In the first installation of my two-part look at plant hardiness, I discussed the way hardy plants prepare for and deal with the onset of cold weather: an appropriate topic as I sit here in a freezing cold auto garage waiting for an oil change. This time of year I have to admit my thoughts are already turning to spring even as the first snow has falls. I wonder if the maples outside could think, would they be dreaming of spring?

Winter is hard on plants, yet surviving the worst of winter’s wrath is only half the battle, for they also need to sense when it is safe to emerge from dormancy and start to grow again. Emerge prematurely and freeze; emerge too late and loose ground and precious growing time to other more timely neighbors. Emergence from dormancy is not like turning on a light switch. It is slow process that begins in winter commences in earnest as the snow melts and the days lengthen. Once the process has really begun, it cannot easily be undone. Thus being able to time the end of winter correctly is a matter of life and death and as such it is honed very precisely by natural selection.

Growing a fur coat?

It is important to understand that every seed comes pre-programmed for a specific place. It has a sort of genetic internal clock that cannot be changed. In other words, plants cannot learn from their mistakes or adapt to a different climate over their lifetime no more than I could grow fur after living in Alaska.

A red oak from eastern Massachusetts is tuned into the seasonal cues of its area because its ancestors were the ones that got their timing right. Plant this same tree in either Alabama or New Brunswick and it would fare much more poorly. This local adaptivity to the rhythms of a particular climate is one of the key reasons why preserving distinct regional populations of native plants is very important and why it is often the Japanese maple and not the native one that suffers from a late freeze.

The Japanese maple whose leaves got nipped by a late frost last spring will likely get hit again next year and the year after that. Likewise, collecting seeds from an Alabama oak and growing them up in a Maine nursery will not make them any more cold tolerant than they would have been if they had been raised down south. The plants that are best adapted to the particulars of your local climate are the ones that have been living there all along.

A drugged sleep

Remember that with the onset of cold weather in late fall, high levels of the plant dormancy hormone abscisic acid (ABA) will prevent dormant plants from resuming growth even if the weather warms for a few days. I think of ABA as a sleeping pill that begins to take effect in late fall and whose effect is insurmountable during the dead of winter so the plant remains slumbering through an occasional January thaw. However, as winter wears on and plant tissues are exposed to a genetically predetermined number of hours below 45 degrees F, the amount of ABA begins to diminish.

By March or early April, the ABA is gone so the plant can spring into growth again as soon as the weather warms. If you have ever forced sprigs of pussy willow or azalea indoors you may have discovered that if you cut them in January, no amount of coaxing will get them to bloom, but if you wait until March the buds swell within a day or two. Though you likely did not realize it, you were witnessing the effects of ABA first hand.

A one-way street

Once plants begin to come out of winter dormancy, they lose more and more of their cold tolerance. A temperature of 18 degrees in January would give the red oak I mentioned earlier no trouble, but 18 degrees in May once the sap as started running again could well be fatal. Accordingly, the oak is very cautious as the weather begins to warm. At this time of year the night temperature more the day temperature determines how fast the buds well and the leaves and flowers emerge. I suppose this is partly because night temperatures are not as volatile as day temperatures early in the growing season. It usually takes a few consecutive nights above 45 degrees for the woods to wake up and for buds to start swelling. As new shoots and leaves emerge they are often red or purple because of high concentrations of anthocyanin pigments in the expanding tissues. Anthocyanins are thought to both retard ice crystal formation and desiccation in the tender young tissues while also shielding them from the damaging effects of ultraviolet light until green chlorophyll pigment develops as the tissues mature.

Wildflowers like Mertensia virginica (Virginia bluebells), Caulophyllum giganteum (blue cohosh), Jeffersonia diphylla (twinleaf), and many others display deep red, blue, or purple that dissipates as the shoots mature and the weather continues to warm. I have regularly seen early spring wildflowers like this freeze solid when temperatures dipped into the mid twenties then thaw and recover completely the following day. This pigment-induced cold tolerance can be lost very quickly if we have a stretch of very warm weather, which allows rapid maturation of the leaves and stems with a concomitant drop in anthocyanins. A drop into the mid twenties is a lot more dangerous after a week of unusually mild April weather, and even cold-tolerant species like Virginia bluebells can get frosted.

The days are getting longer

Just as in late summer and early fall, day length changes become more important as spring progresses and frosts become less frequent. By early June in Massachusetts, most plants have lost all their cold tolerance and are well into growth, flowering, and seed production. Not until the days start to shorten in August will they turn their efforts once again to winter preparation.

Hardiness zones

Gardeners rely on hardiness zone maps to determine whether a particular plant might be suitable for their garden, but hardiness zone maps have an inherent weakness. These maps are compiled from weather station data and are based on the average lowest winter temperature recorded for various regions over several decades. The lowest temperature recorded for Framingham might be -1 one year, +3 the next, -10, the next, +1 the year after that and so on. Average these out over 20 years and you’ll arrive at an average winter low of say -2 degrees, which puts Framingham into USDA hardiness zone 6. If a fully acclimatized plant can survive -2 degrees, then it stands a reasonable chance of surviving the coldest weather experienced during an average Framingham winter.

Yet we know nothing about how well that plant acclimatizes in the fall or how cautious it is about emerging from dormancy in spring, nor how well it might withstand the rare plunge to -10, and all of these factors are just as critical when determining its true winter hardiness. Hardiness zones, therefore are a very imperfect tool for accessing true cold hardiness and this is why they are so often wrong.

Additionally, ratings are routinely applied to a species without any regard for regional variation with in that species. You may read that Helianthus angustifolius (swamp sunflower) is winter hardy to zone 5. It is true that it ranges as far north as New York state and that indeed, if you grow seed from one of these northernmost populations it will survive a zone 5 winter, but most of the plants sold in nurseries come from seed collected in the heart of its range – the Deep South. Most of this southern material is winter-hardy in zone 6 at best and safer in zone 7.

Conversely, many hardiness zone ratings of uncommon species have been determined from experience with very few individuals in very few locations. Since hardiness can vary from plant to plant as well as from population to population, these ratings may be too conservative.

A rare plant thrives in Garden in the Woods

Over the years, many species that were thought to be too tender for Garden in the Woods have proven to be perfectly hardy as we raised up seed from northern populations. A case in point is Muhlenbergia capillaris (hair awn muhly). This wonderful native bunchgrass grows from the gulf coast to the lower Midwest and enters New England only in Connecticut, where it is considered a rare species. After several failures with southern strains (also referred to as southern ecotypes or of southern provenance), we raised seedlings from one of the Connecticut populations for the New England Garden of Rare and Endangered Plants (at the Society's Garden in the Woods) and this group has thrived for more than a decade now.

For cultivation, for sale

Here we have proof once again that often the best garden plants come from nearby. Thus a key mission of New England Wild Flowers Society’s Nasami Farm nursery is to collect and introduce into cultivation New England ecotypes of horticulturally valuable native species. This kind of important work and service gives New England gardeners and landscape designers access to the best adapted plants for our gardens.