We're fixing up a house we bought last year. We just this week planted a couple hundred dollars worth of perennials and shrubs to replace the overgrown foundation plants we pulled out last fall. All of the new plants are small but dear to us since we're on a tight budget. The news says it's going to be cold tonight, and colder the next night, with a chance of frost. These are all hardy perennials and shrubs and they were outdoors at the place we bought them, but will they be okay now? Should we cover them? What should we use to cover them? Do we leave the cover on during the day or just at night? - C.F. -
We're glad you sent pictures.
Above, left: Some of your new plants, including delphinium. Yours came from an outdoor growing area at the garden center so its cells are hardened to the outdoor air.
Above, right: An individual of the same species that's been in a nearby garden for years -- it's established. Your new delphiniums are this same height, just not as full. That's an indication that your "new guys" are in synch with outdoor conditions.
Most of the new plants in your photos do not seem to be advanced in growth, in comparison to plants already established in local gardens. (Sometimes, greenhouse grown plants are weeks ahead in development and height.) Since your perennials are tight buns with all the stems huddled together close to the warm ground, and the shrubs have not fully expanded their leaves, they have as much chance of surviving frost as local garden plants that are already well established.
Above: Some spring frosts don't even reach the ground. When the soil is warm and there is no wind you may see frost on roofs or even lower on the tops of cars yet garden plants are untouched. That's because the warmth that came from the earth remained puddled around the plants in the still air.
However, a hardy plant's survival doesn't always mean coming through spring totally unscathed. Even established perennials may lose some foliage in a late frost and have to replace it. Your smaller plants don't have as much reserve starch to draw on so losing leaves is a bigger set-back.
Above: Leaves blackened by frost. You can cut them off. Don't cut the wood, if it's a shrub or tree. Just cut off the leaves.
So it's a good idea to protect new plants, even if they're hardy species in step with established field-grown plants.
Cover each plant or bed by the end of the day with something that will trap ground warmth. Weight or pin the cover so it won't blow away. Keep plants covered until all frost is dissipated the next day.
Paper, lightweight cloth and cardboard are good insulators. Do NOT use plastic sheets. Such plastic has no insulative value so the plant will frost as if in the open.
You can make each plant a "hat" of newspaper and push a wire pin through the paper into the ground to hold each one in place. You can make a pin from a section of wire coat hanger bent in a "U."
Above: If you were a Curious George book fan as a kid you probably learned how to make newspaper hats...
You can cover each plant with a cardboard box or bushel basket. Put a brick or hefty stone on each to hold it in place. An upended garbage can or tote box can work, too.
Old bed sheets or very lightweight blankets or canvas tarps can be thrown over shrubs with branches thick enough to bear that weight.
You can buy row cover cloth from a garden center. It's a very light fabric that traps heat without crushing plants, and which can be left in place even during the day because air and water can penetrate it. The row cover edges must be pinned or weighted.
Floating row cover is sold in sheets big enough to cover a row or even a whole bed. It's reusable and lasts for many years. So if you're in a frost prone area or like to push the season, it's a good investment. Call your local garden center and ask for row cover, floating row cover or frost blanket. (Over half our own local garden centers carry it -- the half that also sell "hard goods", not just plants. So far this year we've confirmed that English Gardens and Ray Wiegand's Nursery has it in stock; we'll update as we get the chance to call around, if you call other places and find it available, please note it in the row cover topic on our Forum!)
If a new plant is too large or there is some reason you can't cover it against frost, you can always dig it out and move it into a garage or shed where the roof is its frost cover.
Check plants early in the morning. If a cover came off and frost has formed, you may yet save those leaves by shading them or keep them misted with a sprinkler until all the frost is gone from your yard.