Round Robin: The Myth of Hardiness Zones

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Denver, Colorado–My garden is my laboratory. There are certainly days (the bad-hair kind) when I must seem like a mad scientist. The challenge is to make a series of experiments look like a garden. It’s tough when experiments fail, and some of them do.

It’s not because I’m a bad gardener. I’m a good one, but I’m always pushing the limits. I’m in zone denial, refusing to recognize the limitations ­imposed by the almighty USDA Plant Hardiness Zone Map. Certainly, there is value in evaluating plant hardiness by minimum winter temperatures, but there’s much more to it than that. My Denver garden is officially in Zone 5, but so are a lot of other places, such as Chicago. Gardeners there can grow many of the same plants that I do–some better than I do, others not–but there are important differences.

Denver’s dry air and low rainfall make it possible for me to grow many perennials that would ordinarily rot out in Chicago. One such is snow daisy (Tanacetum niveum), a cousin of feverfew with finely cut silver foliage covered by a flurry of small blossoms in June. On the other hand, Chicago’s climate is heavily influenced by its proximity to a great big lake that tends to moderate rapid temperature changes in winter. The tiny little lake in the park across the street from me doesn’t do the same, even though I joke about “the lake effect”. Denver’s winter temperatures swing wildly and rapidly, taxing the constitution of woody plants, especially broad-leaved evergreens. Hollies, yews, and rhododendrons may not be completely impossible to grow here, but I’ve already killed more than my fair share.

My experimenting thus has been largely confined to herbaceous plants, many of which I grow from seed. My favorite source for really weird, unheard-of perennials is the members’ seed exchange of the North American Rock Garden Society (it’s not necessary to have rocks to be a member–for membership information, write to the Executive Secretary, NARGS, PO Box 67, Millwood, NY 10546). Last year, members received a listing of more than 6,000 species, which took me two weeks to go through. I request seeds of odd plants that I’ve been trying to track down as well as any species from favorite genera such as Salvia, Allium, Thalictrum, and Geranium.

Early winter is the ideal time to sow perennial seed. That may sound odd, but it makes sense as seeds from many perennials native to temperate climates germinate best if sown in pots outside. I use quart-sized pots filled with gritty potting mix. I sow the seeds on the surface and then cover them with about a half inch of fine gravel (like that used in aquariums) to keep the seeds from blowing away or getting washed out. I put the pots in a dishpan and add water; the pots soak up moisture from the bottom, and the surface is undisturbed. Then I set the pots in plastic trays with drainage holes and move them outside in the shade of a massive juniper.

By sometime in March or early April, tiny seedlings will have appeared. I leave them in their pots under the juniper until June, when I pick them out and move them into individual 2-1/4-inch pots. I plant them out in late summer or autumn wherever I can find space in the garden. Not all of them make it through the following winter, but about 90 percent do.

Many of these unusual perennials can’t be found in standard reference books, so I have little idea what to expect. The descriptions in the society’s seed list are cryptic, usually just giving the color, height, and seed donor. I often get a clue about a plant’s hardiness by where its donor lives. If he or she can grow it in Connecticut or Michigan, I probably can grow it here.

Speaking of reference books, most are works of fiction for western gardeners. At least half the plants in my garden aren’t hardy here, according to the authorities. Since I’m also one of the so-called authorities who writes gardening books, I feel I should caution readers that we make a lot of this stuff up. Well, actually, I do my best to be accurate, and then I make stuff up.

I wish that reference-book authors would call me up once in a while and say, “Hey, Rob, by any chance is Vancouveria hexandra hardy for you?”

I’d reply, “Why, yes it is–has been for more than a decade even though this shade-loving perennial that looks like a cross between a maidenhair fern and a barrenwort is usually listed as hardy to Zone 7.”

“That’s good to know,” the auth­ority would say, “and I’ll bet you grow other perennials that are perfectly hardy despite what my book says, right?”

“Absolutely!” I’d respond. “You’re probably talking about burgundy pincushion flower (Knautia macedonica), which is taking over my garden even though you say it’s a Zone sixer, or giant sea kale (Crambe cordifolia), another Zone sixer that is seeding itself to excess, or even regular sea kale (C. maritima), which you’ve pegged as a Zone 7 plant.”

That’s what I’d say if someone were to call. Trouble is, I don’t even answer my phone when I’m up to my elbows in potting soil.

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