The asparagus is coming up here in zone 5, and my mouth is already watering. Asparagus is the quintessential seasonal food, only available from the garden for a month or two in early spring. But this quirky perennial tastes so good that it more than earns its space in my garden (see below for planting, care, and harvesting tips).
Flavor aside, asparagus provides impressive health benefits. According to this 2007 analysis, asparagus likely has the highest antioxidant activity of any vegetable, higher even than broccoli. Antioxidants protect your cells from free radicals, so eating lots of antioxidants from fruits and vegetables may help prevent some cancers and other diseases.
One 2016 study from the Journal of Food Nutrition and Research found that in addition to being high in antioxidants, asparagus is a great source of prebiotics. Humans can’t digest prebiotics, but the beneficial bacteria in our large intestines love them. A gut full of good bacteria (specifically bifidobacteria) correlates with a lower risk of colon cancer, lower cholesterol levels, and improved mineral absorption. But asparagus begins to break down soon after harvest, so having a source close to home will increase its nutritional value as well as its taste.
Asparagus grows differently from other plants in the garden. Recently reassigned to its own family (Asparagaceae) from the Lily Family, it is more closely related to onions than to anything else in the vegetable garden. Each plant is either male or female, producing only pollen (males) or seeds (females), whereas most garden plants make both pollen and seeds. The female plants grow thicker stems and pretty, red berries late in the summer. These berries are not edible, but also (as a bemused poison-control employee informed me after some research) are not likely to harm a two-year-old who happens to confuse one with a strawberry. Some varieties of asparagus are all-male hybrids that grow more uniform stems and no berries.
Asparagus stores carbohydrates in root crowns; all the above-ground stems and leaves die back in the fall. The familiar, edible spears are the first thing to appear each spring and can be harvested for 4-8 weeks. Then the stems must be allowed to grow without being cut for the rest of the summer to let the plant produce and store carbohydrates for the following spring’s growth. Summer visitors to my garden are often surprised to learn that the tall, fern-like foliage belongs to asparagus plants, as it bears so little resemblance to what we eat.
Asparagus requires patience: you won’t get to harvest for two years. But planting asparagus is a garden investment that rewards you with delicious flavor and good nutrition every spring for 15 years or more.
Asparagus Planting and Care:
- When: Plant asparagus root crowns in April. Asparagus can be started by seed, but root crowns are simpler for home gardeners.
- Where: While asparagus will grow in almost any climate, it only produces edible spears after a cold winter or a drought forces it into dormancy. Asparagus needs full sun: take care to site it where it won’t be shaded or shade other plants (it can grow 6 feet tall or more). Consider incorporating asparagus in the back of a sunny perennial border if you don’t have room in your vegetable garden.
- How: Buy root crowns from your local plant nursery. Plant at least 10 crowns. This extension specialist recommends 20 crowns for each family member, but perhaps you should only count true asparagus lovers in this calculation since this would yield quite a lot. Dig a ditch 8 inches deep and about 6 inches wide and place crowns 12 inches apart. Position the crown so that the stem faces up, spreading the roots out below. Allow several feet between rows. Cover the crowns with a few inches of soil. As the stems grow, fill in the rest of the ditch around the stems with compost-enriched soil, being careful not to bury the leaves.
- Care: Weed throughout the growing season. When the mature stems and leaves turn brown in the fall, remove them and add a layer of compost and then straw. Unless you plant male hybrid asparagus, lots of baby asparagus will germinate by seed. These won’t produce as much as your established asparagus and will need to be weeded out.
- Water: Water asparagus a few times a week for the first 4 years. Since mature asparagus roots can grow up to 10 feet deep, after year 5 you only need to water once every 2-3 weeks. Drip hose irrigation is preferable to sprinklers.
- Harvest: Do not harvest any asparagus the first year after planting. Harvest for up to 4 weeks the second year after planting. Every year after that, harvest for 6-8 weeks (see below for more detail), and then let the plants grow without pruning. To harvest, cut or break all spears at ground level when they are 9 – 12 inches tall, before they begin to grow branches and look like ferns. Do not let any spears grow beyond this stage during the harvest period.
- More on the Last-Harvest Date: One 2006 experiment published in the Journal of Vegetable Science used carbohydrate levels in root crowns to establish the ideal date for the last asparagus harvest in the spring. The authors caution that harvesting for too long reduces yields the following year. While calculating the root carbohydrate levels would be impractical for the home gardener, the authors suggest an approximate last-harvest date of May 31st in Washington State, supporting another study in Michigan that suggested the same last-harvest date. A harvest period of 50-60 days is standard, but if in doubt, it’s better to end the harvest early than to risk a lower harvest in later years.
Cembali, T., Folwell, R. J., McCluskey, J. J., Huffaker, R. G., & Wandschneider, P. R. (2006). Economic analysis of the inter-year effect of alternative harvesting strategies for asparagus. Journal of vegetable science, 12(1), 29-50.
Drost, D. (2003, August). Asparagus in the Garden. Retrieved from http://extension.usu.edu/files/factsheets/asparagus.pdf.
Fritz, V. A., Rosen, C. J., Hutchison, W. D., Becker, R. L., Beckerman, J., Wright, J. A., Tong, C. B. S., Nennich, T. (2017). Asparagus Production Guide. Retrieved from http://www.extension.umn.edu/garden/fruit-vegetable/asparagus-production-guide/index.html.