On April’s Fool’s Day, it was a balmy 65 in Chicago—with a forecast for 27 two nights later.That kind of grim joke is why it’s handy to have a greenhouse or a cold frame to even out the crazy temperature swings of springtime.
One task that makes a greenhouse useful is starting bare-root plants. Perennials, roses and even some shrubs can be a bargain to buy bare-root and dormant in early spring. They’re cheaper than potted plants by mail order because there’s no heavy soil to ship.
But you need to be able to get them into soil right away before their roots dry out. And in a Chicago spring, planting in the ground in early spring, when a frost may be right around the corner, is a risky business.
If I only need to keep bare-root plants for a day or two, I will keep their roots wrapped in moist newspaper. But if it’s going to be longer than that, I pot them up in potting mix. Once they’re watered, they will come out of dormancy and start growing roots, and they will need sun. A greenhouse can give them the benefit of ample sunlight but keep them out of the weather.
The tricky thing is keeping them sheltered but not overheated. If a conservatory is heated for orchids or other tropical plants, it will be too hot for hardy perennials.
That’s where a cold frame is handy. You can keep the lid open to the air most of the time, breaking the wind but keeping the plants not much warmer than the outside temperature. If a frosty night is predicted, you can close the lid to hold in the day’s warmth. It’s important to open it first thing in the morning before it starts to trap the day’s heat.
The timing for planting these new plants in the garden depends on the species. Right now, I have some hardy geraniums, sedum, and bleeding heart started in pots. They’re all fairly hardy plants, and I’ll probably set them out by the third week in April.
If they were tender hybrid tea roses, I would keep them sheltered longer. I’d wait to plant until after our average last frost, which is about May 15, so they would spend a month or six weeks in a pot. And a greenhouse or a cold frame would be a handy spot.