The Importance of Greenhouse Sanitation and Sterilizing Soil

French garden
Updated: June 23, 2019

OK, I get it. Greenhouse sanitation isn’t sexy and cleaning pots on the weekend isn’t your idea of a good time. I understand. I’ve been you, I know what you’re thinking: this stuff only matters in commercial greenhouses, diseases don’t happen to hobbyists. I’m sorry to be the one to say it, but if you don’t keep your greenhouse and everything in it clean, disease can and will strike when you least expect it. So what’s a gardener to do in a small operation? Read on and I’ll walk you through it.

Start out sterile

The best and easiest way to be clean is to start out clean. From the day you set up your greenhouse, if you take care to keep pathogens out you’ll likely never have to deal with the problems they can cause. That means sweeping up regularly, keeping benches as dry as possible, curing puddling water problems and using sterile medium and pots. Washing the walls and ceiling of your structure are regular chores that help prevent fungal spores from taking hold.

In older structures, it may pay to completely tear down your benches and other equipment and clean them well with chlorine bleach or a commercial product like Green-Shield or ZeroTol. These kinds of tear-downs are good for that time between seasons when you’d love to be in the greenhouse, but have nothing in particular to grow. Don’t forget to wash any lighting equipment you’ve got while you’re at it.

If you have old wooden benches or use a wood-framed greenhouse, a fresh coat of paint will give you a clean, new surface—wood and fungal spores are like peas and carrots. If you use a sturdy outdoor-rated semi-gloss paint, you’ll be able to more easily wash the wooden parts of your operation. When your budget allows, replace those benches with perforated plastic or one of many non-reactive metal options, the maintenance will be less and you’ll have an easier time keeping them clean.

Pots, medium and sterilizing soil

It’s probably pretty obvious when I say that soil is dirty, but I feel compelled to stress this fact because soil, garden soil especially, can be incredibly filthy. Don’t ever bring it into the greenhouse. Though it’s likely that your garden dirt is full of good bacteria and other organisms, it’s just as likely there are diseases and pests in that soil. In the greenhouse, always start with sterile growing mediums or mixes and keep them tightly sealed in sterilized plastic containers when they’re not in use. A lot of experts will tell you to never reuse medium, but you can if you make sure it goes into a dedicated container for dirty medium and sterilize it again before use.

Sterilizing soil is touchy business, so before you even begin, have a meat thermometer handy and plan to pay close attention to what’s happening. Also, it smells—really bad—so make sure to pick a day you can leave the windows open because you’ll want to regardless of the outside temperature. Start by spreading your used soil out on a cookie sheet or other large, flat, oven-safe surface, making a layer less than four inches deep. Set your oven to its lowest setting and move the rack to the middle, or use two racks if you have a lot of medium to sterilize.

Carefully watch the temperature in the center part of your soil container. Once it reaches 180 degrees Fahrenheit, start a timer for 30 minutes. Remove your soil as soon as it reaches the 30-minute mark—toxic by-products are produced in some mediums if they’re heated above 200 degrees. Let your soil cool before moving it to a plastic container that has been freshly sterilized with bleach or an agricultural cleaner and allowed to dry fully.

Pots are another sticking point. A lot of gardeners reuse them, and there’s nothing wrong with that unless you fail to clean them. I’m not talking about knocking the dirt out or giving them a good rinse, I mean really clean those bad boys. My dishwasher has a sanitation cycle, so I cheat and run mine through that, much to the chagrin of my husband, who is thoroughly convinced I can’t tell the difference between the kitchen and the garden. If you don’t have a sanitary cycle or you’ve got a pot you’d rather not risk in the dishwasher, soapy bleach water will do a good job if you allow your pots to soak for a half hour. Rinse them good before dropping them into the pool. Dirt makes it harder for cleansers to get into those tiny scratches and pores of many pot materials.

Label, clean, repeat

I know everybody says not to reuse gardening stuff. I know they say it and I know we do it. So, if you’re going to do it anyways, do it safely, OK? Keep containers around for used medium and used pots, seal them and keep them clearly labeled if you can’t immediately sterilize the contents of your various containers. Move freshly sterilized medium and pots immediately to sealed containers to help prevent spores and bacteria from colonizing those teeny scratches and cracks that are so hard to keep clean.

Why? What’s the big deal? I can hear you screaming at me through the Internet. Maybe you’re even thinking I’m just a neat freak with a greenhouse. Common problems in greenhouses large and small include impossible to treat fungi like Phytophthora, Fusarium and Verticillium, stubborn bacteria like Erwina and about a zillion different kinds of plant-borne viruses. Even if you’re careful and always quarantine new plants before exposing them to the general population, the reproductive structures of these pathogens can still blow in from outside.

Once inside, once germinated, disease burns through a greenhouse much more quickly than in an outdoor garden. Your best bet at the point when symptoms become obvious is to destroy everything that even looks remotely sick. You really don’t want to do that, so keep your greenhouse clean, OK? Sanitation isn’t sexy, but aggressive plant diseases are even worse. Even those that are treatable like powdery mildew and rust can drive you mad trying to eradicate them from every plant on your bench.

Kristi Waterworth
image: nosha (Creative Commons BY-SA)
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