Winter survival of marginally hardy plants
Yes, I know it's early July and miserably hot, but I just got the Plant Delights Nursery Update June 2011 wherein Tony Avent talks about siting marginally hardy plants near trees and shrubs to keep the soil from getting too moist in the winter. Cold, wet soil in the winter can be deadly to some plants, particularly those that like full sun and dry conditions.
"Have you ever wondered why a plant died even though the catalog listed it as winter hardy in your zone? Undoubtedly, there are some catalog errors in zoning, but blaming all failures on improper zoning often keeps us from discovering the real problem, which may take more study. The rating of a plant's winter hardiness indicates that the plant can, under the proper conditions, survive these temperatures...not that it necessarily will survive.
We've written quite a bit about winter survival of marginally hardy plants, and I'd like to share some additional recent observations. Through the years, I would notice a plant that would survive in one part of the garden, while the same type of plant would die only a short distance away. We've all heard of microclimates in the garden, where one area stays warmer than another, but in most cases we didn't observe that to be the distinguishing factor. What we observed was that survival of marginal plants was always better when they were growing near a larger shrub or tree. This indicated to us that winter survivability was actually tied more to soil moisture than microclimates. We've stressed for years the importance of well-drained soils, but since our soils here at PDN are sandy loam, there had to be something more at work.
To confirm our theory, last spring we installed a new replicated planting of some new, marginally hardy lantanas. One row was planted just on the south side of a hedge of Ilex 'Nellie Stevens', while the other was planted 75' away in the open garden. Drainage was equal in both sites and neither had a microclimate advantage over the other. After a winter low of 14F, which included several weeks of alternating cold, wet and frozen ground, we got some great results. In the row by the hollies, 14 of 16 lantanas survived with 12 of the 14 growing vigorously and flowering. In the open row, only 5 of 16 survived, with just 2 growing vigorously and flowering. If we had experienced a dry winter, we would have undoubtedly seen a less dramatic difference. We have previously noticed similar results in the garden with marginal salvias and agaves, but without a replicated trial, we were just guessing.
In using this information to help you better site marginally hardy full sun plants, it is important to locate them far enough away from the shrub/tree that the sun will not be obscured, but close enough that the roots keep the soil dry. This will take some experimentation such as sticking a shovel in the ground outside the drip line and continue moving outward until you see the soil moisture change...best done after a heavy rain or irrigation. I hope this helps you to better site marginal plants in your garden and consequently have better success."
Claire (who right now is enjoying thinking about cold weather)