by Sally Colby
Growers who use greenhouses or tunnels strive to keep crops in them for as much of the year as possible. Whether a greenhouse is used for a continuous cropping cycle or to extend the season, sanitation is critical to minimizing disease and ensuring a healthy crop.
Steve Bogash, Penn State University horticulture educator, says managing weeds is an important first step when it comes to sanitizing houses and materials associated with growing between crops.
“Western flower thrips love to live in the weeds outside a greenhouse as well as in the weeds inside the house,” said Bogash. “When you’re out of a cropping cycle, it’s time to manage weeds so you can start with a clean house. Whether you’re laying down geotextile outside and putting stone on top or whatever else you’re doing, you need to create a barrier around the house so that no weeds are growing. That helps with all of the tiny insects that can carry disease.”
Weed management can be challenging during the growing season, but once the season is over, it’s time to get serious about weed control. “There are very few herbicides you can use inside a greenhouse in production,” said Bogash. “There’s no control for perennial weeds, so now is the time to get them under control. Get all of the weed debris out of the greenhouse.”
Weeds are especially notorious for harboring aphids. Bogash recalls one year, after missing just a few weeds that were growing behind a bench, he found himself dealing with an aphid infestation in peppers by mid-April. “You can almost draw a concentric ring where they come from,” he said. “Now is the time to crawl under the benches and find all the weeds.”
Drainage issues inside the greenhouse can create areas where shoreflies and fungus gnats like to hide and proliferate. Look for wet spots on the floor of the greenhouse, and eliminate any algae growing on irrigation lines, benches or other areas.
Wooden benches may seem like a good economical move for a new greenhouse, but Bogash says they are a problem because they never dry out completely and become a place for bacterial diseases to thrive. Growers who start out with wooden benches to cut costs should always be looking forward and planning to replace them with metal or fiberglass.
“A lot of bacteria form biofilm,” said Bogash. “They get into the cracks and pores of wooden benches. Because wooden benches stay wet for so long, we can often trace disease problems back to those benches. It is impossible to thoroughly disinfect and sanitize them.”
Bogash noted that in the past 10 years or so, growers have seen an increase in problems in tomatoes with bacterial spot, bacterial speck and canker. “Having a pristine house is one of the first lines of defense in that battle,” he said.
Wooden stakes can be another source of disease. “The order of events (for tomatoes) is to use new, fresh-cut wooden stakes the first year, then those wooden stakes go to another crop the following year or are burned,” said Bogash. Bogash strongly recommends against reusing wooden stakes for tomatoes because the economics of tomatoes allow growers to discard stakes. “The only good way to sanitize a wooden stake is to bring them up to 130 to 140 degrees at their core in a kiln.”
Growers who use string training systems often reuse string and clips. “Because they’re plastic and not porous, they generally aren’t a problem,” said Bogash. “When I take the strings down, I drop them into a bucket of Green-Shield® and let them soak for about a half hour before I put them back up. I also disinfect vine clips between crops.” Bogash added that bleach and alcohol can also be used to disinfect.
Remember to look for anything in the greenhouse that could be a source of disease. Bogash says something as minor as cardboard storage boxes full of pots shoved under a bench can harbor disease. “Wet cardboard breeds disease,” he said. “It makes sense to sanitize any part of the greenhouse you can. If you can shut the house down at some point and seal it so it can solarize, you can kill a lot. If you can add moisture, the steam will get into pores. A little steam goes a long way.”