A hundred caveats follow a statement like this. The Internet is full of advice against such a prospect, and most of it is correct. Excellent production requires ample and fertile ground. If you’re OK with a modest harvest, however, the doors of possibility fly open and even reveal a few advantages.
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Asparagus is a temperate crop that requires good drainage. For those who garden in places with a tropical climate or a low water table, the ability to manipulate the irrigation of container-grown stalks may be your best bet. Learning how to grow asparagus in containers also eliminates most perennial weed problems, making organic veggies much easier to cultivate. But no matter how you grow it, you need patience. It takes two years for plants to settle in and build up enough reserves to produce harvestable quantities.
Since container gardening changes the game in a few key ways, we’ve compiled a list of the most important things to consider when raising asparagus in this unconventional way.
Start With Crowns
Potted spears have a much shorter lifespan than their garden-grown counterpart—three to five years as opposed to upwards of twenty for in-ground asparagus. The goal for container asparagus is to maximize output right from the start rather than build a strong foundation for future harvests. Purchase 1-year-old crowns (dormant, bare-rooted starts sold by seed catalogs and nurseries in the spring) to save yourself a year of waiting, and choose a high-yield variety such as Jersey Knight or Jersey Supreme to ensure your small growing space will produce the most spears it possibly can.
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Select The Right Container
The good news is you don’t have to dig trenches. However, if you choose an inappropriate container, you may find yourself starting over again next year. Mature, in-ground plants reach a rooting depth of more than 10 feet, spreading as widely as 6 feet in diameter. A window box fit for pansies would stunt them, producing wire-thin stalks if anything. To grow in the confinement of a container, you must simulate soil depth. Keeping in mind that you will place your crown 6 inches below the soil surface, so your container needs to be about 18 inches (or more) tall, and at least a foot wide.
As long as they are large enough, provide proper drainage, and are made of an appropriate material for growing food, most containers are suitable. Glazed pots, terra cotta, wooden boxes, or molded plastic will all get the job done and can make a decorative addition to your growing space. For those with more utilitarian aspirations, a 30-gallon plastic storage tote with holes drilled into the bottom is an economical DIY option. Smart Pots or other lightweight cloth containers are the easiest and most affordable way to go big. Their 30-gallon planter fits several asparagus crowns, and the 100-gallon planter offers space for 12 crowns with a mere 3-foot diameter footprint.
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Asparagus should be planted in early to mid-spring. Place your container in a spot that gets an average of eight hours of sunshine and fill it with a quality organic potting soil a few scoops at a time until you’ve reached a depth of about 6 1/2-inches from the top of the container. As you go, press your fingertips into it to help the soil settle, taking care not to compact it. The soil at the top of this layer will be the root zone for your first-year crowns. Incorporate a complete organic fertilizer of your choice at the rate recommended on the packaging into the top few inches. Asparagus is a heavy feeder and regular fertilizing ensures strong, healthy specimens.
Now you’re ready to plant. Asparagus is sensitive to crowding: putting too many crowns in one pot is one of the biggest mistakes an eager container gardener can make, and it will result in unsatisfyingly skinny spears. However, because potted asparagus will not live long enough to grow as large as in-ground plants, you can get away with slightly tighter spacing than is typically recommend. Place your crowns so that their roots are spread into a sunburst, equally spacing their centers 6 inches apart from each other and the walls of the container. If you have a bit of extra room but not enough for another properly spaced plant, redistribute it among the crowns you’ve already placed. But resist the urge to crowd!
Top your crowns with 3 inches of soil, patting it down with your palms to make a level surface. Water it thoroughly and keep the soil moist but not wet. Once the plants have grown 3- to 4-inch shoots, add another 3 inches of soil, leaving a 1/2-inch space at the top, so water and soil don’t spill over the rim when you water.
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Though we grow it for its tender spring shoots, we must let some of those shoots stretch into mature plants so they can gather the energy needed for next spring’s flush. Asparagus tops, called ferns, are tall and ungainly like a floppy pine forest or feathery patch of bamboo. Let their 4- to 6-foot height work to your advantage by using the plants as an ornamental screen or backdrop. In most cases, especially in small spaces, you will need to provide support to keep them from slumping over—encircle them with string held up by bamboo poles, or purchase welded metal supports.
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Be sure to adequately water your pots. Uneven moisture availability weakens plants, especially in their first two years. Container soil tends to dry up more quickly than garden soil, so in a hot spell, you may need to water every day. Check to see if the soil is too dry by inserting your finger a couple inches below the surface—eyeballing isn’t always a sufficient indicator.
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For the first two years (the year you plant plus the next one), your containers will only grow whimsical ferns. In subsequent years, after the harvest period is over, allow each crown to keep six or eight ferns through the summer. Once the ferns have been killed by frost, cut them down and use them as protective mulch over the soil. Remove them completely once the plants come to life the following spring.
Each spring after you remove the winter mulch, scratch a side-dressing of complete organic fertilizer (in the quantity instructed on the product’s label) into the top 2 inches of the soil, removing any weeds that may be popping up. If the soil has settled, top it off with a layer of compost up to 1/2 inch from the lip of the pot.
The first harvest year, pick all asparagus spears once they reach 6 to 8 inches for two to three weeks, before allowing them to grow into ferns. In subsequent years, you can harvest spears for six to eight weeks. When harvesting, be sure to remove everything, even the spindly shoots or those too damaged to eat, as this will keep the plants productive.