With the sweltering heat and humidity of summer upon us, the havoc of winter may be but just a memory. Many plant problems caused by the devastating cold of winter, however, are still persisting in the garden.
Believe it or not, a majority of the calls that I and my fellow extension agents are receiving are all weather related — mostly from winter, but also from recent heat and dry spells. Even my garden was not immune. My dear Camellia sinensis plant (that is really a zone 7 plant) that I wrote about two years ago suffered the wrath of winter chill.
This past winter we saw temperatures dip well below our normal winter temperatures. This is especially bad since we have had relatively mild (increasingly warm) winters over the last two decades. Last winter was cold but not off the charts. While most of the state is in hardiness zone 6 (and some 5 in the mountains), the cold temperatures we experienced this past February were in the zones 5 and 4 ranges.
So what does this mean? Many plants that are specifically rated for zone 6 or above sustained damage or even died. There are a few that top the list, based on the calls and emails that I have received.
It turns out that extension agents should really have training in grief counseling before they start their jobs.
The Leyland cypress is tops on the list. I can’t tell you how many distraught homeowners have called me over the past month, bemoaning the fact that their Leyland cypress trees have turned brown. Some have one or two of the trees, but others have had 25 or more of the trees — all turning brown. The culprit they suspect is pathogenic in nature — whenever they call, everyone always say that their trees have a blight.
It is true that Leylands can get a disease that can kill, but not this year. This year it was the winter. Unlike other evergreens that we grow, Leyland cypress grow only at zone 6 and higher — any colder and you could get damage or even death.
These trees have become popular over the last few decades because they are a fast-growing evergreen that can form a hedge or provide quick privacy screening. Unfortunately, if we continue to have swings of extreme weather (which climate scientists predict will only get worse), they may not be the best plant for this area. All it will take is one winter below zero to wipe them out.
According to WVU Extension Service plant pathologist Dr. Mafuz Rahman, the cold of the past two years have also stressed the Leyland cypress trees to the point of making it easier for diseases such as Seridium canker to now run wild on the weakended trees, extending the damage into the summer months. While there isn’t a known treatment for the canker, he suggests pruning out affected limbs and fertilizing to improve overall plant health.
This is especially devastating for evergreens due to their growth habits. Unlike other trees that will rebud and grow out if the wood is still living, if all of the needles on a branch die, an evergreen will not regrow. And if a branch dies out, usually a new one will not grow back to replace it.
So once you prune out all of the brown patches of the tree you need to take a step back, look at the tree, and decide if you are really happy with what is left. If not, it is time to replace it.
Should you not plant Leyland cypress? That’s a tricky question. If we don’t get those frigid, below-zero temperatures, you might be OK to plant them. However, unpredictable extreme weather and their penchant for contracting fungal diseases (which is escalated in drought conditions) would make me think twice about considering them for the landscape.
Another plant on the top of the list is crape (or crepe) myrtle. This popular shrub as been gracing landscapes with its pretty, colorful blooms (and interesting bark) for the last few decades.
However, these Southern belles are also not accustomed to temperatures below zero. In most cases, branches will be totally killed out, but the shrub will regrow from the base. If temperatures were especially harsh, or the base wasn’t mulched, the whole plant could be killed.
Cold weather can also delay leafing out in crape myrtle, so you will want to test branches to make sure they have truly been killed out before pruning out the dead matter. Try to bend a twig. If it bends, it is still living; if it is dry and breaks, it is gone.
Japanese maples have also received their fair share of damage, and I have taken more than a few calls on the tree. Unfortunately for homeowners, this expensive landscape tree is also a fussy one. I’ve received several calls about dead branches and new growth from the base. Unfortunately, it will take a bit of work to get it retrained to its former glory.
Another classic winter damage case is that of the grafted rose. You always hear gardeners exclaim about their roses that were one color (yellow, purple, etc.) and then the next year they started blooming red. Some people think that this is the result of some miraculous genetic crossing, but really it means that the scion (the part grafted on) has died and the plant is now exclusively growing from the rootstock. This can be avoided by mulching the base of the rose to cover the graft union in the winter (but be sure to remove the mulch before new growth starts in the spring, or you could get roots growing from the scion that will change the way the rose grows).
Some winter damage, like that from plants drying out in the dry winter air, can be mitigated by watering plants in the late fall or protecting them from wind. Damage from unusually cold temperatures, however, is harder to control. Mulching can help make sure plants that die back to the crown will survive to grow back. Unfortunately, there isn’t a lot to do to protect plants if we get well below the temperatures they tolerate.