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How to Grow Asparagus, a Perennial Vegetable

Caren White is a Master Gardener and instructor at Home Gardeners School. She has been associated with Rutgers Gardens for over a decade.

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Everyone knows that those succulent spears of asparagus are a vegetable, but not everyone knows that they are a perennial vegetable that grows every year with little care.

What is Asparagus?

Asparagus (Asparagus officinalis) is a perennial flowering plant that is thought to be native to Europe. In the US, it is hardy in zones 4 through 9. Asparagus has been eaten since Roman times. The part that is eaten, the spear, is actually the spring growth.

The plants are dioecious, meaning there are male and female plants. The male plants are needed to fertilize the flowers of the female plants, which produce berries containing seeds. Modern hybrid asparagus is bred to be all male so that the plants do not waste energy making seeds. Most gardeners start their asparagus from “crowns” which are one year old plants rather than from seed. The crowns are sold in the spring. They look dry but they are just dormant. Asparagus roots can tolerate the dryness of being out of the soil. Look for crowns that are not mushy or withered. They should smell of asparagus.

Female asparagus plants produce berries

Female asparagus plants produce berries

What is the Difference Between Green, Purple and White Asparagus?

Asparagus spears were originally green. New hybrids have been developed that have purple spears. Purple asparagus has a higher sugar content than the green variety and it is less fibrous. The spears turn green when cooked.

White asparagus is just green asparagus that has been blanched while growing. Blanched means that it has been covered in soil or in a shade tunnel so that it is not exposed to sunlight. Plants that have not been exposed to sunlight are white because they lack chlorophyll which develops in the presence of sunlight. Chlorophyll is what gives plants their green color. White asparagus has no fibers and is sweeter than green asparagus.

White asparagus is just asparagus that has been blanched while growing.

White asparagus is just asparagus that has been blanched while growing.

How to Prepare Your Garden For Planting Asparagus

A bed of asparagus can be productive for 25 or 30 years, so preparing the bed is of the utmost importance. To determine how large the bed should be, decide how many plants it will contain. A good rule of thumb is 10 plants for each member of your family. So if you are a family of 4, you will need 40 plants. Each plant needs to be 12 to 18 inches from its neighbor. Rows should be 4 to 5 feet apart. Mature plants are 5 feet tall and 3 feet wide.

Choose a sunny spot in your vegetable garden that is well away from any areas that are tilled every year. An asparagus bed must remain undisturbed for the long life of the plants. Have a soil test done. Based on the results, amend the soil appropriately, making sure that it has a neutral pH of 6.0 to 7.0.

Remove all weeds from the bed. Asparagus plants when they are young cannot compete with weeds. A good idea is to cover the planting area with black plastic the summer before you plant your bed to smother and kill all the weeds.

Lastly, remove all large stones from the bed.

Mature asparagus plants are 5 feet tall and 3 feet wide

Mature asparagus plants are 5 feet tall and 3 feet wide

How to Plant Asparagus

Asparagus is planted in trenches and hilled up as the plants grow, like potatoes. Early in the spring as soon as the soil can be worked, dig a trench in your garden that is 8 to 10 inches deep and 18 to 20 inches wide. Pile the soil that you have removed from the trench on either side of it.

Place each crown in the bottom of the trench, carefully spreading out the roots. Space the crowns 12 to 18 inches apart. Cover with 2 inches of soil and water well. When the plants start to grow, gradually fill in the trench using the soil you piled on either side of the trench leaving just a few inches of plant above the soil until you have completely filled in the trench, just like you would do for potato plants. Once the trench is filled, allow the plants to continue to grow. Keep your plants well-watered. Young asparagus plants need lots of water.

How to Harvest Asparagus

You will have to be patient with your new asparagus plants. They won’t be ready to harvest for 3 years. The first year, the plants are getting established in their new home, mainly growing out their roots. Side dress with compost in the spring and the fall to keep the soil fertile. In the fall, when the foliage turns yellow, cut it down and compost it.

Some gardeners like to leave the dead foliage until spring. Leaving it can provide places for harmful insects to hibernate during the winter so I always remove all dead plants and foliage from my vegetable garden. Cut the dead foliage down to 2 inches from the soil.

Fall is also a good time to give your bed a good weeding. If you allow weeds to take hold in the bed, they are nearly impossible to pull out once entangled in the roots of the asparagus plants.

The second year, keep up with the side dressing in the spring and fall. You may be able to harvest a few spears this year. In the fall, cut down the dead foliage.

Your patience will be rewarded in year 3. This is when you can begin harvesting spears. The harvest will only last about a month because the plants are still young. In later years as they mature, the harvest can go on for up to 2 months.

Every other day, check your bed. When you see spears that are 8 inches tall and as thick as your finger, you can either snap them off or cut them below the soil line with a sharp knife. Most gardeners prefer to snap them off. Using a knife has two drawbacks. The first is that it can spread disease from plant to plant. The second drawback is that if you are not careful, you may cut new spears that haven’t yet emerged from the soil yet.

The reason you want to check so often is that the spears grow quickly. Once they start to get woody and sprout foliage, they are no longer edible. So check often!

Continue your harvest until the new spears that appear are very thin and spindly, about the circumference of a pencil. Asparagus is a cool season plant. When the weather warms in the late spring, the plant stops growing vigorous spears that are good to eat. Stop harvesting and allow the spindly spears to mature into plants for the summer. Cut down the dead foliage in the fall.

Don’t forget your spring and fall side-dressing of compost! The soil needs to be fertile to continue to produce healthy plants for decades.

Harvest spears that are 8 inches tall and as thick as your finger.

Harvest spears that are 8 inches tall and as thick as your finger.

How to Store Asparagus After Harvest

The minute you cut asparagus spears, they start to get fibrous. This is a natural reaction of the plant to protect it from invading disease and insects. Freshly harvested asparagus should be cooked and eaten immediately. In the real world, this is not always possible. Most of us choose to store our asparagus in the refrigerator up to a week until needed.

Wash your freshly harvested spears and dry them carefully. Any water left on them will cause them to rot. Wrap the cut ends in a wet paper towel to keep them moist and discourage formation of fiber. Place your spears with their paper towel in a plastic bag in your crisper drawer. Alternatively, you can place them in a cup of water in the refrigerator. The water will keep the ends moist and free of fibers.

You can freeze asparagus. Blanch your freshly harvested spears in boiling water for 3 to 5 minutes. Immediately immerse them in cold water to stop the cooking process. Wrap them in plastic and freeze. Frozen asparagus remains edible for up to one year.

© 2019 Caren White

Comments

Caren White (author) on January 22, 2019:

Thanks, Pollyanna! I hope your asparagus produces for many years.

Pollyanna Jones from United Kingdom on January 22, 2019:

A very helpful guide! I have some asparagus plants that hopefully should be producing his coming spring as they will be two years old then. I've just laid down an inch of good rotted compost over the bed, so hopefully it will give them a real boost. I can't wait to taste the result!