Why evergreen shrubs look toasty in winter
Q: Some of my evergreen shrubs are looking pretty toasty. I don’t think we got below a zone 7 winter though, and these shrubs are supposed to be hardy to that zone. Is something else going on? A:
Probably not, since few diseases, pests, or other issues would mimic that type of leaf damage this time of year. Winter hardiness can depend on more than just an absolute minimum temperature, which is what the USDA cold-hardiness zone map refers to. Indeed, I don’t think many areas in Maryland experienced a zone 6 winter this year so far (where temps drop below zero) outside of our westernmost counties where this is more normal.
However, when we have a mild spell followed by a relatively quick plunge into colder temperatures, even cold-tolerant plants can become damaged. Plus, evergreens that are experiencing drier conditions due to insufficient precipitation or moisture loss from a wind-swept location or unmulched soil are more vulnerable to leaf drying during these cold snaps.
I’ve seen a lot of winterburn around Central Maryland so far this season. This happens when the foliage can’t replenish the moisture it’s losing fast enough, often due to soil moisture being frozen and therefore unavailable to the roots. I think the first cold snap around late December caused some of this damage, and more recent repeat occurrences likely worsened the situation. Fortunately, many of these otherwise-hardy shrubs will recover once they can
produce new growth, and their damaged foliage will gradually fall off on its own.
What remains to be seen is how well more marginal plants recover, and this is the risk we take when experimenting with less reliably-hardy species. Although climate change trends will warm our winters overall, they can also become more unpredictable when it comes to mild spells and cold snaps. Examples of more vulnerable evergreen shrubs still worthy of garden use — but with careful siting — include distylium, illicium, gardenia, and loropetalum. Even so, we routinely
can see winterburn on commonly grown evergreens like camellia, cherry laurel, azalea, holly, and Southern magnolia.
While the foliage may have been badly desiccated on some species, the branches and dormant buds themselves could be OK. Microclimate can have a big impact on either protecting or increasing the vulnerability of plants that are more borderline hardy. Microclimates are the more subtle, highly-localized differences in ambient temperature, soil moisture, or other environmental conditions that impact plant growth and
performance. A garden bed planted next to a foundation or the base of a retaining wall, for example, will be more insulated from cold snaps and winter winds than a site more exposed to the elements.
Shrubs developing winterburn are usually those located in areas that are windy, have little supplemental irrigation, or have to deal with salt accumulation from ice melt, which impedes root function when they need to absorb moisture. Periodic monitoring for watering needs and using a thick layer of mulch (about 3 inches) over the root zone can help insulate the soil to reduce this risk, but in some situations it may be unavoidable.
Q: Something caused holes in my callaloo crop last year, but I didn’t see any bugs on it. Is it slugs? A:
It could be slugs or snails, but more likely is the pigweed flea beetle. Named for its host plant, this native striped beetle can chew leaf holes as both an adult and a larva. (Pigweed is a weedy member of the amaranth family, the same plant family callaloo belongs to. This leafy green originates in southeastern Asia and is popular in the cuisines of
Asian, African, and Caribbean nations.) Flea beetles are so-named because most are great jumpers due to muscular hind legs, so they may be hard to spot, especially if watering or tending to the plants startles them and the adults hop off. The larvae can’t jump away or fly, but are paler in color and might blend-in well enough to escape notice unless you look closely at both sides of the leaves.
While not widespread garden pests because many of our other vegetables don’t suit them, this particular flea beetle can still hamper the productivity of callaloo. Excluding beetles by using either a lightweight floating row cover or fine-mesh insect netting (which won’t block much of the plant’s light or smother it with heat buildup) to cover transplants after putting them in the ground is the simplest approach. Keep enough slack in the material to cover plants as they grow taller by summer.
A feeding deterrent like a treatment of kaolin clay — a type of organic pesticide that uses a fine coating of natural clay particles to discourage pests — may work, but you’d have to put more work into thoroughly cleaning off the foliage.
As a last resort, a pesticide formulation using the ingredient spinosad instead might be effective, though heed any label precautions about timing of application as it relates to harvesting.