Most gardeners in cold regions (hardiness zones 7 and below), even beginners, know you’re supposed to dig up gladiolus (Gladiolus x hortulanus) corms in the fall. You then roughly clean off the corms (bulbs), cut off the leaves and store them cool and dry for the winter. (Read Soon Time to Bring Tender Bulbs Indoors for more details on this technique.)
That’s pretty classic information and generations of gardeners have done just that with their gladiolus. No one will deny that it’s simply the best way to keep them alive and well. Even so, if you leave them in the ground, it just might happen that your gladioli survive the winter to bloom again. They can even persist that way for many years.
Most gardeners who discover this phenomenon do so by accident. They forgot to bring in their corms or just didn’t have time. Either that or they decided all that digging and replanting just wasn’t worth the effort. For whatever reason, they left their glads out for the winter and the following summer, most grew back and flowered, just like before.
The accepted hardiness zone for the classic Grandiflora gladioli (your typical garden glad) is zone 8, but it’s well known that by covering them with a good mulch, you can keep them alive in zones 6 and 7. But what’s more surprising is that sometimes (and there is no guarantee!), they survive the winter zones 4 and 5, even zone 3 with only the slightest bit of protection.
Before thinking of leaving gladiolus corms in the ground over winter, it would be wise to reunite the best possible growing conditions, including:
- a location in full sun;
- loose, sandy or well-drained soil;
- extra deep planting (6 inches/15 cm rather than the usually recommended 2 to 4 inches/5 to 10 cm);
- 6 to 8 inches (15 to 20 cm) of well-aerated fall mulch;
- a good layer of snow.
Under those conditions, I find that I can grow glads outdoors with reasonable success (I do lose a few and their numbers slowly decrease over the years) in my USDA zone 3 (AgCan zone 4) garden … and that’s really cold!
Some Varieties Are Hardier Than Others
Of course, some varieties are hardier than others. To start with, dwarf gladioli (often sold under the name Gladiolus nanus, although in fact there is no such species) are well known to be hardier than the usual Grandiflora hybrids and can be safely grown in zone 6 without protection. With a good mulch, they’re pretty much certain to thrive in zones 4 and 5. Among other gladiolus cultivars known for extra hardiness are ‘Boone’ and ‘Carolina Primrose’.
Then there are also species gladioli that are naturally very hardy. Originating in Europe and Asia (hybrid varieties are mostly derived from South African species), some grow in regions with very cold winters indeed, such as on the Russian steppes. Authorities usually say they’re hardy to zone 6, but I suspect that some at least are much hardier than that. These glads are proving hardy in many zone 4 gardens … and that’s with no special protection. In this group are Byzantine gladiolus (G. communis byzantinus, syn. G. byzantinum) and Turkish marsh gladiolus (G. imbricatus), both modestly available by mail order if you know where to look.
I’ve been growing Byzantine gladioli in my garden (again, USDA zone 3/AgCan zone 4) with no special attention for almost 20 years, although they do profit from good snow cover. They’re actually proving a bit weedy and I have to pull out excess ones when they start to spread too widely.
Note that these Eurasian gladiolas need to be treated like hardy bulbs and planted in the fall, not in the spring like tender glads. In other words, you have to treat them like tulip or narcissus bulbs. They flower around at the same time as the tulips too, well before the classic gladioli. Spring-planted corms (and some shady suppliers do sell them in spring) will not adapt well: they need a cold winter in order to thrive.
On the opposite end of the spectrum, some gladioli are less hardy than Grandiflora glads. The popular Acidanthera or Abyssinian gladiolus (G. murielae, formerly Acidanthera murielae) won’t tolerate much colder than zone 8, perhaps zone 7 with good winter protection. Then there are out-and-out tropical species, too, like Gladiolus pole-evansii, rarely grow beyond zone 9.
Should You Take the Risk?
There will always be a risk when you plant anything beyond its accepted hardiness zone. I therefore suggest you bring indoors in the fall any gladioli that are of great value to you. However, if you have extra plants you could live without, experimenting by pushing gladiolus hardiness limits could definitely be worthwhile.