For several years in the 1980s, my wife and I lived just north of Germany's Black Forest, where one culinary highlight in late spring and early summer is white asparagus. More tender, milder, and nuttier in flavor than green asparagus, it quickly became our favorite vegetable, though we'd never particularly liked the green type. In general, white asparagus is preferred over the green type in Europe, though some Americans find it less sweet. White seemed to be the only kind of asparagus grown in southwest Germany, and today, it's the only kind I grow in southwest Virginia.
Actually, white asparagus is not genetically different from the green kind. It's simply any asparagus grown in the dark, or blanched. The traditional way to blanch asparagus is to mound mulch or sand around the spears as they emerge. But that technique is a lot of trouble, because it must be done daily, and it makes the spears grubby. All that's really required to blanch them is to keep them under an opaque cover.
I grow my asparagus in a 4-foot-square raised bed, framed by 2 x 12's of lumber. I use pressure-treated lumber, but if you're leery of pressure-treated wood in the garden, consider applying a wood preservative to untreated wood or using rot-resistant cedar. The resulting structure looks like a sandbox. During each spring harvest, I cover the entire bed so the spears are in complete darkness, except during the few minutes required to cut spears every couple of days.
I use a simple method to keep the plants in the dark: I cover the raised bed with another box that's basically a second 4-foot-square frame with a top made of 1/2-inch plywood. The asparagus shoots grow inside this box. I simply set the top on the raised bed, without hinges or other attachments.
Untreated pine is fine for this, because it's lighter and cheaper and doesn't rot during its brief period of annual use. (The top stays in the basement the rest of the year.) However, the top is fairly heavy (about 75 pounds) and awkward to maneuver. Many gardeners would need help. I wanted the cover to withstand the strong spring winds here in the Appalachian foothills. An alternative in other regions might be to use 1 x 12's, then weigh down the top with bricks if necessary.
Even if I were growing green asparagus, I would still use a deep raised bed filled with rich soil to ensure vigorous plants and harvests. In fact, I use the same kind of bed for my strawberries and all my annual vegetables.
Here are some tips for constructing sturdy raised beds out of 2 x 12's. First, use treated, preserved, or rot-resistant lumber as described above. Untreated wood rots in a few years, and replacing it around a bed like this is awkward. To avoid gaps and soil leakage, make sure your cuts are straight and square. Use three 4-inch deck nails (the ribbed kind used in deck construction) per joint. Smaller or nondeck nails don't hold as tightly.
If you like, you can trim the top edges of the bed with 2 x 4's. Attach them wide-side-up to the edges of the raised bed frame. The molding makes it easier to set on the top cover.
Early spring is the best time to plant asparagus in most regions, except if you live where winters are mild. In that case, plant in late fall. Start by amending the soil with compost. Asparagus prefers soil that is neutral to slightly acidic, so if necessary, add lime to correct acidity or sulfur to correct alkalinity. The plants come as crowns, which are clumps of roots. Nine crowns, planted 2 to 4 inches deep and about 15 to 16 inches apart, will fill a 4-foot-square bed in about three years. Both male and female plants produce edible spears, though all-male varieties are said to be more productive. (I haven't noticed much difference.)
'Mary Washington' and 'Jersey Knight' are good varieties, and plants are readily available from mail-order catalogs. 'Mary Washington' may be more rust-resistant. 'Jersey Giant' produces such large spears that the largest among them may be a little tough and stringy. Based on experience, I would avoid no-name bargains. They may be inferior strains being unloaded by a breeder.
Each year, when the first spears begin to emerge (around the first of May in my area), I apply a cheap, fast-release lawn fertilizer, such as 30-3-3, and water it in. This encourages good top growth for the harvest period. Then I put on the top. The bed won't receive rain, of course, while the top is on, but I seldom have to water, because the covered soil doesn't dry out much.
Surprisingly, air circulation isn't a problem, either. During my first harvest, I created some openings for ventilation. Not only did this turn out to be unnecessary, but it let in a little indirect light, which made the spears pale green instead of white.
Japanese beetles are the main pests in my garden, so I put out a trap. The other common pest is asparagus beetle. Fortunately, though, in both cases, my harvest is over before the beetles begin theirs, and by that point they cause relatively little damage compared to the size of the plants. I have rarely had to apply a pesticide.
An occasional problem is rust, which shows up in a few wilted spears. A standard vegetable-garden fungicide seems to cure it (but maybe the problem would clear up on its own). After cutting off the infected spears, I drench the soil with the fungicide, trying to avoid getting any directly on the good spears that remain. (To be safe, I wash those spears thoroughly before cooking.)
Be sure to harvest -- or at least peek under the top -- every two days or so throughout the cutting season. The spears grow fast in the dark, and if you go too long without checking, some will grow too tall and bend after reaching the plywood ceiling.
Many growers say it's best not to harvest any spears in the first two years. That's probably true, but raised beds like these are so efficient that it really doesn't hurt to harvest for two to four weeks the second year. After that, you can harvest for four to six weeks each spring, or until most of the spears start getting too spindly to be worth cutting.
When the harvest is over (early June in western Virginia), I remove the cover. The plants need full sun in summer. I apply a complete fertilizer, such as 10-10-10, immediately after the harvest and again in midsummer.
Uncut asparagus spears eventually grow into 6-foot-tall fernlike bushes, with attractive red berries on female plants. An important point: Let the plants grow as big as they want. Don't cut them back, because top growth is proportional to root growth, and you want maximum root growth if you also want a maximum harvest. Little weeding is needed due to the dense shade provided by the cover.
In all but the hottest climates, plants die back to the ground in the fall. Either way, cut them down then. After that, because the soil level usually sinks a little during the growing season, I top it off in late winter with an inch of fine bark mulch. It's attractive and eventually decomposes into soil. But because pine bark acidifies soil, I check the soil pH annually and sometimes add some fireplace ashes or lime to bring the pH closer to neutral.
Several dishes, such as veal Oscar, call for white asparagus. In Germany, cooks make a mouthwatering cream of white asparagus soup. But we usually just steam ours until barely tender, then add butter, salt, and freshly ground black pepper. (Children often prefer asparagus to other vegetables, because it's one veggie that proper etiquette permits us to eat with our fingers.) We try to save our white asparagus for special meals, though. If you've never tried it -- and especially if you think don't like asparagus -- you're in for a treat.
Christopher O. Bird is the author of Modern Vegetable Gardening. He lives and gardens in Riner, Virginia.