It's hard to believe it's already June. The plants are coming alive with flowers and green leaves.
That is, most plants. The trouble with January's short cold snap was that there was no snow and several nights of below-freezing temperatures killed or stunted many plants.
So far I've heard of damage to lavender, butterfly bush, roses, laurels and tender bulbs. There may be others affected so please let me know if there are any plants that don't seem to be recovering. If you're trying to determine if they've died completely, please be patient. Plants that are recovering from the roots may be slow to sprout.
According to the USDA hardiness map, this region is in Zone 6, with minimum temperatures between -10 and 0 degrees Celsius. In reality, most experienced gardeners here don't attach much importance to this designation. The USDA system doesn't take into account the topography of the region and how it affects cold air flow. Cold air is denser and heavier than warm air, so it sinks from higher elevations to lower elevations in the terrain, creating pockets of very cold air.
A difference of 5 to 10 degrees is common and enough to damage plants you thought were hardy. For example, my garden in the Painted Hills area is exposed to cold air that drifts in from the Palouse foothills. As a result, my garden gets much colder than gardens further down in the valley. During one cold snap in January, temperatures dropped to minus 8 degrees over three nights.
Another crucial factor in the cold snap was the absence of snow cover to provide an insulating blanket for plants: Air pockets in the snow layer trap warm air and block the wind, protecting plants and their roots like a warm down jacket.
What about plants that may have been damaged by the cold? Start with roses. Cut the stems back to the green part where the leaves are emerging. For grafted roses, remove any stems extending beyond the graft. For own-rooted roses, leave any stems extending. This cold snap has confirmed the advice to mulch roses in the fall.
For butterfly bushes, cut dead stems down to the ground and wait for new stems to emerge, which can take several years to fully recover.
Most of the lavender I've seen looks dead. Unfortunately, many of the lavender varieties sold here are labeled Zone 6 but fall at the upper end of that designation and appear to be cold sensitive. Lavender cannot be cut back to regrowth, so repotting may be best. The evergreen laurel leaves were badly burned and the shrubs will need to be cut back heavily to regrow. Delicate bulbs like crocosmia may be damaged and will need to be repotted.
