Garden Myth: Wait Until Frost Blackens the Foliage of Tender Bulbs Before Bringing Them In

Bulbs of dahlia, tuberous begonia and gladiolus ready to be brought indoors for the winter. Photo:

If you garden, you’ve probably heard the recommendation to wait until frost has blackened the foliage of tender bulbs like tuberous begonias, dahlias, gladioli and cannas, etc. before digging them up to store indoors over the winter. After all, these bulbs aren’t hardy in cold climates and do need to be stored in a frost-free spot. But the term “blackens the foliage” can be a bit misleading. Let’s say it’s more of a little white lie than outrageously incorrect.

You see, freezing need not be the decisive factor in harvesting bulbs.

This advice comes from the idea that leaving the bulbs in place as long as they have healthy foliage allows them to store a maximum amount of energy for the next blooming season. Hence the advice to wait for the first severe frost (which kills the foliage and puts an end to them storing energy) before bringing the bulbs in. And this makes perfect sense in areas where there are inevitably frosts in September. However, due to global warming, the first killing frost of fall has been pushed way back in many areas, to November or even December.

In many places, the first killing frost often now corresponds to a thick covering of snow that will make harvesting the bulbs difficult or even impossible. By that time, too, although their foliage may still appear healthy and green, it’s usually been cold long enough that the leaves are no longer contributing much energy to the bulbs below.

Not all tender bulbs have leaves that blacken after frost. They may need to be harvested when the leaves are still green. Photo:

Also, if you wait for the foliage of certain bulbs to actually blacken, you’ll wait forever! True enough, some react instantly to frost and the leaves actually do turn black. That’s the case with cannas, tuberous begonias, colocasias and several other truly tropical bulbs. However, the leaves of some of bulbs are much hardier. Those of gladioli, acidantheras and crocosmias, for example, change very little in color, even after a severe frost.

A better way of putting the situation would be to say that it’s time to dig up bulbs and bring them indoors after the first hard frost or in mid-October, whichever comes first in your area. And yes, you can bring in tender bulbs even if their leaves are still green.

To learn more about how to bring in and store tender bulbs, read Soon Time to Bring Tender Bulbs Indoors.20170927B

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