Protecting Bulbs for Winter

Autumn in Minnesota is the time to prepare your bulbs for next year’s growing season, whether that is tucking in your hardy bulbs for their winter sleep (and protecting them from foraging critters!) or retrieving your tender bulbs for indoor storage to spare them from the harsh cold. Read on to learn more about what to do this season for beautiful blooms next year!

By Jessica Nguyen, Master Gardener

Tubers, rhizomes, corms, and true bulbs are commonly referred to using the overarching colloquialism “bulbs.” Bulbs are underground storage structures that carry both the genetic material for a plant for propagation and a food supply to kickstart plant growth. Some examples of bulbs you might have in your garden are daylilies and cyclamen; irises; gladiolus flowers, crocuses, and freesias; daffodils and tulips.

Know your plant:

There are generally two routes for winterizing bulbs, depending on whether the bulb is hardy or tender. Hardy bulbs require the cold winter temperatures to kickstart biochemical processes necessary for their next bloom cycle. They generally bloom in early spring to early summer (think crocuses or daffodils.

Crocuses are some of the earliest-blooming flowers that have a bulb structure underground (it’s actually a corm!)

Tender bulbs can’t survive the cold temperatures and are planted in the spring to bloom during the summer months (e.g. dahlias, elephant ears, begonias, amaryllis.)

Amaryllis hippeastrum Candy Floss

In-ground hardy bulb overwintering: for that pleasant spring surprise!

Hardy bulbs need the winter chill to flower in the spring, and should be planted in the fall in order to get that necessary cold cycle. Plant your bulbs according to the specific recommended planting depth and spacing for the plant, mark where you’ve planted your bulbs so you don’t forget about them, then watch and wait.

Many hardy bulbs are perennials that you may have planted in previous seasons! For hardy bulbs that are already in the ground, trim away foliage in the fall after it has had plenty of time to die back naturally, clear away debris from the previous bloom season, cover the bulbs back up, and leave them be!

Planting Crocus Bulbs

You should water hardy bulbs up until the first frost, even if the foliage has died down. The wet soil helps to ensure the bulbs don’t dry out or wither before they are able to freeze for the winter.

Hardy bulbs can survive the winter chill because even though the air temperature in Minnesota winters can reach incredibly low temperatures and can fluctuate wildly, soil temperatures can stay much more consistent at just below freezing. At these temperatures, moisture in hardy bulbs freezes without damaging the cells of the plant. It is important, therefore, to plant your bulbs at the appropriate depth, to make sure that they are under enough soil to appropriately buffer against the winter cold. See this article by Dominic Christensen on soil temperatures during winter and the buffering effects of snow.

Sometimes, bulbs will emerge prematurely due to random periods of warmer weather in late winter, damaging tender growth of the bulb when the temperatures eventually dip back below freezing. For areas that are subject to freeze-thaw cycles (think, that weird February-March time when it is 70℉ one week and -30℉ the next) covering the soil with a 6-inch layer of mulch or straw will protect the bulbs from moisture and temperature fluctuations. Container gardens are more susceptible to freeze-thaw complications, so a protective layer is a must.

Critter considerations for bulbs left in the ground:

No matter how busy we gardeners are as we prepare our gardens for the winter months, critters are even busier foraging for food to bulk up before the snow falls. There are a couple of ways to keep animals from nibbling on bulbs in autumn, such as:

● Using physical barriers such as row covers, wire mesh, or cages around your bulbs to prevent animals from accessing them. For gardens susceptible to animal attacks from underground, lining the interior of your planting area with a barrier of wire mesh before burying bulbs, may offer protection. Just make sure that the material that you use has openings large enough such that root production of the bulbs is not hindered, while still small enough to block underground wildlife.

● Utilizing planting practices such as mixing in crushed stones into the dirt when you plant your bulbs to deter digging and chewing.

● Hiding evidence of bulbs by clearing away debris from dead foliage; or, by covering newly planted bulbs with a groundcover plant or a layer of mulch.

● Planting only bulbs that are unappetizing to critters, or mixing in your desired bulbs with bulbs that are unappetizing. Some examples of bulbs that animals tend to avoid are daffodils[1] , alliums[2] , hyacinths[3] , grape [4] hyacinths (Muscari varieties,) squills[5] , and snowdrops[6] .

TLC for tender bulbs:

Many varieties of bulb plants cannot survive our severe Minnesota winters. Some examples of tender bulbs include canna lilies, calla lilies, and caladiums. Dig these bulbs up in the late fall before the first frost, but after the greenery has died off naturally (important for allowing the bulb to gather energy for its next blooms!) They should be stored indoors in a cool, dry place.

For these bulbs, avoid freezing temperatures, which can damage the cells of the bulb. It is important to also avoid excess moisture, which can cause mold and rot, while allowing the bulbs to stay plump and retain enough water to survive the duration of storage. Dig up your bulbs and spray or brush off soil that clings to the bulbs. This is important: dry or cure your bulbs for three to seven days at room temperature in an area with plenty of airflow to remove excess moisture before storage. When you touch the cut end of the stem coming out of the bulb, it should feel dry. Otherwise, continue to cure the bulbs. Optional: dust with an antifungal.

Store only large, healthy bulbs, as damaged bulbs may not survive storage or may invite rot, and smaller bulbs may dry up before spring. Store bulbs in a container that allows plenty of airflow, such as a wicker basket, or a mesh bag. Bulbs should be stored at an average of 40 degrees without freezing, such as in a cold corner of a basement, a warm fridge, or a temperature-controlled garage. Check on your bulbs periodically (at least once a month) to remove any rotting or molded bulbs, as one bad bulb can spoil the whole batch!

Photo credits: University of Minnesota Extension (1,5), Creative Commons Crocus, Anatomy (2), en.wikipedia.org, all creative commons (3), www.wilsonbrosonline.com, all creative commons (4)