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Ask a Master Gardener Cool spring allows for delayed asparagus planting

by Peter Sutter | May 8, 2022 at 3:59 a.m.

Over the last week, I've had a couple small asparagus harvests. I always look forward to spring asparagus in my garden and a few wild sprouts in the fence rows. Although it is a little late to plant, most gardening has been delayed by our cool, wet spring, so I would say go for it.

David Trinklein with the University of Missouri Plant Science & Technology had an excellent article to get you started, here are some highlights:

March though mid-April is an ideal time to establish a planting of asparagus in Missouri which, with proper management, should last 10-15 years of more.

Because asparagus is a perennial vegetable, attention should be given to choosing the best planting site. Like most vegetables, asparagus will not tolerate wet, soggy soil. Choose well-drained soil, or use raised beds to promote drainage. Since weeds are a major problem, try to select a site with as few weeds as possible.

A soil sample should be taken the fall or spring before planting to determine its nutrient status. The optimal pH for asparagus is 6.5-7; lime may need to be incorporated into the soil before planting. Before planting, broadcast and incorporate fertilizer about 20 pounds of 10-20-10 or a similar fertilizer per 1,000 square feet of garden area.

For best results, asparagus should be planted in the spring as early as the soil in the garden or field can be worked. In Mid-Missouri, that's usually early to mid-April.

Normally, asparagus is started using dormant crowns. When available, male crowns should be used since male plants are up to three time more productive than female plants. Examples of all-male asparagus varieties include Jersey Giant, Jersey Supreme and Jersey Knight. Select healthy, 1-year-old crowns from an inspected nursery. Separate crowns by size, and plant similar-sized crowns together to encourage uniform growth. If crowns cannot be planted immediately, store them in a refrigerator.

To plant, make a 4- to 6-inch-deep furrow. Cover the fertilizer or mulch with an inch of soil, and space the crowns 12-18 inches apart in the furrow. If a variety produces large-diameter spears, you should reduce spacing within the row to decrease spear size. Each row should be no less than 5 feet apart so the ferns can close canopy and shade weeds during the summer. If rows are spaced too close together, spear size may be reduced. Cover the crowns with about 2 inches of soil and, as the plants emerge and grow, gradually fill in the furrow throughout the summer.

Weed control is the most challenging component of successful asparagus production. Asparagus is a poor competitor with weeds. On small plantings, very light cultivation with a hoe may be used to remove weeds, but avoid using a power rotary tillers or any other tillage implements that can damage the crown, reduce yields and promote diseases. Organic mulches such as grass clippings, wood chips, straw/hay or compost can be applied 4-6 inches thick to suppress weeds.

Several herbicides are labeled for weed control in asparagus. Glyphosate can be used as a contact spray to control winter annual and biennial weeds early in the spring before the spears emerge and after the last harvest. Cover crops such as rye or wheat may be spring-seeded in row middles to suppress weeds. Common rock salt once was used to control shallow-rooted weeds in asparagus because asparagus is deep rooted and can tolerate some salt. This practice is no longer recommended because the salt can damage soil structure by creating a crust that impedes water infiltration.

Asparagus can be harvested for a limited time (two weeks) the second year after planting crowns. Overharvesting one year can weaken the plants and decrease yields the following year. Three years after planting the crowns, asparagus can be harvested for five to eight weeks. Each year during the first several years of production, yields will increase if the planting is managed properly.

Asparagus spears are best harvested by snapping them off by hand near ground level. Most gardeners prefer to snap the asparagus spears when they reach 7-9 inches in length in cool weather (less than 70 degrees), or 5-7 inches in warmer weather (more than 70 degrees), and the spear tip is tight. Cutting with a knife is generally not recommended because it may spread diseases. Expect to harvest every one to three days as temperatures increase. Harvesting should stop when the majority of spears are the diameter of a pencil (less than 3/8 inch). Harvesting for a longer period will weaken plants and lead to poor production the following spring.

After harvest season has ended, the asparagus planting should be fertilized to stimulate summer and fall fern growth. A complete, balanced fertilizer (like 13-13-13) can be applied at a rate of about 1.5 cups per 10 feet of row. Herbicides can be applied after harvest to control weed growth. Frost will desiccate the ferns, and they can then be cut in late fall or early winter. Do not mow ferns in early fall while they are still green because this will reduce the following spring's harvest.

This may sound like a lot of work, but once you get the asparagus bed started, it will bring years of rewards. Happy gardening!

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]

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