We Get Greener Kit Cassingham & her Bigger Half

Greenhouse Lessons

After two years of operating a greenhouse I have learned more about growing organic vegetables than I thought there was to learn! Growing produce indoors is nothing like growing it outdoors. There are pros and cons to each growing location.


With my indoor growing I don't have to worry about weather or wildlife. With my outdoor gardening I don't have to worry about sun or humidity. Ah, life's balancing acts!

The problems I experienced the first winter in my greenhouse were too high a humidity (Sandy, you were right on with your question) and too low a light. The humidity caused powdery mildew to form on the dirt.The peas and beans were the most susceptible to the mildew, though all vegetables suffered a bit. The humidity also ruined the insulation, building up enough the insulation was down right wet. The low-light situation was approached by running full-spectrum fluorescent tubes and CFLs 12 hours/day; an expensive and only moderately successful approach. I feared I'd have too cold a space so put shutters up each night and ran a space heater, making for an expensive hassle.

To try to minimize the humidity I reduced my watering volume. You can't water in a greenhouse the same way you do outdoors in the sun and wind. I also added a fan, which moved the air around to reduce stale areas and move heat and moisture around. And I added a dehumidifier. That seemed to help, but too much damage was done before I implemented those plans so never did have a successful crop of peas or beans. In the summer I keep the windows, door and ceiling vent open a lot, stirring up the air and modifying the humidity.

I knew I was balancing the low light and the insulation, but I didn't realize just how much insulating the shed to make it a year-round greenhouse was going to cut out light. I'd hoped the south wall windows would be enough, with the lights and the foil around the edge of the planting areas, to give me adequate light. Not even! Hence running the lights every day for 12 hours. I added two solar tubes on the north roof, hoping that would give the north wall planters the extra light they needed. It's not quite enough, though. I haven't figured out if I should have had three tubes, or put them on the south roof.

One other light option, which cuts into the insulation effort, is making the south roof glass, using double pane windows. My handyman says the glass won't hold up under a snow load so isn't inclined to go that route. But, given the lack of snow we've had I'm thinking that's a non-issue! This idea is still percolating on the back burner.

Another mistake I made was planting too densely. That works in my outdoor garden but it magnified the humidity problems. Indoor plants need more air circulation so planting sparsely is the way to go with them. And, trimming the tomatoes more would have helped improve their air circulation and increased solar penetration in the greenhouse.

The approach I took for the second year was to not plant at all during the summer so I could strive to manage the powdery mildew. My hope was that with no water in the soil the mildew would die. I actually bought poison for the mildew but couldn't bring myself to use it, so returned it. Research indicated my soil might need pH adjustment; either make it more basic or acidic. Don't you love that kind of research?! So, I covered the soil with baking soda and left it for a month. My hope was that rest would help take care of the humidity and mildew problems.

I also put the shutters away for the second winter, never putting them over the windows, so that any sunlight that shined on the greenhouse was useful. The space heater didn't get used, except for four days when I unnecessarily worried it would get too cold inside. The solar mass I have is adequate for keeping the space warm enough to grow vegetables. They may grow slowly, but they won't freeze.

My utility bill was much lower last winter by not running the heater or many lights. I still ran the fan and dehumidifier, but they didn't seem to cost that much to run. And they helped keep my greenhouse space healthier.

Last fall, the beginning of the second growing season in the greenhouse, I added compost and planted some seeds in one planter. There was minimal problem with mildew, but I could see it threatening. I didn't have a great crop. No lettuce ever grew, and spinach and Swiss chard did sprout with the second planting.

Three tomato plants went in to a different planter last fall and have slowly produced tiny tomatoes for me to enjoy. I didn't keep them trimmed enough, I have concluded, and let them take over. By severely trimming them back I'm hoping the plants' energy goes into growing big tomatoes instead of big plants. It's my plan to keep these tomato plants going as long as they will produce fruit. That's going to be a fun experiment.

This spring I stirred coffee grounds My Bigger Half collected at his office over the winter, in before seeding in a one of the planters I didn't use last year. I sparsely planted lettuce, kale, spinach, and chard but my efforts to water them seem to have brought the mildew back out. Ok, so it's time to cut back on watering, and maybe add some organic soil before re-planting.

My new challenge is bugs. Something cut the leaves off the kale that did sprout so I better track that critter down and invite it to go outside. I work too hard to let a bug eat my seedlings.

So, I'm starting planting season three and look forward to succeeding at growing vegetables year round. Stay tuned for the next lessons learned. Time to go check on the seedlings!

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