It has been a bountiful year in the asparagus patch, and as the harvest starts to wind down, here are some tips from David Trinklein, of the University of Missouri Division of Plant Sciences, to maintain or improve next year's productivity.
From now until the end of the growing season this fall, asparagus plants (ferns) must produce and store compounds that are a result of the photosynthetic process. In essence, they are "plant factories" that supply the energy to the crown and storage roots for next year's crop. Anything a gardener can do to keep healthy, vigorous ferns throughout the post-harvest growing season will increase asparagus yield and quality the next season.
If an established asparagus planting begins to produce small, thin spears, one or more problems might be the cause. This condition may be a sign of low fertility, an extended harvest of spears the previous growing season or severe winter temperatures.
A new asparagus bed may take several years before abundant, thick spears are produced. Well-planted and established asparagus beds can be productive for as long as 20-30 years or more, if soil fertility is maintained and weeds are controlled.
To maintain soil fertility, liberal amounts of fertilizer should be applied as soon as each harvest season has ended in June. Well-rotted manure or compost can be applied at the rate of about one bushel per 30 square feet of bed area. Alternatively, a compete, balanced fertilizer such as 10-10-10 or 12-12-12 may be added at the rate of about 1 cups per 10 feet of row. Scatter the fertilizer lightly and uniformly, but do not allow it to touch the stems of the asparagus plants.
Since vigorous growth of ferns is important from the time harvest ends until fall, good weed control is essential as well. Although cultivation and hand pulling always are options, in larger plantings, there are several herbicides labeled for the control of weeds in asparagus that might be employed. Pre-emerge herbicides with long residual activity (e.g., trifluralin or simazine) can be applied in the early spring before spears begin to emerge to control annual weeds. Pre-emerge herbicides will not control established weeds. To accomplish that, a post-emergence herbicide is needed.
Post-emergence herbicides may be applied either before asparagus spears emerge, during harvest or after the harvest season ends, depending on the herbicide used. For example, glyphosate, a non-selective herbicide, can be used to control established weeds after harvesting the final cutting of spears and no asparagus foliage remains above ground. It is important not to allow non-selective herbicides to contact asparagus spears or ferns, since plant injury may occur. Whatever the method employed, weeds are easier to control with post-emergence herbicides when they are relatively small (less than 6 inches tall).
Table salt once was commonly used to kill weeds in asparagus plantings, since asparagus is fairly salt-tolerant and most weeds are not. This practice is now discouraged since the prolonged use of salt leads to the deterioration of the physical properties of soil and a reduction in plant growth.
Another pest that warrants monitoring is the asparagus beetle, a common insect that feeds on asparagus. Both adult and larvae must be kept under control for maximum plant growth and future spear production. Pesticides labelled for control of asparagus beetle (e.g., carbaryl) should be used whenever the pest becomes problematic. Normally, chemical control is warranted when 5-10 percent or more of the ferns show beetle infestation. When using any pesticide, always read and follow label directions carefully.
Peter Sutter is a life-long gardening enthusiast and a participant in the MU Extension's Callaway County Master Gardener program. Gardening questions can be sent to [email protected]