Step into your greenhouse. Do you feel like sneezing? Coughing? Do you have an itchy nose? These symptoms could be signs of an allergy to mold, which is a form of fungus. The moist, enclosed environment of a wintertime greenhouse that is filled with plants can be a veritable welcome mat to mold.
Three main types of mold infect greenhouse plants. The most common one is grey mold or Botrytis cinerea. You’ll see this ash grey fungus growing on leaves and small fruits, extracting nutrients from the plant tissue it kills with its toxic secretions. You’ll also see Botrytis cinerea on plant stems just before the plant topples over and dies. Never underestimate the ability of grey mold to spread. If you find any plant with grey mold on it, remove it from the greenhouse immediately before it infects other plants. But Botrytis cinerea is not the only type of mold to guard against. Sooty mold (which looks like black soot on plants) and powdery mildew (which resembles a white or greyish powder) can also be very troublesome in a greenhouse.
Knowing the conditions conducive to molds can help prevent them from taking hold in your winter greenhouse. Molds thrive in greenhouses that have high humidity, relatively low temperatures, plants that are overwatered and overcrowded, and generally poor air circulation. Having a thermometer and a hygrometer in your greenhouse will allow you to keep track of two of these factors: air temperature and humidity. When the humidity rises to over 80% in a winter greenhouse, mold is likely to develop. The ideal humidity for a wintertime greenhouse is between 30% and 45%, especially if you have lots of plants. Keeping the temperature on the warm side – around 60˚F – will also help to keep mold at bay in winter. But most important of all to reduce the chance of mold growing on your plants is to keep the air inside the greenhouse moving. This can be done simply with a fan at each end. If these fans have heaters, they’ll also be able to keep your winter greenhouse warm.
How you water your plants in winter can also affect the likelihood of them developing mold. To keep mold away in my wintertime greenhouse, I never water on cold, wet days when humidity is high. I only water on sunny days when the warm air can dry the plants quickly. I also try as much as possible not to wet the plant leaves when I’m adding water to the soil. That’s hard to do with more than 200 plants in my greenhouse, but I try! At the same time, good plant hygiene also helps to discourage mold from forming. This means removing all dead leaves, rotting matter, and dying plants as soon as you see them.
Of course, once you find mold in your greenhouse you must move from prevention to removal. Empty the greenhouse as much as you can and wipe all surfaces with a solution of water with bleach. Wearing rubber gloves for this job is strongly recommended. You can also use vinegar and hydrogen peroxide as a solution to wipe away molds. If you have mold on plants, you can use a vinegar spray (two or three tablespoons of cider vinegar to a gallon of water), or you can buy a commercial mold remover. Test the product on one or two leaves before spraying the entire plant to make sure that the plant isn’t overly sensitive to it. Although mold in a wintertime greenhouse can be a challenge to eliminate completely, these steps will help you to get these fungal invaders under control.