the time the weather's warm enough to plant tomatoes, it will be three to four feet tall and beginning to flower. The day before planting tomatoes, use hedge shears or a hand scythe to cut the vetch an inch or two above the ground and lay it in place on the bed. You should end up with a dense mulch that is four to five inches thick. It is important to cut all the vetch and cover the stubble. Stragglers that survive can become tangled in the tomato plants. Plant the tomatoes down the middle of the bed by parting the mulch. Set the plants in place and then tuck the mulch back around them.
Where weekly rainfall is not dependable, it is important to irrigate over the vetch mulch. About 85 percent of the nitrogen in the vetch is in the tops, with the remaining 15 percent in the roots. When cut at the prebloom to bloom stage, vetch is very tender and decays readily, but it cannot rot and release its nutrients without moisture. The researchers lay drip irrigation down the center of the bed on top of the mulch to speed up its decomposition. In gardens, wetting the mulch with a sprinkler of some kind would be ideal. "We are developing a farming system, so we use drip tape, which wets the mulch at individual points," says Dr. Abdul-Baki. Drip emitters that deliver a small spray either in circles or half-circles are available.
The researchers use the stake-and-weave method of training the tomatoes (set a stake every two plants and weave twine back and forth around the stakes and plants). But you can use any tomato training method you like. Enjoy the harvest and the freedom from feeding and weeding that the vetch-mulch method provides. By the end of the season, the mulch will have decayed so much that only traces remain. Some late-season weeds may begin to poke through, but they are easily pulled or cut by hand in the soft soil. Besides, it's almost time to plant vetch again.
Before the first light frosts arrive, prepare to plant more vetch, disturbing the soil as little as possible. Cut the tomato plants and take them to the compost pile. Pull or lightly cultivate to destroy any small weeds. Then plant vetch seed, either in furrows or by broadcasting.
Abdul-Baki and Teasdale have some plots where they've been growing vetch every winter and tomatoes or other vegetables every summer for four seasons now. Any vegetable that is grown from transplants or has large seeds will work well in this system. Cucumbers, squash, melons, peppers, eggplant, sweet corn and even green beans are candidates. Preliminary work at Beltsville and elsewhere hints at success that will equal the results with tomatoes. It's a good idea to rotate tomatoes with other vegetables.
The method is not for all crops. Small-seeded, closely spaced vegetables like beets, carrots and lettuce would have trouble growing through the thick mulch. Where the season is very short or when you really need the extra 10 to 14 days that only plastic mulches can provide, then the vetch-mulch method may not apply.
But surely there is a place somewhere in your garden for this hardy and productive legume that does so much growing while you are waiting for things to warm up enough to plant the main-season vegetables. If we can all start growing at least some of our own mulches, we won't have to work so hard to find the right materials, then haul them home.
"This is the production method of the future," says Dr. Abdul-Baki. "And I think it can work as well for gardeners as for farmers, maybe even a little better. All I want is for gardeners to try it, on just one bed. Try it and make the judgment for themselves."
Jack Ruttle is a former senior editor at National Gardening.
Photography by Suzanne DeJohn/National Gardening Association
Article published on June 23, 2008.