Winter survival of marginally hardy plants

claireplymouth z6b coastal MAJuly 6, 2011

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 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)

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carol6ma_7ari(zones 6 & 7a)

I too am enjoying thinking about cold and snow and bitter wind-- I'm eating an orange popsicle as I write.

I don't completely agree with Tony Avent about the effect of planting marginals near trees. It seems to me not a good comparison, those 2 locations: 1 in an open field, and the other on the SOUTH SIDE of a line of hollies. The 2nd group had the advantage of protection from cold north and northwest winter winds. What would have happened if they had been planted on the north side of the hollies?


    Bookmark   July 6, 2011 at 1:05PM
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claireplymouth z6b coastal MA

Carol: He said they planted the lantanas in the spring, so they would have had a whole growing season before winter hit. I think that planting them on the north side of the hollies probably would have given them too much shade so they would not have grown well.

You make a good point about the winds from the north and northwest. He doesn't say whether that is a windy site, but lantanas are perennials that die down to the ground in the winter so I don't think they would be exposed to the winds. I've never grown lantanas so I don't know much about them.


    Bookmark   July 6, 2011 at 8:42PM
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It's downright chilly here at the moment; my DH called from Boston at about 6 PM and it was about 15 degrees hotter there. Love that fog; please let's not go back to cold weather just yet!

Anyway, this is an interesting concept and one that I may try. I recently had really good luck with one of the marginally hardy pink agastache, planted just south of a young tree; it has rarely come back for me even when planted on a little hill. We'll see if that keeps coming back or not...

    Bookmark   July 6, 2011 at 9:21PM
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carol6ma_7ari(zones 6 & 7a)

a-HA! I looked up his nursery and it's in Raleigh NC. Which is inland enough to experience a freeze in winter, but a fairly short winter, compared to us.

I bet most of us are already siting our borderline zone plants to take advantage of shrubs for winter protection. For example, I'm growing some antique climbing roses which are supposed to be OK to zone 6. I've planted them on the south side of tall cedar posts (to eventually grow up those posts) and also along the north side of a fenced garden, to benefit from the high hedge-y tangle on the other side of that north fence.

For herbaceous perennials (such as hostas, ferns and daylilies) I'm letting them survive facing north, with a high concrete wall to their south; and they seem to do OK there. But those perennials are not borderline for this zone.

--change of subject: it's pouring down rain right now and maybe that'll cool off things outside. Pleasant here, with the AC on.


    Bookmark   July 6, 2011 at 9:34PM
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I like browsing the PDN catalog, but I always get frustrated because the amount of zone 4 and zone 5 plants is limited. Too much zone envy!!! I do like that you often get an enhanced statement of hardiness (e.g. zone 6 at least).

My winter is longer than most,well inland, close to the NH border, so I avoid anything marginal, no matter where it is sited. Especially now that I am winding down the adventurous side of creating new gardens. No more trial and error. Just trying to tread water to maintain what's here.

But I do like the debate. THanks for posting. The cynic in me wonders if he is he trying to sell more marginally hardy plants, but I do appreciate the technical merits he poses.

    Bookmark   July 6, 2011 at 10:06PM
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loomis(Z6a Western MA)

I think a lot depends upon where the plants were grown. When buying perennials, I much prefer those grown in my area, rather than a southern state. Even though the plant may be labeled as hardy in Zone 5, for instance, I find that southern grown plants don't always make the grade, so I try to purchase plants from nurseries either in my local area or more north of my location. Then I can be sure they'll make it.

Most of the plants I find at local nurseries now have labels listing where they were grown.

    Bookmark   July 6, 2011 at 10:44PM
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