Bulbs Gardening

Gladiolus: Hardier Than You Think

20171104A westcoastseeds.com.
Maybe you won’t have to bring your gladiolus corms back indoors for the winter! Source: westcoastseeds.com

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.

Happy Accident

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.

Winning Conditions

20171104B gardenwise.co
Planting the corms deeper than usual and in well-drained soil can help perennialize them. Source: gardenwise.co

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

20171104C www.jparkers.co.uk
Dwarf gladioli are hardier than standard gladioli. Source: jparkers.co.uk

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’.

20171104D Meneerke bloem, WC
Gladiolus communis byzantinus is a naturally hardy species of gladiolus. Source: ;Meneerke bloem, Wikimedia Commons

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.

20171104E Jitka.karlikov, WC
Gladiolus murielae is less hardy than standard gladioli. Source: Jitka.karlikov, Wikimedia Commons

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.

7 comments on “Gladiolus: Hardier Than You Think

  1. I’m so glad you brought this up! I accidentally discovered that my showy commercial gladioli overwintered when I planted a bed of them on top of my septic tank lol. I missed a few in the fall dig-up, and didn’t replant the following spring, as I was planning to move. Guess what sprouted like magic that year? Apparently the residual heat from a decomposing mass of septic tank muck, plus a heavy snow load, was enough to keep them happy.

    Now that I’m in my new house, I’m experimenting with planting them right up against the foundation , along with some African torch lilies, and mulching heavily. Let’s see how this crew does in zone 4a!

    Cheers!

  2. I have had gladiolus several years in northern VA. I left them in the ground two winters, covered with Christmas tree boughs. Came back well. Last fall I dug them up, stored , and now have planted twice to stagger bloom. All are coming up.🤞But in the area I now have many tiny sprouts from, I assume, corms left in ground. They are very crowded, I don’t have the room to propagate in flats or inside. Should I thin & see what happens?

  3. Daniel Hauptfleisch

    The above is true. I am Zone 4 Catskill Mountain, and discovered by accident (did not dig up the corms in the fall) that the gladiolas will winter over. I have six bed with about 50 corms per bed that come back each year. The blades poke up early June and by late July I begin to harvest. As an aside, the gladiola flower makes a nice flower wine.

  4. a wiggins

    Yah, missed digging up some glad corms last year, and was shocked to see them popping up. Here is Northern Illinois, zone 5 (B). I will leave them in this year and throw some mulch on them late Fall. I have tall red and white ones that I bought at Menards dirt cheap. They are gorgeous – pure white and a vivid red. So easy! Only problem that I have with them is that the wind tends to blow them over somewhat when they are in full flower. Would encourage others, even in Wisconsin to give it a try.

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