How should I prune winter-damaged Phormium, Hebe, and Spanish lavender? The Phormium leaves look wilted. They are folded over and discolored but not blackened. The Hebe ‘Autumn Glory’ and ‘Great Orme’ are blackened, while ‘Shamrock Purple’ is mostly brown. The Spanish lavender has some blackened foliage, but mainly just got weighed down with snow.
My advice for right now would be only to prune any branches which were broken under the weight of the snow. We may yet have more cold weather, so you don’t want to make your plants any more vulnerable.
You may want to wait until early spring or at least all danger of further frost or snow and ice to prune your Spanish lavender (Lavandula stoechas). Under normal circumstances, Spanish lavender can be pruned back by a third to one half in early to mid-autumn, according to Lavender: The Grower’s Guide by Virginia McNaughton (Timber Press, 2000). I grow Spanish lavender, and I usually tidy the plants a bit when most of the flowering is done. However, because of the extreme cold to which your plants were exposed, you may want to assess them in spring, and prune out any dead areas, or replace any mostly-dead plants.
I recommend giving the Hebe plants a chance to recover. I have heard of very sad looking Hebes coming back, perhaps not the first summer, but by the following year. By mid- to late-spring, you should be able to tell what is truly dead and prune it, though Hebes sometimes dislike hard pruning. The Hebe Society of New Zealand suggests pruning frost-damaged shoots in spring.
An article in the Kitsap Sun from May 2011 by Kitsap County Extension agent Peg Tillery mentions hard-pruning winter-damaged Hebes which manage to recover.
You may find this information from Oregon State University useful, as it evaluates the cold hardiness of individual species of Hebe. Here is an excerpt:
“Major cold damage will cause browning of most of the leaves on the canopy, followed by dieback. Sometimes, plants will recover over a 2-3 year period from this damage if subsequent winters are mild. Very severe, sudden cold often turns the entire plant brown and sensitive cultivars do not recover from this damage and require replacement.”
There is a discussion of winter-damaged Phormium in Houzz, an online gardening forum. Again, I suggest waiting until spring to see if only parts of the plant are dead. Keep in mind that gloves are essential when pruning this plant!
The Oregonian has an article about the effects of the December 2008 cold and snow on tender plants. Here is an excerpt:
“I hope, unlike me, you’ve had better luck with your phormiums surviving the storms. Not to be negative, but chances are you didn’t. On the positive side, though, they’ll most likely only die to the ground. If you cut them back, phormiums often come back from the roots.”