If you’ve ever wondered how a patch of weeds seemed to appear overnight in early spring, fully grown, flowering, and already setting seed, the answer is that they didn’t grow overnight at all. They started last fall.

Winter annuals are plants that follow an unusual life cycle. They germinate in late summer or autumn, spend the cold months as small, low-growing rosettes tucked close to the ground (often sitting under snow or frost), and then resume growth as soon as conditions warm up. By late spring or early summer, they flower, set seed, and die, completing their entire life cycle in one continuous stretch that just happens to span two calendar years.

How this differs from other plant life cycles

• Summer annuals germinate in spring and complete their whole life cycle, including growth, flowering, seed set, and death, within the same growing season. Most classic garden annuals fall into this category.
• Biennials also overwinter after germinating, but they need a full second growing season of vegetative growth before they’re ready to flower.
• Winter annuals skip that second season of vegetative growth. They’re already primed to flower as soon as the weather turns, which is exactly why they seem to spring up so fast.

Where you’ll find them

Winter annuals are common across Canadian gardens and disturbed ground, including roadsides, field margins, gravelly or sandy soils, and neglected corners of the yard. Familiar examples include shepherd’s purse, chickweed, and cleavers, all of which regularly overwinter as small rosettes and shoot up rapidly at snowmelt.

One especially notable winter annual is Arabidopsis thaliana (thale cress), better known as the workhorse “lab rat” of plant genetics and the first plant to have its genome fully sequenced. In the wild, though, it’s an inconspicuous little weed of disturbed soils, found across much of southern Canada, that most people would walk right past without a second look.

The same pattern in a major crop

If you listen to farm reports, you’ll know about winter wheat. It is essentially a cultivated version of this same life cycle. Sown in fall, usually September or October, it germinates into a short tuft of leaves before winter sets in. It sits in that low, semi dormant state through the cold months, protected by snow cover, then resumes active growth as soon as the ground thaws and is ready for harvest by midsummer, months earlier than spring sown wheat.

Many winter annual plants, winter wheat included, actually require a period of sustained cold before they’re able to flower at all, a process called vernalization. Skip the cold exposure, and the plant tends to stay stuck in vegetative growth even if planted in spring instead. It’s a genuine physiological requirement, not just a matter of timing. Winter wheat is grown across the Prairies, though spring wheat remains more common in Canada overall, since our winters can cause significant winterkill without reliable snow cover.

Ornamentals gardeners grow this way on purpose

Several garden favourites are deliberately sown in fall to take advantage of the winter annual pattern. Larkspur, bachelor’s buttons, Shirley and corn poppies, calendula, and sweet alyssum are commonly scattered directly into the garden after the ground has cooled but before it freezes solid. They germinate that fall or very early the following spring, overwinter as small seedlings, and flower weeks earlier than they would from a spring sowing. Larkspur and poppies in particular tend to resent root disturbance, so direct fall sowing suits them well since it avoids transplanting altogether.

Pansies and violas are grown this way too in milder climates, planted in fall and flowering through a mild winter before fading with summer heat, though in a colder climate they’re more reliably grown as early spring plants instead. Forget me nots also frequently self-seed and behave as winter annuals or biennials, overwintering as small rosettes before flowering the following spring.