Why Bulbs Don't Freeze in Winter

Bulbs are designed by nature to withstand cold winter temperatures. Indeed they rely on winter’s cold to trigger the biochemical process necessary to bring the bulb to flower in spring.

While winter soil may actually freeze to depths beyond which the bulbs are planted, soil temperature will rarely fall below 29° F or 30° F (-1°C). At these just-below-freezing temperatures, water in the cells of the bulb may freeze but the cells will not be harmed. Also, as is true for many hardy plants, cold temperatures trigger starches in bulbs to break down into glucose and other small molecules. This simple sugar or glucose, interacting with other small molecules, acts in much the same way as salt on a winter sidewalk. The sugar in the bulb, like the salt on the sidewalk, lowers the temperature at which water freezes.

This fortunate chemistry helps to keep bulbs safe and snug in their winter beds. Other factors that help keep soil temperatures within tolerable limits include an insulating snow cover and, in colder areas, a nice layer of mulch over the bulb bed once the ground temperatures have dropped.

Who's a Deadhead?

Despite the opinions of certain music aficionados, dead head is an ancient term that has nothing to do with a certain group from San Francisco. It refers to the act of removing withered flower heads after bloom to discourage flowers from going to seed. The act of setting seed can use up as much as 30 percent of the energy of tulips in spring. That’s why it’s smart to dead head tulips, encouraging subsequent bloom while providing a tidier look. Daffodils, on the other hand, reproduce differently from tulips. There is no need to dead head them after bloom. In order to regenerate for new growth next spring through photosynthesis, the foliage of all spring bulbs must be left in place to die back for a minimum of six weeks after bloom. After this period, the withered leaves may be cut back to ground level.

Yikes - Forgot to Plant These!

Each year, a certain percentage of busy gardeners belatedly realize that the flower bulbs they bought in September or October have not yet been planted -- and they are left wondering, guiltily, what to do with the little rascals. If this sounds familiar, take heart. For one thing, it may not be as late as you think.

Rule #1: When in doubt, plant the bulbs. If bulbs should be planted in October in your area and you’re looking at unplanted bulbs in December or January – get them in the ground. It’s not optimal but it’s certainly not impossible. Plant the bulbs if it’s at all possible to dig in the ground (look for mulched beds which don’t freeze as quickly). Bulbs are programmed by nature to “want” to grow and late-planted bulbs generally still grow and flower, though not always at peak performance.

However, if you’re looking at unplanted spring-flowering bulbs in March or April, you’re probably out of luck.

The reason is simple. Though flower bulbs look like nothing more than brown lumps, they are actually living things. Nestled inside each is a tiny embryonic flower complete with leaves, surrounded by layers of plant food ready to nourish the bulb to bloom. All that a spring-flowering bulb needs is to be planted in a somewhat timely manner for rooting and a period of sustained cold to activate a bio-chemical process that stimulates it to send forth its glorious spring blooms.

Spring bulbs? Summer bulbs?
Flower bulbs fall into two main categories: spring-blooming and summer-blooming bulbs. Spring bloomers include tulips, irises, crocuses, narcissi (daffodils), snowdrops, grape hyacinths, hyacinths, alliums, and a whole lot more. Summer bloomers include gladioli, dahlias, freesia, canna, begonias, nerines, and others.

What makes them different? Spring bloomers are known as “hardy” bulbs, programmed by nature to need a cold period before they can bloom. Not only can they survive the cold, they need it. That’s why they must be planted in fall. Fall planting allows bulbs time to root, then settle in for a long, cold “beauty sleep” prior to spring growth. For tulips, for example, this translates to 12 to 16 weeks in the dark with a sustained soil temperature below 50° F. In most areas, the optimal planting time is when autumn night-time temperatures drop down to the 40° to 50° range.

Most summer bloomers, on the other hand, are not hardy – and cannot survive sustained temperatures so low. They are known as “tender” bulbs. Summer-flowering bulbs are available for sale in spring and should be planted outdoors after the local threat of frost is past. In the fall, many people dig up summer bulbs to store them indoors in a cool dry place over the winter for planting again the following summer.

Among the exceptions: lilies! These summer bloomers are winter-hardy perennials and can be planted in either spring or fall.

The Squeeze Test
About those bulbs you forgot to plant in fall: The best way to tell if they are still viable is to gently squeeze them. If they are firm, not dry or spongy, they are probably still okay – though no guarantees. Plant them immediately (if there’s any chance, they’ll grow). After all, nothing ventured, nothing gained. If the ground is too hard to work, plant the bulbs in pots and keep them in a cool unheated area with temperatures between 38°F and 50° F – a home refrigerator works just fine. Water them – and keep the soil moist but never soggy. After eight or more weeks (depending on the bulb type), bring a few pots into the warmth each week to initiate growth indoors. Or, once spring begins to warm things up, move the pots outdoors to bloom.

Flower bulbs are among the easiest plants to grow successfully. But that ease comes with a few caveats. Planting in a timely manner is one of them.

Mid-Winter Thaw Won't Nip Buds

If bulbs in the garden sprout during a mid-winter thaw, will the next cold snap damage the flowers? Probably not. Healthy, spring-flowering bulbs that have sprouted prematurely are pre-programmed by nature to shrug off the return of extreme cold and even snow. In a hard frost, the buds may be blighted, or the tips of the leaves may get frost burn, but in almost every case they will flower. Smaller bulbs, such as snowdrops, crocuses, Eranthis, winter aconite, and mini-narcissi are actually meant to bloom early, often peeking through the snow.

About Winter Mulch

Spreading mulch over fall-planted flower bulbs is a good idea. However mulching isn’t advised for the reasons commonly thought. Most people we talk to think you mulch bulbs as soon as you plant to keep the soil warm so the bulbs won’t freeze over winter. Actually, mulch is applied later, once the ground gets colder, to keep the soil temperature consistently cool over-winter. The goal is to minimize damage from frost heaves and help retain moisture in the soil through the winter.

Plant bulbs approximately six weeks before local hard frosts typically start but wait until the cold weather is upon them to mulch their bulb beds. If you mulch too early, overly warm soil conditions can promote disease and mildew. Also, premature mulching invites mice, voles and other unwanted critters to nest in your bulb beds – poor you – and lucky them to find such warm cozy dens for the winter!

Courtesy of the International Flower Bulb Center (www.bulb.com)