Mini Horticultural Myth: Dormant Bulbs Need Darkness

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Sleeping bulbs are indifferent to light. Ill.: owips.com, http://www.mycutegraphics.com & http://www.wpclipart.com, montage: laidbackgardener.com

I want to bring up a very minor detail on the cultivation of tender bulbs (gladioli, cannas, dahlias, tuberous begonias, etc.), but one still worth clarifying: the concept that bulbs need darkness during their dormant period.

Typically, the explanation offered to gardeners is that when you bring tender bulbs indoors, you need keep them in a spot that is cool, dry and dark, such as a basement, a slightly heated garage or a root cellar. But in fact, darkness itself is not a factor.

When a Bulb Is Sleeping

As long as you keep dormant bulbs dry (and, for many, cool as well), they don’t care about the lighting conditions. Photo: http://www.dutchgrown.com

Dormant bulbs are just that: dormant. They’ve stopped all growth. And when they’re in that state, they’re perfectly indifferent to light. Sun, partial shade, darkness: it’s all the same to them. True enough, you don’t want to store them in a spot that gets really hot as that could cause them to dry out and intense sun can indeed heat things up quite a bit. However, if you have potential storage space that receives light part of the day or even all day, yet remains cool, that would be a perfect place for bulbs.

Other Dormant Plants Too

Also, the same information applies to any plant that goes dormant at some time during the year: amaryllis, cyclamens, desert cacti, etc. Yes, do stop watering them and yes, do put them in a cooler place … but there is no need to keep them in the dark unless there is some reason that would be convenient for you.

I tried to come up with even one exception: a single plant that must necessarily experience darkness 24 hours a day for a long time when it is dormant, but I couldn’t think of one.

And, when you think this over, this is quite logical: when a plant goes dormant in the wild, it doesn’t dig itself up and take refuge in a dark cave nearby. It stays where it is and puts up with whatever natural light is found in that spot.

So, if there is a window in the garage or basement where you overwinter your bulbs, you don’t need to block it nor to seal the bulbs in an opaque box. As long as this light doesn’t overly heat the room, its presence or absence is irrelevant.

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Bringing Frost-Tender Bulbs Indoors for the Winter

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When frost kills the foliage of your bulbs (here, dahlias), it’s time to bring them back indoors for the winter. Source: canoecorner.blogspot.com

Most summer-flowering bulbs other than lilies, and that includes dahlias, gladiolus, cannas, callas, colocasias, acidantheras and tuberous begonias, are considered “tender bulbs,” that is, they are not hardy enough to survive the winter in cold climates. Many of these bulbs may be able to overwinter outdoors in USDA zones 7–8, especially if well mulched (indeed, some gladiolus thrive in zone 6!) and certainly in zones 9 and above, but most gardeners living in colder zones can only keep them alive from year to year by bringing them indoors in the fall.

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Gladiolus corms freshly dug up. Source: natureworksct.blogspot.com

Usually the signal to bring them indoors is when the first frost damages their foliage. If there is no frost before mid-November, though, bring them in any way: you don’t want them to be still in the ground when it freezes solid!

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Let the bulbs (here, dahlia tubers) dry out for a week or so, then clean them up roughly before storing them. Source: thepropagatorblog.wordpress.com

To do this, dig up the root ball and shake it to knock off excess soil, then let the bulbs dry on newspaper or cloth for a week or so in a frost-free spot (a garage or shed, for example). Now clean them roughly with a brush to remove most of the dirt. Don’t rinse them, though: they must stay dry.

On some bulbs, stems and foliage will fall off all on their own at this stage. If not, cut them off about 2 inches (5 cm) from the bulb. Also cut off any lingering roots and remove any bulbils (baby bulbs). (You can save the bulbils if you want, but be forewarned that they are usually 3 to 5 years from blooming!) You can also sprinkle garden sulfur on the bulbs: this will help prevent rot over the winter.

Make sure you properly identify your bulbs at this point. There is nothing more confusing than looking at a pile of gladiolus corms or dahlia tubers in the spring and trying to remember which were the red ones and which were the yellow ones! Either write the name on their storage container or add a label. Some people write the name on dahlia tubers with a felt pen!

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You can layer bulbs one on top of the other as long as you cover the previous layer with vermiculite, peat moss, wood shavings, or shredded paper. Here, the first layer of dahlia tubers was covered in peat moss. Source: www.thatbloomingarden.com

If possible, store the bulbs in a cardboard box or a plastic container (the latter should be have holes so there’ll be a bit of air circulation), covering them with vermiculite, peat moss, wood shavings, or shredded paper. Some gardeners like to store them in a mesh bag.

The ideal location for winter storage is a cool but not cold spot that remains between 40 and 55˚ F (5–12° C) for much of the winter. That means that a fridge or a cold room may be too chilly for some tender bulbs, but a basement or lightly heated garage should work well.

You have nowhere cool to store them? Don’t worry: they can be kept at room temperature if necessary, but if so, the bulbs will tend to dry out over the winter, so check them monthly, giving them a spritz of water if they start to shrivel.

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If you grow your bulbs in containers, you can simply pile the up in a cool, dry spot. Source: thebikinggardener.com

One final note: if you grow your tender bulbs in containers, overwintering is even simpler. Just bring the pots indoors, cut off the foliage, let the soil dry out, then pile the pots up in cool, dry spot until spring.

Storage Temperatures for Tender Bulbs

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At what temperature should you store your tender bulbs? Source: lopatkina.wordpress.com

You’ve brought your tender bulbs (gladioli, tuberous begonias, dahlias, cannas, caladiums, etc.) in for the winter? Perfect! The bulbs are either in their original pot or bare root, surrounded by peat, vermiculite, newspaper, etc. Also perfect! Now where to place them for the winter?

On the Cold Side

The ideal location for winter storage is cool, but not cold, between 40 and 55 °F (5 to 12 °C). That means a refrigerator or a root cellar is generally too cold for bulbs, but a barely heated basement or protected garage ought to be ideal. At such temperatures, there will be little evaporation and that will allow the bulbs to stay dry yet well hydrated until spring. There should be no need to humidify them in any way.

On the Warm Side

You don’t have a cool spot where you can store your bulbs? I don’t either … and that’s not a problem! You can easily store them at room temperatures as well. However, warmer air means there will be evaporation and probably some dehydration. That means you’ll have to take an extra step and moisten them occasionally through the winter.

I recommend a monthly inspection, followed by rehydration if needed.

How to handle this inspection, though, depends on how the bulbs are stored.

In Pots or Loose

For example, I store many of my tender bulbs in the same pot they grew in all summer. If so, you can’t really examine the bulbs themselves, but you can take a look at the soil. If it’s very dry, add just a dribble of water, say a few teaspoonfuls, to the top. Let the water penetrate the potting soil (that may take a few minutes, as very dry potting soil will repel water at first), then store the pot away again.

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If .loose bulbs (here, dahlia roots) look dehydrated, simply spray lightly with plain water. Source: forums.gardenweb.com

If the bulbs are loose (bare root), you really can inspect them individually. If they don’t look shriveled (often the case during the first months of storage), they’re doing fine: just put them back. If, on the other hand, they’re turned soft or starting to shrivel, just spray them with clear water and put them back into storage. This tiny amount of water will be enough plump them up again and prevent a potentially fatal dehydration.

A Reminder

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Add “check bulbs monthly” to your agenda. Source: thetomatos.com

As soon as you bring your tender bulbs indoors in the fall, add a monthly note to your agenda: “check dormant bulbs, moisten as needed.” Mine is electronic, so beeps at me to let me know when it’s time. You don’t have to be precise: check them a week early or a week or two late, but do give them a look-over.


As you can see, when tender bulbs are dormant, they’ll adapt to almost any non-freezing situation. Finding the right place to store them couldn’t be simpler!