Watch Out for Winter Annuals

By Andrew Scott | The Gardens on Spring Creek Horticulturist

This is the time of year you think you would usually be curled up inside with a warm drink planning your garden aspirations for spring, but it’s also a crucial time to get ahead on your gardens’ weed management. Winter annuals tend to fly under the radar (or under the snow) and can end up wreaking havoc on a garden if left alone. These plucky plants don’t tolerate the intense heat of summer well, so they finish setting seeds in the spring which lay dormant in the soil until fall rolls around. 

Once the weather cools off and gets wetter, the seeds germinate and get to work establishing shallow roots that can take advantage of any seasonal moisture before being insulated from winter’s cold under a blanket of snow. Come springtime — or during a very mild winter like we had earlier this season — the annuals are already established and ready to bask in the sun, drink in the spring rains, make more seeds, and die before most perennials even start to come out of dormancy. This makes controlling seed production the primary goal when dealing with these weeds.

Many winter annuals start off as leafy rosettes collecting the waning autumn sunlight. One such annual is redstem filaree (Erodium cicutarium) which is easily identified by a rosette of lightly hairy, oak- or fern-like leaflets. Asters like horseweed (Erigeron canadensis) and prickly lettuce (Lactuca serriola) have similar looking rosettes to each other — elliptical leaves with notched/toothy margins — and can be easily confused, but if you must tell them apart, look for prickles under the midvein of prickly lettuce or sparse hairs on horseweed leaves. 

A number of annual mustards can also be identified by their winter rosettes. Field pennycress (Thlaspi arvense) has lightly wavy spoon-shaped leaves with rounded fruits, while shepherd’s purse’s (Capsella bursa-pastoris) leaves are more elliptical and heavily notched with heart-shaped fruits. Hairy bittercress (Cardamine hirta) has lots of rounded leaflets and is especially important to remove as soon as you can. As its seed pods dry, they explode with the slightest touch, flinging hundreds of seeds several feet away from the parent plant.

Mints like henbit (Lamium amplexicaule) and red deadnettle (Lamium purpureum) are especially opportunistic weeds, sometimes pushing two waves of germination in early spring and again in the fall if conditions are right. Both have square stems and purple two-lipped flowers, although their leaves are quite different: henbit leaves are kidney-shaped or completely encircle the stem further up, while red deadnettle leaves are more triangular to heart-shaped, shingling over one another.

We can’t forget annual grasses either: annual bluegrass (Poa annua) and cheatgrass (Bromus tectorum) are what you’re most likely to see emerge in Northern Colorado in early spring. Vegetatively, annual bluegrass is more or less smooth and will form a small, patchy sod, while cheatgrass looks like multiple hairy stems coming from a shared point.

If you have seen these weeds in your garden before, now is a great time to take a close look at your beds and get out there to do some hoeing. If it never occurred to you that winter is also a time to garden, you’ll definitely want to keep an eye out for these wee weeds popping up!

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