A hard freeze in November poses danger for the garden. But a mantle of soft snow actually protects plants. (Getty Images)

A hard freeze in November poses danger for the garden. But a mantle of soft snow actually protects plants. (Getty Images)

How cold is too cold for the Northwest garden? It depends

Overnight temperatures in the teens, and daytime highs that don’t exceed the mid-20s, can spell doom if gardeners don’t take action.

As we move into winter, I am often asked when we should worry and provide protection for our landscapes.

There are, of course, many variables that can affect winter hardiness in a plant, but in general, my rule of thumb is as long as the mercury stays in the 20s at night and rises above freezing during the day, our plants should be just fine. Once we drop into the teens or lower and there is no thawing during the day, we can experience problems.

That’s exactly what happened to us in January of this year. In my yard it got down to 11 degrees and I had several friends that recorded single-digit temperatures. When it gets that cold, gardens often stay frozen during the day (especially in shaded areas), and that is when some plants, broadleaf evergreens in particular, can exhibit extreme freeze damage or even death.

It is hard to predict just how much cold a plant will tolerate in a garden setting. One or two nights in the teens is one thing, a whole week is quite a different case. Even if the tag that came with the plant indicated that it would tolerate a certain minimum temperature, other factors will often play into account. Plants in containers are less hardy because the whole soil mass is exposed and can freeze, potentially damaging the root system — a problem not typically experienced by plants in the landscape.

Another factor is the timing of a hard freeze. Freezes in early November can be quite damaging, whereas a freeze later in the month or into December can be completely harmless. This is simply due to the fact that the later we get into the fall and winter, the more our plants have had time to acclimate and “toughen” themselves up. Along this same line, too much fertilizer during the growing season can leave a plant in a “too soft” state. Similarly, late summer pruning that stimulates new growth runs the risk of winter damage. While I would not recommend stressing out our landscape plants, it is good to let them slow down their growth and “harden off” going into the fall/winter season.

What about snow? In most cases, snow is a gardener’s friend. Except for the damage it can cause on limbs due to the weight, snow on the ground will help insulate the soil and actually protect our plants. Plus, whenever it is snowing it is almost never as cold as when it is clear and the stars are out. Unless you are worried about breakage, leave snow wherever it falls and let it melt naturally.

As for protecting our plants when it does drop into the teens, we have several options. Leaf litter makes for a good mulch and can be banked up around shrubs and tender perennials to provide insulation. I just did this with my newly planted banana grove, along with wrapping the trunks with foil covered bubble wrap. If you have already raked up all of your leaves then purchase some mulch — you can spread it around the beds come springtime, too. Covering broadleaf shrubs, like Escallonia and ‘Spring Bouquet’ Viburnum, with sheets, blankets, burlap sacks or floating row covers will help immensely — just be sure to take it all off when it warms up. Avoid plastic, as it it can cause more problems than it solves. As for containers, bring them close to the house or into the garage, but again, be sure to take them back outside as soon as the temps moderate. Follow this advice and you should have minimum loss over the winter.

Stay warm and keep on gardening!

Steve Smith represents Sunnyside Nursery in Marysville, and can be reached at sunnysidenursery@msn.com.

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