Q. I understand why not to mound mulch around the base of trees, but even viticulturists who agree with that stance strongly recommend mounding grape vines for the winter—and I've been getting newsletters from state ag agencies telling me that it's time for me to start mounding. Why the different practices between trees and grapes? Please advise me on this—I lost all my French clones (which were grafted onto American rootstock) during last year's severe winter weather and would like to avoid future tragedies.
- ----Tony in Pittsburgh
A. First, I'd like to thank Tony for helping us remind people NOT to pile mulch up against the base of their trees. As we've warned many times, any mulch should begin six inches away from the trunk, should be no deeper than two inches, and the root flare of the tree should be visible above ground.
Ah, but THAT advice is for normal plants, like trees and shrubs. And even in those cases, "mounding" only refers to the despicable practice of wrapping the base of the plants in little 'volcanos' of piled up mulch. In the case of Tony's tender grape vines, mounding literally means "buried alive".
Yes, completely. In regions where this practice is necessary, you prune the vines back hard enough to bury them entirely with soil right after those vines have gone dormant. The PA state ag websites that Tony refers to have some great photos of the machines that do this in large scale operations. The lanes in these vineyards are designed to be the perfect width to allow huge machines to till up large amounts of soil from the pathways in between the vines and then pour that soil over the pruned vines, all in one pass. Truly buried alive.
On a related note, some people in the frozen North have to do the same thing with some of their roses. There are even 'rose cones' designed to be put overtop of the pruned and buried roses for the winter.
Now to many of us, these kinds of things probably seem crazy. After all, a large portion of the country just had one of the most severe winters on record (the dreaded "Polar Vortex"), and the only plants that suffered death or disability in large numbers were somewhat tender specimens, like hydrangeas and figs—not roses!
But 'we' don't live in one of the USDA Hardiness Zones where the average winter minimum temperature is minus 35 F. (zone 3) to minus 15 F. (Zone 5). Those intrepid gardeners often have to take drastic steps to protect some of their plants; and in some cases, burying really is a good thing in those zones.
But only in some cases. The risk of fatal winter damage is very low if you're growing plants that are rated as being reliably hardy in your region—and there are entire classes of grapevines and roses that can survive winter without protection in those low zones. And those plants typically flourish when they're naturally buried under snow for most or all of the season, as deep snow is the best winter protection. A deep 'mulch' of snow keeps the temperature at the soil line from fluctuating, and protects the plant from damaging winter winds.
But snowfall isn't reliable enough anywhere for a gardener to depend on it to be there all winter for plant protection. (Except maybe in Buffalo this year). And most gardeners don't want to be limited to 'reliable plants'—they want to grow roses they've fallen in love with while travelling, or grapes that produce fine dry red wine--like Tony in Pittsburgh.
His deceased 'French hybrids' produce the fine wines that characterize regions of France, California, Washington State, Australia, Chile and the like. It is very difficult to grow them well outside of those regions, and winter is a big reason why. Yes, the desired varieties are grafted onto tough hardy roots, but those top parts—the parts that produce the actual grapes—are not hardy, and are in fact easily killed over winter. When that happens, any new season's growth comes from the rootstock, not the desired, grafted variety.
…Which means that these people have to bury the actual graft--where the two plants are joined. Which, of course, we tell you to NEVER do with grafted plants. We're breaking every rule today, including the 'no fall pruning' rule, because—as with just about every aspect of fine grape growing—timing is crucial. You have to let the plants go naturally dormant before you prune them, but you also have to prune and bury them before harsh cold without snow cover can kill the top graft.
Same with roses grown outside their recommended regions. You can't prune them too early or you risk stimulating new growth. You can't bury them too early or you greatly increase the risk of them being attacked by rot and mice. And of course you can't wait too long either, or you risk losing the part of the plant that produces the flowers or fruits you want.
With tender varieties of grapes, our resident expert Lee Reich suggests in his great book, "Grow Fruit Naturally" (The Taunton Press) that you plant the vines at an angle to start with, and then use a trellising system that allows you to easily untie and lay the pruned vines down on the ground at the end of the season for covering with soil.
The advice for hybrid teas and other grafted roses in dicey climates is to keep as much of the plant intact as you can, mound soil up around the entire height of the pruned rose—little foot-high mountains are the norm—and then cover it with a rose cone or burlap.
Either way, it's vitally important to uncover the grafted area before the plants start actively growing again in the Spring. So no late winter vacations for you.
All this timing hubbub may be one big reason why Lee Reich concluded our conversation by saying, "and this is why I only grow grapes that are hardy in my very cold climate."