After a mid-April snow storm, things in the prairie may be on hold, but the greenhouse is abuzz with activity.
One of the nice things about growing native plants in the greenhouse is that the temperatures can get below freezing in there, as they are at night even in April this year, but that doesn't bother the native grasses and wildflower plugs. For most vegetables--but not all--below freezing temps means death.
This means that I don't have to turn on the heat and worry about keeping things above freezing, which this winter and spring would be quite expensive. The greenhouse warms up nicely during the day but gets below freezing at night.
One of the challenges of growing plants in a greenhouse in Central Minnesota is how to deal with snow. If this were a hoop house, meaning a greenhouse with just a plastic sheet cover, we would have to be running the heater so that the snow would melt and run off. Without the heat on, the snow might accumulate and collapse the roof of a hoop house.
Because this is a polycarbonate greenhouse, it can withstand a lot of snow before it would collapse, which means we don't have to run the heater. Polycarbonate is a thick plastic material, much like plexiglass, but has channels that run through it like corrugated cardboard. These channels give it strength and add insulation value.
I took this photo on Sunday, April 15, during a break in the 3-day snow storm. The temperature outside was around 32 degrees. But even though there was dense cloud cover, the temperature in the greenhouse was about 50 degrees, even without running a heater. Those temps inside the greenhouse had the effect of melting the snow that accumulated on the roof.
The temperatures may get below freezing in the greenhouse at night, but they stay above freezing in the little galvanized structure you see in front of the greenhouse. That structure is where the water pump is. Pumps must not be allowed to freeze; if they do, the ice in the pump will expand and break the pump (something that's happened to more than a few of my pumps). I have a small heater in that house, which allows me to keep it from freezing, so that I can water the plugs during the day when the temps in the greenhouse can be quite high, but then close it off in the pump house so it won't freeze when the temps drop at night.
Each one of these pots has been filled with a potting mix, and then seeded with wildflower or native grass seeds. Even though they've been watered, some for a month now, only the cool season plants are germinating.
This time of year, with the sun getting higher, the temperature in a greenhouse will get over 100 degrees. So, we must have a thermostatically controlled ventilation system that kicks in to prevent overheating. Oddly, a big challenge to managing a greenhouse is keeping it from overheating.
It is worth noting that many plants (including forage crops and vegetables) fall into two categories: cool season plants or warm season plants. It is something many of us are unaware of, but it is a big deal in the plant world. Think, for example, how green the turf grasses of the average residential lawn are in October, and how beautifully the grass contrasts with the brown corn stalks we see on farm land. This is because Kentucky bluegrass is a cool season plant, and therefore thrives in the cool weather of October, but corn is a warm season plant that thrives in the heat of summer and dies after a frost.
This flat was planted with Lupine seeds in March. Lupine is a cool season plant, which means it geminates and thrives in cooler temperatures.
When it comes to weeds, it is important to know whether they are warm season or cool season plants. For example, Reed Canary grass is a big challenge to prairie restoration work. It's an aggressive non-naive grass that is hard to get rid of. But knowing that it is a cool season plant allows us to, for example, spray it in the fall (even in October) to get a good kill. Herbicides are most effective when plants are active, and are much less effective when plants are inactive. So spraying Reed Canary grass in September and October allows the herbicide to be more effective.