Solarization is a method of killing pests, diseases and weed seeds near the surface of the soil by covering the soil with clear plastic for six weeks or more in hot weather. We use this method to help control nematodes in our hoophouse. Nematodes are only active in warm weather, and we have not had problems with them outdoors, but of course, it’s warmer in the hoophouse!
I’ve written before about solarization to fight nematodes in our hoophouse.
In my Book Review: The Organic No-Till Farming Revolution: High Production Methods for Small-Scale Farmers, Andrew Mefferd, I wrote a little about solarizing:
“Solarization uses clear plastic (old hoophouse plastic is ideal). In a summer hoophouse, solarization can be as quick as 24 hours, Andrew says. When we’ve done this, one of our goals was to kill nematodes and fungal diseases, not just weeds, so we waited a few weeks. Outdoors it takes several weeks. You can see when the weeds are dead. Bryan O’Hara poked a thermometer probe through solarization plastic and found a 50F degree (28C) difference between the outside air and the soil immediately under the plastic; a 10F (6C) difference at 1″ (2.5 cm) deep and little temperature gain lower than that. Solarization does not kill all the soil life!”
Extension offers Solarization and Tarping for Weed Management on Organic Vegetable Farms in the Northeast USA which can, of course, be modified for those of us in other regions.
I’ve written here before about our struggles with root knot nematodes in our hoophouse, and you can read everything I know about nematodes in the Year-Round Hoophouse.
My article on nematodes in Growing for Market in November 2014 describes our discovery of the beasties and our first attempts to deal with them.
My most thorough blogpost about nematodes was for Mother Earth News Managing Nematodes in the Hoophouse.
My post Good news – great hoeing weather! Bad news – more nematodes in the hoophouse August 2014 includes a photo of our first attempt at solarizing – a bit of a How Not To!
There is info on dealing with nematodes from Garry Ross in Hawaii, where nematodes are a fact of daily life, in my post Cold weather, snow, thinking about nematodes from February 2015.
Cover Crop Choices
In June this year I wrote about using marigolds, sesame, Iron and Clay cowpeas as nematode resistant cover crops. We’ve also used winter wheat, and white lupins. See Our Organic Integrated Pest Management . Other cover crops that suppress nematodes include some other OP French marigold varieties (but avoid Tangerine Gem or hybrid marigolds); chrysanthemum; black-eyed Susan; gaillardia (blanket flower, Indian blanket); oats; sesame/millet mix. We decided against sorghum-sudangrass (too big), winter rye (harder than wheat to incorporate by hand), bahiagrass, Bermuda grass (both invasive), castor bean and Crotolaria (sunnhemp) (both poisonous, although newer varieties of Crotolaria have lower toxin levels, and I’ve been rethinking my opposition to using that), partridge pea, California poppy (both require at least one full year of growth) and some obscure vetches that weren’t available locally. We might have included Pacific Gold mustard (B. juncea), if we’d found it in time. Don’t confuse this with Ida Gold Mustard, which kills weeds, and is susceptible to nematodes.
Food Crop Choices
This list starts with the crops most resistant to Root Know Nematodes and ends with the most susceptible. I’ve included some “bookmarks” between categories, but it can also be read as a continuous list:
Onion (? not certain)
West Indian Gherkins
Some Sweet Potato varieties
Radishes (? not certain)
Garlic, Leeks, Chives
Brassica juncea mustards
Brassica rapa var. japonica greens (? Uncertain)
Broccoli, Kale, Collards, Brussels Sprouts
New Zealand Spinach
Cucumbers, Muskmelons, Watermelons, Squash, Pumpkins
Beans, Fava Beans, Soybeans
Tomatoes, Eggplant, Peppers, Potatoes, Peanuts
Nematode-resistant winter greens
We came up with a collection of nematode-resistant winter greens, including radishes, Russian kales, Brassica juncea mustards (mostly salad greens like Ruby Streaks, Golden Frills, Scarlet Frills), and Brassica rapa var. japonica greens, mizuna and Yukina Savoy. We have since learned that Yukina Savoy is a Brassica rapa, not B. juncea as we thought, and that mizuna is Brassica rapa var. japonica with a less certain resistance, or perhaps Brassica rapa var. niposinica, or perhaps B.juncea after all (integrifolia type). We also grow scallions in the nematode-infested areas. Now I am looking for more nematode-resistant cold-weather greens.
After the winter greens this spring, we transplanted two beds of tomatoes, one each of peppers, squash and cucumbers, and put two beds into Iron and Clay cowpeas. The eastern ends where we had found evidence of nematodes, we transplanted French marigolds and sesame as stronger fighting forces.
When we pulled up the squash and cucumbers we found no sign of nematodes on the roots. One of the tomato beds produced no sign either, but the other one did. Our first response was to sow Iron and Clay cowpeas instead of the planned soybeans, but before the plants were even 2” (5 cm) high, we decided to solarize that whole bed. We now have small patches of nematode infestation in almost every bed, calling for a more nimble approach to crop planning.
Brassica juncea mustards to try
According to Wikipedia, Brassica juncea cultivars can be divided into four major subgroups: integrifolia, juncea, napiformis, and tsatsai. I did some searching for more B. juncea, especially large leafed ones. Some promising looking crops include these:
“Green-in-Snow” mustard, Serifon gai choi type Chinese Mustard, Suehlihung.
“Red-in-Snow” mustard (sorry, no details)
Giant Red, Osaka Purple, Southern Giant Curled Mustards, all quite pungent
Yanagawa Takana broad leaved mustard