At this point in the season, gardeners growing asparagus may be wondering when to cut down those tall, skinny stalks that have grown from their asparagus plants. Wait until the ferns are fully brown and dormant to maximize the next year’s harvest.
First, a brief biology lesson
Those tall stalks with thin, wispy leaves are called “ferns.” The ferns are the part of the plant that emerges from the “spears” during the summer if they are left to grow rather than being harvested.
Asparagus spears are harvested for 6 to 8 weeks in the spring. The harvest is over when we stop cutting the spears and allow them to grow into ferns.
It is important to let the ferns grow instead of harvesting spears all summer long. Every time we remove a spear, we remove a stem of the asparagus plant. So after a few weeks of harvest, it is important to stop harvesting and let the ferns grow.
The ferns stay green until they go dormant in the late fall or early winter. During this time, they are photosynthesizing - harvesting light - and producing energy that the plant needs in order to keep producing spears the next year.
In the fall as the plants prepare for winter, they move sugars (energy) from the ferns down to the roots. The more energy they have stored over the winter, the healthier they will be the next spring.
Don’t be too hasty cutting down the ferns
The longer the ferns are allowed to grow in the fall, the more energy and nutrients they are able to move down to the roots for the following year.
Therefore, the key is to leave the ferns alone until they are totally brown. This change in color signals that they are done for the year and can be removed.
If possible, it is best to remove the dormant, brown ferns in the late fall or early winter (mid-November to December). This helps control asparagus beetles that would otherwise overwinter in the ferns and damage the next year’s spears.
Sometimes in Minnesota snow begins covering the ground before the ferns turn brown. Snow can make it impossible to cut off the ferns, forcing gardeners to wait until the spring to remove them. When that happens, remove the ferns as soon as possible in the spring.
What to do with cut ferns
After cutting down the ferns, the best practice is to remove them from the garden and destroy them. Again, this helps control asparagus beetles that may be taking up winter residence inside the ferns.
Rather than placing the ferns on a compost pile where asparagus beetles and diseases can still survive under cool winter temperatures, either burn them or place them in your yard waste containers for removal.
For more information on growing asparagus in Minnesota, see Growing asparagus in home gardens and the Minnesota Asparagus Project, a collaboration between UMN Extension and the Sustainable Farming Association.