A friend of Frank’s used to say, “This would be a great business if it weren’t for the customers.” We thought about him the other day while running the farm stand. “Do y’all have any vegetables?” a woman asked us. She was standing right in front of a wonderful spring display of spinach, onions, herbs, snow peas, swiss chard, lettuce and radishes. We weren’t sure what she meant, exactly. “I mean tomatoes and peppers,” she said, a bit exasperated, “ things like that.” It was early April – even here in Texas, tomatoes were still a long way off. She left without buying a thing. Not even a radish.

She didn’t know just what it took to get those radishes in! Radishes, of all things. The day before they just about pushed Frank over the edge! “Here I am trying to run 15 acres of cut flowers and I’m picking radishes! Radishes! It’s late Friday afternoon, everyone else is kicking back with a cold one and I’m picking radishes! Radishes…!” – he went on and on. That lady was skating on thin ice with Frank in earshot! It was times like this that Frank’s friend would have had enough of the horticulture business. “I’m going to put a tortilla on my antenna,” he’d say, “and drive north till someone asks me what that is”.
Actually, our market has been very successful and is a lot of fun, but it does take a lot of time away from the cut flower business. Before our crew arrived we were both running ragged trying to run a cut flower farm and a vegetable operation. Now that they are here, we can relax a bit (just a bit) and get caught up on some projects that have gotten away from us.

One of the projects we need to do this summer is to clean up the soil in our winter cold frames. We have been growing crops in the ground beds almost continually for more than four years, five years in some of them. Over time we have built up a population of weed seeds and diseases that we need to eliminate. This past winter we started to have a real problem with a nasty little fungus called sclerotium rolfsii – known by the all-encompassing name of Southern Blight.

According to the literature we’ve turned up, this pernicious fungus attacks over 500 species in 100 plant families throughout the warmer latitudes of the world. (Up north you get a different, but similar species-sclerotium delphinii.) It attacks the plant at ground level, causing the plant to suddenly wilt and die. The crown of the plant will be totally rotted and you will see a cottony white mass of mycelium at the base. Within this mass the fungus forms small seed-like bodies called “sclerotia”. These sclerotia are about the size, shape and color of a morning glory seed. The sclerotia are not exactly spores, but more like compressed pads of fungal mycelium. They can survive in the soil for years to reinfect a crop when conditions are favorable.

Conditions in our greenhouses were certainly favorable for it last winter. After a five-year drought we had rain every week all winter long. Last March was the darkest (least amount of sun) and coolest on record. The next thing you know, the anemones were melting down. We always have had a small amount of this disease present, but this year it gave us a scare. It took out about half the anemones, and started to get into the ranunculus before the weather brightened and we were able to dry things out. But now it has a good foothold in our beds. Add that to the henbit and amaranth seed that has built up, and we see that it is time to clean up these beds.

In years past, greenhouse growers would simply divert steam from their heating system into beds covered with tarps. The steam would sterilize the soil. Many growers still use steam, and steam units for bed preparation can be bought for greenhouse use, but that’s out of our league. Another method used by greenhouse growers to sterilize the soil is chemical fumigation with something like methyl bromide or Vapam. No way, Jose! Not on this farm. Luckily, here in Texas we have a pretty obvious answer: soil polarization.

We found out about solarization the hard way when, a few years ago, a greenhouse full of celosia was forgotten and left closed in the July sun. Not only did we cook a spectacular crop of celosia, but we also blew out the thermometers and destroyed the circuitry in the thermostat! It even melted the PVC pipe so that the pipe drooped like spagetti! It was HOT! At least 140 degrees.

Most readers will probably be familiar with the concept of soil solarization in field situations. In general, a plot of ground is watered well, and then covered tightly with a sealed sheet of clear plastic. The plastic traps the sun’s heat and raises the soil temperature up to a point that suppresses or kills pathogens and weed seeds. It can be a very effective way of sterilizing soil. A study in Florida showed that soil solarization, while slightly less effective than methyl bromide, is still an effective method of soil sterilization and that disease suppression and yield increases were comparable to methyl bromide.
There are several inherent limitations with solarization. First of all, not everyone is lucky enough to live in a place like Florida (or Texas, where we don’t even have to use plastic to solarize anything!) Obviously it is not going to be as effective in a place like Maine. Secondly, it takes the growing area out of production for a minimum of six to eight weeks, right in the middle of the growing season. And then there is the problem of scale. If we solarized even 20 percent of our outdoor growing space each season, that would still be 3 acres of plastic.

But it is a different story inside the greenhouse. We already know just how hot we can get it in the greenhouse. Solarizing would be done in mid-summer, when the greenhouses are empty. And it is not an overwhelming amount of space to cover with plastic. In fact, you can save your old plastic when you change the covers out and you would have more than enough without any additional investment.

We went surfing on the internet to find out if there had been any work done on greenhouse soil solarization and found out that there was quite a bit of information, most of it from overseas. One study from Brazil looked at enhancing the solarization of field beds in the cooler regions of southern Brazil by building low tunnels over the solarizing soil. In effect, you have a double solarizing effect. The soil is covered with a flat piece of plastic, and then the surrounding air is heated up by the greenhouse.They called it “Intensified Soil Solarization”. It doesn’t seem very cost effective to build greenhouses over field beds and then remove them, but the study does point out that this double covering is an effective solarization system for cooler areas. They reported “a significant increase in carrot yields resulted by solarizing, even in the absence of known pathogens.” If we were going though all that trouble though, we’d grow something other than carrots!

Another study, this one in Crete, looked at verticillium control in permanent tomato houses. The houses were described as “unheated” – a lot like our winter cold frames. They gave more details than the Brazilians. They covered the beds for a 10-week period from June through August. Irrigation pipes remained in the beds, and the beds were watered every 20 days under the plastic. The greenhouses remained tightly closed during the period.

They found that the average high temperature of the soil was 58 degrees C at 10 cm deep and 44 degrees C at 40 cm deep. It was apparently hot enough to do the job. They reported that “no pathogens (We assume they mean the verticillium they were looking at) could be isolated from treated soil to a depth of 40 cm”. They also reported a yield increase of 112.4% over the control plots!

Another interesting thing we turned up was the use of organic amendments to enhance solarization. As one study in the journal Phytoparasitica put it: “Pathogen sensitivity to biotoxic organic compounds is enhanced by increased soil temperatures”. Several studies mentioned an increase in effectiveness when composted chicken manure was added to the soil prior to covering. But the most interesting thing to us was the use of cruciferous type (Cabbage family) plants to provide these biotoxic compounds. Many studies mentioned using broccoli residue.

According to the article in Phytoparasitica (now there’s some light reading for y’all), “Toxic volatile compounds including alcohols, aldehydes, sulfides, isothiocyanates, and others were detected in soil amended with cruciferous residues during heating”. Another website (a gardening site called www.markw.com) gave a summary of using brassica residues. He mentioned that the isothiocyanates(mustard oil compounds) and the sulfides were the effective agents, and that “ brassicas with soil solarization killed 99.4 % of the targeted pathogens, spurred the development of microflora and microorganisms, and prevented weed development for several months.” No source was given for this info.

The concept of using brassicas to clean up soil is amazing and seems to hold a lot of potential as a future replacement for soil fumigants, but there are some caveats that need to be pointed out. First of all, these results are all still very experimental. There are no standardized procedures or recommended rates available yet. The compounds mentioned are indeed “toxic organic compounds” and an overdose may do more damage than good. A study from UC Santa Cruz said that broccoli residue by itself was not very effective, but under plastic it was. They believed that the volatile compounds were trapped under the plastic tarp. This would lead one to believe that the toxic compounds would dissipate after the plastic removal, but how much time is enough is not known. Mark Whitelaw at markw.com says that tomato plants planted just after treatment “withered and died within a day:” The fact that he said earlier that brassicas and solarization prevented weed development for several months indicated that some toxicity could carry over. So be careful.

Another issue is a legal one. Plowing in a crop of green broccoli plants is considered a green manure or cover crop. But some studies used dried brassica residue that was removed from the field, dried, and then applied to the soil surface and worked in. This technically makes it a pesticide, and to use it legally one would have to have an EPA registration number. In theory dried broccoli, for instance, would need to be tested and evaluated like any other pesticide before being given EPA approval. LD 50’s would have to be established, as well as re-entry intervals.

So where is all this leading? Well, we are going to try solarization in our greenhouses this year. If there is one thing we have an abundance of, it’s sun! This is the first year that we have enough outdoor growing space, and we don’t have to cram our greenhouse full of celosia all summer. So we are going to cook those babies! Even without ground plastic, one paper from North Carolina pointed out that simply closing up your greenhouse for a short time can kill pathogens on wood benches and structural materials. But we are going to go the whole nine yards. We intend to put out a cover crop of inexpensive, open pollinated broccoli seed and plow down the plants when they are big enough. We are not sure when that will be, but markw.com mentioned a dried rate of 1/8 inch of dry brassica on the soil surface. (This guy recommends that you make the dried brassica in a microwave or dehydrator, but we can’t imagine making enough for 10,000 sq. ft!) We could cut a patch and dehydrate a small amount to see how thick it would cover. In reality though. we’ll probably never get that scientific, and we’ll probably just plow it in when we have time!

One more interesting thing that we turned up, this one from the University of Hawaii, was that by using Trichoderma harzianum and Gliocladium virens beneficial fungi the effectiveness of soil solarization on sclerotium was increased. These fungi are available commercially as Plantshield and SoilGard, respectively. Both fungi actively attack the sclerotium fungus. We assume that the Hawaiians applied the treatment after the solarization, although they didn’t say.

If you decide to solarize your greenhouses, remember to remove or protect anything that would be sensitive to the high temperatures. All of our controls and heating equipment are near the front of the houses, by the door, so we plan to drop a plastic curtain in the greenhouse just in front of the heater to separate that area from the heated beds.

For more information on the actual mechanics of solarization, such as how to lay the plastic and so forth, check out the web. We like the search engine google.com. It came up with pages and pages of references, ranging from some extremely technical articles (Like the 1997 conference on soil solarization in Jordan) to articles for backyard gardeners. And while you are surfing check out our new website. Yes that’s right–Frank and Pamela are finally on the web (or at least we will be shortly after you read this). As Jim Hightower says, “a Luddite with a website”! We’ve got our friend Joe Caputi (check out his ad in the classifieds) putting our site together–www.texascolor.com. Look out amazon.com, the Arnoskys are in cyberspace.