It was one year ago this week that we were experiencing a bout of winter, Canadian style. You remember it don’t you? Several inches of snow and bitter cold that made the term “Napa Valley north” seem a little bit ludricous.
The fallout from that sudden and deep drop in the temperatures came several months later when plants in the garden began to show the effects. Broadleaf evergreens fared the worst, especially rhododendrons. Several customers told me that their plants went into rapid decline as the spring weather warmed and by the end of April branches or entire plants were dead.
The problem is that rhododendrons and other broadleaf evergreens continue to transpire and lose moisture through their foliage during the winter. This water loss is especially prevalent during warmer windy days, and we get a few of those in the winter don’t we?
Transpiration is not a problem if the ground is full of moisture and not frozen. In the Okanagan Valley this rarely occurs during the winter. Frozen ground means that roots are unable to take up moisture, and a lack of snow cover down in the valley results in dry soils.
The resulting injury to plant tissue doesn’t appear immediately, but ultimately the damage does become apparent. Rhododendrons are valuable flowering shrubs for shady areas in the garden, providing spring colour and winter foliage, and they will grow quite successfully here if they’re looked after. How can we keep them healthy through the winter so we don’t have any surprises come next spring?
It’s too late now, but watering the soil through the fall is very important. Keep the water coming right up until the soil freezes so that the ground is soaked to a depth of a couple of feet around the root zone of not only rhododendrons, but other broadleaf evergreens like pieris, pyracantha and boxwood. If we should be blessed with a warm spell in the next few months take a bucket or two of water out to your plants and give them a good drink.
Rhododendrons like mulch for a host of reasons, but renewing the mulch layer in the fall will help limit the penetration of frost into the ground and allow the deeper roots to continue to take up moisture. It will also protect the surface roots from alternate freezing and thawing through the winter. Use shredded leaves, pine needles, bark or straw.
Products known as antidesiccants will help to lessen the amount of moisture lost through transpiration. Ideally they’re applied before the first frost, but when the weather warms above zero you can spray your plants with Wiltpruf or Cloud Cover. Either will form a thin film on the foliage that seals in moisture. These products can also be used right now to keep evergreens used for Christmas decorating from drying out.
Finally, for plants in exposed locations consider building a windbreak. Drive three or four stakes into the ground and fasten burlap or shadecloth to the stakes. The cloth will offer some much-needed protection from drying winds.
These simple steps apply to the other plants I’ve mentioned, although rhododendrons are more at risk simply because of their larger leaves which result in more surface area for moisture to escape.