A few days ago (October 18, 2017), I wrote a blog called Overwintering Plants in Containers and it discussed perennials, shrubs, hardy climbers, small trees, etc. It didn’t cover hardy bulbs (tulips, daffodils, hyacinths, crocuses, garlic, etc.) and for good reason: the same rules just don’t apply.
According to Climate
Container Bulbs in Cold Climates
In fact, growing hardy bulbs in containers outdoors in areas with cold climates (hardiness zones 1 through 6) is difficult, because soil in containers tends to freeze solid. Of course, that’s also the case for shrubs, perennials, etc., and they survive. What’s the difference?
Shrubs, perennials, hardy climbers, etc. go into full dormancy in late fall and stay there until spring. Unless the temperature drops so low it kills the dormant cells, it doesn’t much matter if the ground freezes: they’re asleep!
Hardy bulbs, on the other hand, start to grow in fall and continue through the winter. Yes, even under the snow! Deep in the ground, their roots start to grow and lengthen and the shoot that will give next spring’s foliage and flowers starts to extend upwards. Often, when the snow melts, you discover the bulbs’ sprouts are already emerging from the soil, proof they were growing in the middle of the winter while most other plants were dormant.
Soil outdoors, aided by bottom heat that constantly rises from the depths of the earth, rarely freezes very deeply. That’s way, even when it does freeze, it usually doesn’t harm the growth of the bulbs, most being planted well below the surface. And where the ground does eventually freeze deeply, that takes weeks and so bulbs have time to prepare for spring beforehand. Also, frozen soil is not necessarily all that cold. At the depth at which most bulbs grown, temperatures are often 20 °F (10 °C) warmer than the air above and sometimes much more.
In a pot, on the other hand, the soil soon reaches about the same temperature as the air and, in many regions, remains frozen from top to bottom for much of the winter. When the thermometer says the air is -5? F (-15 ° C), it’s also -5? F (-15 ° C) in the middle of the pot … and that’s just too cold for bulbs!
Moreover, in regions with milder winters, the soil in pots tends to freeze and thaw repeatedly, which is no better.
Bulbs are not able to grow under such harsh conditions. Come spring, you’ll find them dead and rotting.
A Frost-Free Location
To successfully grow bulbs in a container in a cold climate, you really need to find a location where the soil won’t freeze at all! Ideally, the temperature would remain between 33 and 46? F (1 and 8 ° C) throughout the entire winter.
It’s almost impossible to maintain such a temperature in exposed pots, so pots should be placed where they can get good shelter, for example by planting them in the garden, placing them in a garage or other lightly heated structure or putting them in the refrigerator.
The result is that it’s no longer a question of “overwintering hardy bulbs in containers outdoors.” What you’re really doing is carrying out an age-old technique called “forcing.” You can read more about this subject in the article Forcing Bulbs Without Twisting Arms.
Container Bulbs in Mild Climates
Given the difficulty growing bulbs outdoors in containers in cold climates, you’d think it would be easier in mild ones, but … no dice!
In mild regions, let’s say zone 8 and above, it’s not the deep frost that hinders bulb growth, but insufficient cold. Most hardy bulbs (tulips, narcissus, crocuses, etc.) need prolonged periods of cold temperatures (less than 48 ° F/9 ° C) in order to bloom. These “cold spells” have to last at least three months.
(Hyacinths and Paperwhite narcissus will bloom at warmer temperatures—13 ° C at most—and require only about 8 weeks of cold.)
All mild climates are not equal and in some zone 6, 7 and 8 regions, notably those where there is a maritime effect, temperatures remain relatively cold and stable throughout the winter (33 to 48 ° F/1 to 9 ° C) and only fall below freezing for short periods of time. If so, it will be possible to grow hardy bulbs in containers without too much trouble. However, that’s more the exception than the rule. Usually, gardeners in zone 8 and above have to place their bulbs in a refrigerator if they want to see them bloom, while those in zones 6 and 7 will have trouble with containers freezing solid before the bulbs are ready unless they protect them to some degree.
Hardy bulbs: you can grow them in the ground or force them indoors, but overwintering them outdoors in containers is generally not going to work!