Seven steps to growing superior asparagus spears, year after year.
Photo courtesy DAVID CAVAGNARO
Fresh from the garden, asparagus is the very essence of spring. The sweet, slender spears are at their best lightly steamed and topped with a bit of butter and maybe a drop of lemon juice. Of course, you can enjoy your bountiful crop in plenty of other ways, too: sautéed, roasted, grilled, in sauce or microwaved, alone or with other veggies in soups, stir-fries and salads. Any way you slice it, asparagus is one of the most delicious vegetables you can grow.
But the beauty of asparagus is more than skin deep. One of the few perennial vegetable crops, asparagus comes back year after year, producing pounds of succulent spears for 15 to 20 years or more. In fact, when it comes to productivity, asparagus is difficult to top. A modest planting of 25 all-male crowns (dormant roots) will yield up to 20 pounds of edible spears per year — that’s 400 pounds or more over a 20-year period! And it’s simple to store your bounty of spring asparagus in the freezer — just blanch it in boiling water, then chill in ice water before you pop it in the freezer.
Asparagus is not difficult to grow, either. While it takes some time to get a crop started, if you choose your variety and site wisely, then provide basic care as outlined here, you’re on your way to decades of good eating.
1. For highest yields, choose “all-male” varieties. Until about 20 years ago, all asparagus varieties were a mixture of male and female plants. But Rutgers University researchers developed a method for propagating only the male plants (the female plants produce seeds). These “all-male” asparagus varieties — including ‘Jersey Giant, 'Jersey Supreme’ and ‘Jersey Knight' — produce up to three times more than older, open-pollinated male/female varieties, such as ‘Mary Washington.’
That’s because they put all of their energy into producing spears rather than seeds, according to Chee-kok Chin, a professor of plant biology and pathology at Rutgers. That also means male plants do not produce volunteer seedlings, which compete against the established plants and reduce yields. All-male hybrids also are more disease resistant than older varieties. In fact, one of the best ways to avoid asparagus rust, a fungal disease that reduces yields, is to plant rust-resistant varieties.
2. Take time to make a cozy bed. Remember that your asparagus will call this site “home” for many years to come, so it’s best to choose a good location and prepare it carefully, says Mathieu Ngouajio, assistant professor of horticulture at Michigan State University.
“To avoid disease problems, choose a site where corn or asparagus did not grow the year before,” Ngouajio says. If you can plant a cover crop, such as sorghum or rye, in the area where you plan to plant asparagus, you’re in luck. “A cover crop [turned into the soil before planting] will increase the soil’s organic matter, which is very beneficial to asparagus,” he says.
Native to warm climates, such as Asia Minor and the Mediterranean region, asparagus thrives in full sun and rich, well-drained soil. Incorporate plenty of high-quality, aged compost into your asparagus site to improve drainage, boost soil fertility and reduce the chance of soil-borne fungal diseases, such as fusarium crown and root rot. If you don’t have compost, add grass clippings or shredded leaves. Planting in raised beds also will improve drainage.
You may want to test your soil to be sure the pH level is in the neutral range of 6.5 to 7.5. Plant pathologists have found that lower pH values may promote fusarium disease in asparagus. If the tests show a low pH, the testing lab can tell you how much limestone to add to your soil in order to neutralize the acidity.
3. Get a jump on weeds. They can be one of the biggest challenges for organic asparagus growers. To reduce perennial weed problems, be sure to eliminate them from the site before planting, and be vigilant about routing them out in years to come.
“Before we planted our new asparagus bed, we prepared the soil carefully,” says Charlotte Johnson, co-owner with her husband, Glen, of Mother Flight Farm in Mt. Vernon, Wash. The Johnsons decided to start a new asparagus patch after their previous, 15-year-old patch became hopelessly invaded by quack grass and thistles, possibly brought in with some straw mulch.
“After tilling the area, we allowed the dormant weed seeds to germinate, then burned them off with a flame weeder before we planted the asparagus.” Now that their patch is becoming established, the Johnsons stay on top of any new weeds by flaming the entire bed in late fall to winter, after the asparagus has gone dormant.
Other growers find that a combination of cultivation and mulching does the trick. In a five-year trial of organic versus conventional asparagus culture, conducted by Mark Hutton, extension vegetable specialist at the University of Maine, weeds were kept at bay by applying a thin layer of bark mulch around plants and cultivating between rows. “After the final harvest this year, we also did a shallow (1 1/2-inch deep) cultivation with a tiller over the entire area to eliminate grasses,” he says.
In cold locales, such as Gene Thiel’s Prairie Creek Farm in Joseph, Ore. (elevation 4,150 feet), black plastic mulch not only stops weeds, but also warms soil and conserves moisture. Thiel says the 4-foot-wide perforated plastic, used between rows, warms the soil enough to allow him to harvest spears two weeks earlier than usual in the spring.
4. Plant in trenches. After the days have warmed to about 50 degrees, plant asparagus crowns in trenches, 6 to 12 inches deep, and about 12 inches wide. (Use the shallower depth for heavy clay soils, Ngouajio says.) By starting your asparagus crop with crowns, you’ll be able to harvest them a year earlier than if you plant seeds. Spread finished compost in the bottom of the trench, then space the crowns about 12 to 18 inches apart inside the trench, mounding the soil a bit beneath each crown. Cover the crowns with about 2 inches of soil. Gradually add more soil (mixed with compost, if available) over the next four to six weeks as the plants grow, until the trench is level with the surrounding soil.
5. Resist the temptation to harvest spears the first season! For the best future harvests, allow the underground crowns to become well-established during the first year or two after planting. If you do not harvest the spears (which are actually the plants’ newly emerging shoots), they will develop into tall, attractive plants with lacy leaves.
“The vegetative part of asparagus — the tall, leafy growth sometimes called ‘ferns’ — is the part that captures sunlight and energy, which is stored in the crowns. The more energy stored in the crowns, the better your crop will be the following year,” explains Thomas Orton, extension vegetable specialist with Rutgers Cooperative Extension in New Brunswick, N.J.
“It’s important that a gardener look at their asparagus plants and not push them before they are ready,” says Robert Dufault, professor of horticulture and vegetable physiology at Clemson University. For instance, if the plants’ ferns grew lush and tall (about shoulder-height) the first year, it’s OK to harvest the spears for about two weeks the second spring (instead of the usual six to eight weeks), Dufault says. He also recommends waiting one more year before harvesting if the spears did not grow that tall, or if new emerging spears look spindly, to ensure robust plants the following year.
6. Stop picking when spears grow spindly. By the crop’s third season, you should be able to pick a full harvest — that means you can pick all the spears that emerge over six to eight weeks. To harvest asparagus spears, simply snap them off by hand where they naturally bend; a knife is not necessary, and can actually spread disease from one plant to another. Plus, by snapping off only the tender stalks in the garden, you’ll save a step in the kitchen by avoiding the need to remove the fibrous stems.
Be sure to harvest the spears before their tips begin to open, however. Once the tips open, the spears become tough. Warm temperatures encourage tips to open faster, so you may need to harvest daily if your area experiences a sudden warm spell. In cool, spring weather expect to harvest taller spears a couple of times each week. As soon as you see most of the spears coming up spindly — about the diameter of a pencil or less — stop harvesting. Spindly spears are a sign that the plants are stressed, says Dufault.
7. Take good care of your “ferns.” Keep in mind that your asparagus plants will continue to grow after you finish harvesting the spears in spring. And it’s that tall ferny growth that the plants produce from June through September that is most critical to the success of next year’s crop. Pamper your ferns by ensuring that they receive adequate nutrients and water (at least a half inch per week). Orton advises applying compost on top of the bed right after harvest is completed.
And again, stay on top of competing weeds and insect pests. Jeff Cantara, owner of New Roots Farm in Newmarket, N.H., uses a homemade portable chicken pen to fertilize his asparagus crop, while controlling asparagus beetles — the quarter-inch long, black or red-spotted insects that sometimes damage ferns. After he’s finished harvesting his spears for the season, Cantara moves the 5-by-12-foot wooden frame covered with poultry netting to the asparagus bed (with young laying hens inside), then moves the enclosure to a different location in his patch every three or four days.
“I add a little grain to the soil to encourage them to scratch,” he says. “We had some asparagus beetles before, but not since the chickens have been cleaning up the beds. They don’t hurt the asparagus ferns, and the chicken manure gets incorporated into the soil.”
Although asparagus aficionados sometimes differ when it comes to the finer points of planting and care, all agree on the fundamentals: Give your asparagus plants a little bit of love and they’ll reward you richly for many years to come!
1. ‘Jersey Giant’: Medium to large green spears with purplish bracts (scaly leaves); resistant to fusarium and rust disease; cold tolerant
2. ‘Jersey Knight’: Similar to ‘Jersey Giant’ in size and appearance with thick, flavorful spears; highly resistant to rust; tolerant to fusarium; adapted to most climates
3. ‘Jersey Supreme’: Slender to medium diameter green spears; high yielding and uniform in size; good rust resistance; adapted to temperate, cool and warm regions
4. ‘Guelph Millennium’: Developed by the University of Guelph in Ontario; high-quality green spears; requires rust control; excellent for cold regions, including Canada and the upper Midwest
In 2008, look for several new all-male varieties — including a purple one — that will offer enhanced disease tolerance and wider regional adaptation.
5. ‘Atlas’: Combines the heat tolerance of ‘U.C. 157’ (an older California variety) with the productivity of the Jersey varieties; tips stay tight in higher temperatures (70 degrees and above) longer than with other varieties
6. ‘Purple Passion’: Large reddish-purple spears that turn green when cooked; said to have a slight nutty flavor and be milder, sweeter and more tender than green-speared asparagus; susceptible to rust
Vicki Mattern is a contributing editor for MOTHER EARTH NEWS magazine, book editor and freelance magazine writer. She has edited or co-authored seven books on gardening, and lives and works from her home in northwestern Montana. You can find Vicki on Google+.