Asparagus is a hardy perennial vegetable that produces spears for 10-15 years. If you choose to grow it in a square foot or raised garden bed, you will need at least 18” (30cm) of high quality topsoil, and a location to dedicate to asparagus long-term.
Plant asparagus crowns 2-4 weeks before the spring frost date. Dig a trench 18”(30cm) deep and 12” (25cm) wide. Combine topsoil, compost, peat moss, and fertilizer, then refill the trenches with 6” of amended soil.
Place a mound of soil every 12” (25cm), and place the crown on the mound, spreading out the roots. The top of the crown should be 6-10” (15-25 cm) below the final level of the soil. The shallower plants will produce stalks sooner, and the deeper plants will produce them later. This is a way you can stagger the harvest.
Cover the crowns with 2” (5cm) of amended soil and water well. As the asparagus plants grow, fill in the trench an inch or two at a time, until it is even with ground level. As you do so, leave at least 2” (5cm) of stem above the ground, and do not cover the ferns (leaves).
In subsequent years, cut off dead ferns from the previous season in the spring.
In the fall, add fertilizer and 6” (15cm) of compost.
Soil and Fertilizer
Since asparagus will grow for so many years, it is important to improve the soil as much as possible before planting. The soil must drain well; if necessary, add sand or other amendments to ensure good drainage. Use high quality compost to mix with the topsoil for refilling the trenches.
Use a fertilizer with high nitrogen after harvesting in the late spring/early summer, and then a complete fertilizer late in the fall.
Fertilizer: 21-0-0 in late spring (after harvesting) 16-16-8 in late fall.
Do not harvest the first two years. It is important to let the plants get established. Harvest young spears 6-10” (15-25cm) less than 1” around by cutting them at ground level with a sharp knife. Be careful not to damage any other spears that are developing.
The third year, harvest for two weeks, then let the spears develop into ferns. Increase the harvest period to four weeks in the fourth year, and six to eight weeks every year after that.
- Asparagus Reference Sheet (Utah Extension Service)