Q. In 2000, we put in two 4' x 8' raised beds. Over the years, we have added beds and now have a total of five. In the beginning, we filled each bed with Leafgro Soil Conditioner. Each spring, we add many bags of Leafgro and use a pitch fork to turn the soil. In addition to adding air and nutrients to the soil, this process raises the soil level as it seems to get compressed during the growing season and winter. We also put plastic tarp over one of the garden beds from April-May to heat the soil and kill the nematodes and other organisms. But last year, the beds did not produce all that well. One problem is lack of sun from trees that have grown dramatically over the years. Another is that we didn't rotate crops until last year because we wanted the tomatoes to get the most sun. Our question: After 14 years of use, is our raised bed soil so old it needs to be removed? Behind our backyard are woods where we can place the soil (and our kids can make levees or whatever).
- ----Susan in Vienna, VA
A. It's interesting to see how people can convince themselves that strange issues are the cause of their growing problems, and often ignore the obvious reasons for a garden's progressive decline. I'll cut right to the chase here and say that the culprit is almost certainly the additional shade. Many sun-filled gardens surrounded by young trees often mature to become too shady for anything other than impatiens, begonias and hostas: "The Holy Trinity of Shade Gardeners".
So the possible answers to this problem are a): have some trees felled or thinned; b): join a nearby community garden where your plants can enjoy more sunshine; c): or move the beds to another location on your property where they'll get good sun.
But absolutely keep the soil if you do move the beds. "LeafGro" is the brand name for a high-quality compost the state of Maryland makes from all their fall leaves. Its great stuff; there's even some LeafGro in my raised beds, dragged back from speaking gigs at garden centers in Maryland.
Now, let's discuss some of the gardening tactics they've mentioned. The yearly 'fluffing up' of their compost-filled beds is pretty much a wash, with slightly more negatives than positives. Tilling soil depletes nutrients rather than enhancing them, but it does introduce some air. It can also cause rampant weed growth.
And some 'settling' of the soil in garden beds is always going to occur. That's why I advocate applying a fresh two-inch layer of compost to the surface of your raised beds every season—to both provide that year's organic matter and to keep the soil at the right height. And of course, don't step on the soil in your raised beds; that's why you have beds whose centers you can reach into from either side--so that you don't compact the soil around your plants' roots.
Now—If you don't step on the soil during the growing season and you still feel it's getting a little 'heavy', mix lots of perlite or a soil free mix into your yearly application of fresh compost to keep things light and airy.
And now I'm going to yell about the tarp. Who ever said it was a good idea to throw a tarp over your soil in the Springtime?! Our shady gardeners say they do it to get rid of nematodes and warm the soil, but both rationales are wrong.
First, Northern Virginia does not have problems with pest nematodes. The dreaded "Southern root knot nematode" (which they probably read about in an article that didn't bother to discuss things like range and location) is a pest only in the deep South. And Southerners know when they're afflicted with that bad nematode because the roots of their plants get all gnarly and knotted up—hence the common name of the microscopic pest.
"Nematodes" in general aren't necessarily good or bad. There is a great diversity of species in the wide world of nematodes; some are pests, but others are beneficial nematodes that rid gardens (and especially lawns) of pests like fleas and grubs. And some are just there; neither good nor bad. (Hey—everybody has to be somewhere!)
AND a month of cover isn't long enough to stop bad nematodes or bad anything else—it takes an entire season of baking soil under CLEAR plastic (not a tarp) to solarize pests, weeds and diseases to oblivion. (You'll find all the details on proper soil solarization in previous Question of the Week.)
Now, you can warm up the soil by covering it with clear plastic for a couple of weeks in the Spring, but it's rarely necessary, especially in a normal clime like Northern Virginia. What you mostly want to do to achieve the maximum soil warmth early in the season is to remove anything on top of the soil that's blocking the sun, like a weed preventing mulch…
…Or a tarp!~ "Oops".
So what we have here is not old, tired soil but a garden that's getting progressively less light due to the growth of mature trees. Fruiting plants like tomatoes and pole beans realistically need eight hours of sun a day—ideally beginning with morning sun for the tomatoes. Good soil containing lots of organic matter—which they do have from applying all that tasty compost over the years—will help a little if they're close to that number, like, say six or seven hours of sun a day. But if we're talking four, it's time for some bed moving or arboreal interference removal.
And lose the tarp!