Plant asparagus where there is plenty of sun in the spring and the soil is rich in organic matter and well drained. As a guide, you will need about 20 to 40 crowns for each person; 20 for the spring harvest and more for a second or third harvest later in the season.
Keeping the asparagus patch productive will require a few simple tasks each season.
Here is a season-by-season guide for asparagus growing and care:
Spring. Asparagus spears will be ready for harvest shortly after the soil temperature reaches 50°F in early spring—that’s when spears will begin to emerge. Two week before spears begin breaking through the soil, pull the winter mulch back and cultivate the asparagus patch lightly loosening the soil and uprooting weeds.
Begin the harvest when spears are 6 to 8 inches long; that’s when they are most tender. Don’t let spears grow taller; they will become tough. Harvest spears daily during the harvest period; hand snap each spear off just below the soil surface.
Harvest will last 2 to 10 weeks depending upon the age of the plants. The first harvest year pick spears for 2 weeks; this will allow roots to become established and grow strong. Each year after, increase the spear harvest by a week.
Once spears have started to grow tall, weed the growing bed once more then place a 4 to 6 inch layer of straw or hay or dried grass around plants for the remainder of the harvest and growing season. Mulching after spears have emerged will keep weeds down and preserve soil moisture through the warm time of the year. Weeds that grow up through the mulch can be easily pulled by hand. An alternative is to sow annual rye grass around asparagus plants in spring. Rye grass will crowd out small weeds and will die off in winter.
Late spring. Plant for future asparagus harvests in late spring. Sow asparagus seed 1½ inches deep and 2 inches apart in loose, well-drained soil. Sow seed when the soil temperature is between 70 and 75°F. Seeds should emerge in 10 to 20 days. Keep the soil moist and control weeds. Later, thin plants to 12 inches apart.
Transplant asparagus crowns (one-year-old rhizomes) in spring about the same time you transplant out tomato plants. Set crowns at the bottom of a trench 10 to 12 inches deep and cover with 2 to 3 inches of soil. Set crowns 12 inches apart. As plants begin to grow cover the new growth with 2 inches of soil every few weeks until the trench is filled. When spears shoot up the first year, let them leaf out and grow on.
After harvest in spring, feed each plant an organic fertilizer to support top growth through the summer; summer growth will determine how good the following year’s spears will be. Aged compost and well-rotted manure will feed asparagus beds. (Add nutrients in spring before spears emerge and again after the last harvest.) A rich asparagus fertilizer is three parts greensand, two parts cottonseed meal, one part dried blood, and one part bone meal applied at the rate of 2 pounds per 50 square feet of bed.
During the warm season, keep weeds down with a thick layer of straw, hay, or dried grass clippings—fresh grass clippings contain too much moisture to use as a mulch without first drying; they can become slimy and moldy if applied freshly cut. Weeds will compete with asparagus plants for nutrients and diminish both yield and spear size.
Summer. After harvest, let the plant’s fernlike foliage grow tall. Summer growth allows asparagus roots to grow large and store energy for the following year’s spears. Stakes and string will keep plants upright. In breezy areas, plant rows parallel to the prevailing wind so that plants can support each other. Crowns grow upwards about 1 inch each year, so spreading compost across the planting bed or along rows in summer will both feed plants and raise the soil surface.
To get a second or more harvests each year, plant twice the number of plants or more needed for each person (40 rather than 20) then harvest only half of the bed in spring and let the other half grow on. In early summer, cut down the top growth in the half of the bed that was not harvested. This section will send up new spears in a few weeks that can be cut in late summer or early fall for a second harvest. If the growing bed is large enough, you can divide the bed into sections and use this method for a new 2 month harvest every 8 weeks, multiple asparagus harvests each year.
Late Fall and Winter. In mild winter regions, cut down ferny top growth after it has turned brown in late fall or winter. (Even in autumn, asparagus tops may look dead but are still storing energy for spear production the next spring.) In cold winter regions, don’t cut back top growth; let it provide extra protection for roots from freezing temperatures.
(You can allow asparagus berries to mature and drop each fall. In spring, thin out new plants started from dropped berries; allow some new plants to establish and replace older plants.)
Where you cut back top growth, mound 3 or 4 inches of soil over crowns or add 3 to 4 inches of aged manure and compost across the bed to protect roots from cold temperatures. An alternative is to mulch the bed with 8 inches of loose straw or hay and add phosphorus and potassium rich cottonseed meal and wood ashes across the mulch. Deep mulching is important in cold regions to protect asparagus from tip-kill in spring and feeding is important for future spear production. A decline in spears is commonly the result of lack of nutrients.
Adding 4 to 6 inches of aged compost over planting beds and asparagus crowns each year effectively increases the depth of the crowns which in turn increases spear size and tenderness the following spring. When spears become spindly, crowns may have grown too close together and older plants should be thinned out.
Leave winter mulch in place until the danger of a freeze has passed in late winter or early spring, then pull the mulch back and ready the bed for the spring harvest.