Wisterias (Wisteria spp.), with their dripping spikes of scented lavender blooms, are probably the most desirable of all climbing plants in colder climates… precisely because they are so difficult to get to bloom. After all, the rarer the plant and the more impossible it is to get it to grow properly, the more you’re bound to want one! Wisteria’s refusal to flower even in mild climates is legendary; in cold areas, it is even less reliable.
If you live north of USDA hardiness zone 4 (AgCan hardiness zone 5), you’re in the “not really a good climate for wisterias” region, but you can be successful… if you choose one of the hardiest cultivars. Here then are some tips that can help you succeed.
The most coveted wisterias are two Asian species, Japanese wisteria (W. floribunda) and its doppelgänger, Chinese wisteria (W. sinensis). You can tell the two apart by the direction the twining stems turn in: clockwise in the case of Japanese wisteria and counterclockwise in the case of Chinese wisteria. Otherwise they’re pretty much identical. Both produce long spikes of grape-scented flowers that drip down to up to 20 inches (50 cm) in length. But they aren’t very hardy.
Although they will grow in USDA zone 3 (AgCan zone 4), but often freezing back severely, their flower buds are formed over the previous summer and overwinter on the plant (they bloom on old wood). And the flower buds are less hardy than the stems, so they almost always freeze where winters are cold. For that reason, you can’t reasonably expect to get Asian wisteria to bloom reliably north of USDA zone 5b (AgCan zone 6b). There are some extra hardy cultivars, such as W. floribunda ‘Lawrence’ and W. sinensis ‘Caroline’, that may bloom fairly well in USDA zone 4b (AgCan zone 5b)… but only after a fairly mild winter.
North American Wisterias
The North American species, American wisteria (W. frutescens) and Kentucky wisteria (W. macrostachya), have gained in popularity over recent years as cold-climate alternatives to the Asian species. In actual fact, they are pretty much equal to Asian species when it comes to stem hardiness (USDA zone 3, AgCan zone 4), but where they shine is in their capacity to bloom after a cold winter. The difference is that they bloom on new wood, that is, new stems that sprout in spring, and that means the plant can bloom even after a harsh winter. Moreover, both species tend to rebloom once or twice during the summer. Of course, these addition flowerings are not as dense as the first wave, but who doesn’t appreciate a few extra blossoms?
That said, it must be admitted that the North American species aren’t nearly as impressive in bloom as the Asian species. First of all, their flower spikes are less than half as long, so they really don’t drip down like the Asian species. Rather, they look a bit more like a lavender pinecone. Also, they don’t bloom as heavily as Asia wisterias often do: you simply don’t seem them “covered with flowers” like an Asian wisteria can sometimes be… in zones 7 or 8, that is! Finally, when the Asian species start to bloom, their foliage isn’t yet fully formed, allowing the flowers to dominate while North American wisteria flowers have to peek out through fully leafed out stems.
On the plus side, North American species will bloom even when they are young. Compare that to Asian wisterias that can often take 7 to 10 years to produce their first flowers. Many North American wisteria will already be in bloom when you buy them and will continue blooming from then on. Also, North American types are less dominant plants, not nearly as likely to take over your landscape with crushing, spreading, invasive stems as Asian wisteria are prone to do.
The Hardiest Cultivars
American wisteria (W. frutescens) is the earliest bloomer of the two North American species, usually in May or early June. As a result, its flower buds often begin to form too early in the season and can be killed by a late frost. The cultivar considered the most suited to northern gardens is W. frutescens ‘Amethyst Falls’, but still, its susceptibility to spring frosts means it really can’t be widely recommended north of USDA zone 4b (AgCan zone 5b), except in the most protected spots.
It’s among the cultivars of Kentucky wisteria (W. macrostachya) that you’ll find the cultivars best adapted to colder climates. They bloom later, in June, and are in fact sometimes called “summer wisterias”. This puts their late-to-sprout flower buds out of range of late frosts in most years.
Two cultivars that have been giving good results are ‘Blue Moon’ and ‘Aunt Dee’, and they can be considered bud hardy to USDA zone 3 (AgCan zone 4). And there is a new arrival, deemed the hardiest wisteria of all: Summer Cascade ™ (W. macrostachya ‘Betty Matthews’). It blooms later than the others, often not until the end of June in colder areas, well out of reach of frost. Studies at the University of Minnesota show it to be the only wisteria that can truly be counted on to bloom annually and abundantly in USDA zone 3 (it hasn’t been officially tested in Canada, but the AgCan equivalent would be zone 4). This is one you might even want to try, at least as an experiment, in USDA zone 2 (AgCan zone 3). It is scheduled to go into continent-wide distribution in 2016, so look for it in a garden center near you very shortly.
Growing Wisterias in a Cold Climate
Of course, once you’ve found a wisteria cultivar that ought to do well in your climate, you still need to give it appropriate growing conditions.
Plant your wisteria in full sun in a warm, protected spot (near a south- or west-facing wall, for example). It needs well-drained conditions and soil that is not too rich. Wisterias live in symbiosis with bacteria on their roots that provide them with the nitrogen they need for their growth. If you plant them in soil that is too rich, or if you add a nitrogen-rich fertilizer, the plant will tend to produce lots of leaves but few flowers.
Note too that wisterias are very strong growers and will need a solid support, easily overwhelming and even crushing small wooden trellises. Consider growing them on a pergola or up an old tree.
Once your wisteria starts to take off, don’t hesitate to cut back its roaming branches. Otherwise it will tend to invade all nearby structures and trees. It’s helpful to know that these long “whips” are juvenile stems and will not produce bloom the following year, so no harm comes from pruning them back to 6 to 12 inches from a main stem. In fact, cutting them back may even help stimulate better bloom. You can prune back wisteria immediately after their main bloom… or as late as at the end of winter.
Best of luck with your wisteria trials. If you’ve failed in the past, the newer, hardier W. macrostachya cultivars might just be what you need for good results this time.