Gardening: January cold snap damaged or killed many plants

It’s hard to believe we are already into June. Our plants have come alive with flowers and green leaves.

That is, most of them are. The ugly side of our brief cold snap in January is that a lot of plants were killed or stunted in those few days of subzero nights with no snow cover.

So far, I’m hearing that there was damage to lavender, butterfly bush, roses, laurels and tender bulbs. There are probably others, so let me know if you have plants that don’t seem to be coming back. If you are trying to decide if something has completely died, be patient. Some plants will be slow to emerge, especially if they are coming back from their roots.

The USDA hardiness map says we are in Zone 6, which translates into minimum temperatures between minus 10 to 0 degrees. In reality, most experienced gardeners here take that designation with a big grain of salt. The USDA system doesn’t consider the topography of our region and how that affects the flow of cold air. Cold air is denser and heavier than warm air, and as a result sinks from higher elevations to low points in our topography, creating pockets of much colder air.

Differences of 5 to 10 degrees are common and that is enough to damage plants we thought were hardy. As an example, my garden in the Painted Hills area catches cold air that flows off the higher Palouse hills. As a result, my garden can be much colder than those farther into the valley floor. It was minus 8 for three nights during our January freeze.

The other crucial factor in our cold snap was that there was no snow cover to provide an insulating blanket for our plants. Air pockets in the snow layer trap warmer air and block the wind, protecting the plants and their roots like a warm down jacket.

What do you do with plants that might have been damaged by the cold? Let’s start with roses. Cut stems back to green wood where leaves are emerging. If you have grafted roses, remove any stems that are emerging from below the graft. If they are own-root roses, leave all stems that emerge. This cold snap reinforced the advice to mulch roses in the fall.

For butterfly bush, cut the dead stems to the ground and wait for new ones to emerge. It will take a couple of years to fully recover.

Most of the lavenders I’ve seen look dead. Unfortunately, many of the varieties of lavender sold here are labeled Zone 6, but I think they fall in the upper range of that designation, so they are sensitive to the cold. Lavender can’t be cut back to regenerative wood, so it may be best to replace them. Evergreen laurel leaves were heavily burned, and the shrubs will need to be cut back heavily and allowed to regrow. Tender bulbs like crocosmia may have taken a hit and will need to be replanted.