How Big Do Asparagus Plants Get?


If you are planning on growing asparagus in your garden this year, you might be wondering how big they will get.  That way, you can plan the number of plants and the amount of space you will need for your crop of asparagus.

So, how big do asparagus plants get?  Asparagus plants have spears that can grow up to 7 feet (2.1 meters) tall!  However, the spears are harvested for eating when they are 6 to 10 inches (15.2 to 25.4 centimeters) tall, before they start to look like ferns.  An asparagus plant has a spread of 2 to 3 feet (61 to 91 centimeters) above ground.  The crown and root system of an asparagus plant can grow up to 6 feet (1.8 meters) in diameter and 15 feet (4.5 meters) deep.

Of course, the quality of your asparagus (if you get any at all!) depends on the care that you give your plants.  Let’s take a closer look at asparagus, including size, growing conditions, and time to maturity.

How Big Do Asparagus Plants Get?

An asparagus plant can grow spears as tall as 7 feet (2.1 meters) tall.  However, they start to produce flowers and fruit long before they get this tall.

asparagus plants
Asparagus spears can grow as tall as 7 feet (2.1 meters) as they grow to look like ferns. However, they are usually harvested for eating at 6 to 10 inches tall.

Also, as the spears get larger, they become tough and fibrous, making them difficult to eat.  Usually, asparagus spears are harvested at a height of 6 to 10 inches (15.2 to 25.4 centimeters).

An asparagus plant can spread out 2 to 3 feet (61 to 91 centimeters) above ground.  The underground crown and roots can grow up to 6 feet (1.8 meters) in diameter, and to a depth of 15 feet (4.5 meters).

Such an extensive root system is apparently great for the longevity of the asparagus plant!  According to the University of New Hampshire Extension, asparagus plants can live for 10 to 15 years or longer.

How Do You Know When To Harvest Asparagus?

You should harvest asparagus spears when they are 6 to 10 inches (15.2 to 25.4 centimeters) tall.  This usually means the spears will be at least 0.5 inches (1.3 centimeters) thick.

asparagus plant
Harvest asparagus when the spears are 6 to 10 inches tall, with a diameter of at least 0.5 inches. You may need to harvest every 1 to 3 days when asparagus is in season.

On average, this will mean harvesting asparagus spears every 1 to 3 days, depending on quickly the plant is growing.  According to Virginia Tech, you can expect 3 to 4 pounds of asparagus for every 10 feet of asparagus plants.

When harvesting, use a knife to cut the spears near the soil.  Be sure to store the harvested spears in a cool place and eat them soon afterwards, since they do not keep well.

If you wait too long to harvest asparagus, the spears will become too tall and thick.  This makes them tough and fibrous (stringy), and they will be difficult to eat.

On the other hand, if you harvest your asparagus too much in one year, it will cause decreased growth and yields in the next year.  This is especially true for young asparagus plants.

The University of New Hampshire suggests that you not harvest asparagus until the third year after planting.  The first two years of growth should be reserved for building up energy reserves in the roots of the plant.

The University of Michigan Extension suggests harvesting spears from mature asparagus plants until June 30.  Then, let the spear grow into ferns, which can produce and store energy in the roots and crown.

The University of Maryland suggests the following schedule for harvesting after transplanting asparagus crowns into your garden:

YearHarvest Duration
0 (seeds planted)None
1 (crowns transplanted, or
1st year after planting seeds)
None
25 to 7 days
310 to 14 days
4+8 to 10 weeks

To increase harvests every year, consider planting all-male hybrid varieties.  These produce more (but thinner) spears and resist disease better than other varieties.

Why Are My Asparagus So Skinny?

Your asparagus spears may be skinny for a few reasons:

  • The plants are too young, and so the spears are thin due to an immature asparagus plant.  You will see this in the first two years after planting seed, or the first year after transplanting crowns.
  • You harvested too many spears in the previous year.  The plant exhausted its energy reserves, and so it will produce fewer and thinner spears this year.
  • You did not allow last year’s asparagus ferns to grow large enough to produce and store energy (carbohydrates) in the roots.  This can happen if the ferns are pruned too early (more on pruning asparagus later).

Of course, asparagus spears may end up thin due to environmental factors such as lack of water, sunlight, or nutrients (more on this later).

What Does Asparagus Look Like?

Asparagus has an extensive root and crown system underground, but you will see long, green spears above ground.  Each plant produces several of these spears in a season.

Although the spears are usually green, there are asparagus varieties with purple spears.  For example, check out Purple Passion Asparagus on the Burpee website.

white asparagus
Asparagus can grow white when not exposed to sunlight.

In addition, asparagus spears can grow white in the absence of sunlight.  For more information, check out this article on asparagus from the Michigan State University Extension.

Do Asparagus Plants Multiply and Spread?

Yes, asparagus plants can multiply and spread out.  As mentioned earlier, their crown and root systems can grow quite extensive, and will get larger in the first few years as the plant ages.

If you want to multiply (propagate) asparagus plants, wait until the fall, after all the ferns die back.  Then, dig up the roots and cut the plant into several pieces, each of which has some of the root system.

Then, replant these parts in different areas.  Just be careful when handling the plants, and try not to damage the roots, which can cause transplant shock and death.

Asparagus can also multiply and spread by their seeds, which are held in small red seeds pods (“berries”). For more information, check out my article on the red berries on asparagus.

asparagus berries
Asparagus “berries” contain seeds of the plant.

Are Asparagus Hard to Grow?

Asparagus are not too hard to grow.  They are a long-lived perennial that prefers full sun (8 or more hours of sunlight per day).

sunlight through forest
Asparagus needs full sun: 8 or more hours of sunlight per day.

Asparagus prefers well-draining soil with a pH of 6.5 to 6.8.  However, there are many other factors that affect asparagus growth, including temperature, watering, fertilizing, and spacing.  Let’s start with temperature.

Temperature for Asparagus

The minimum temperature for asparagus seed germination is 50 degrees Fahrenheit (10 degrees Celsius).  If the soil is any colder than this, you will see low germination rates – that is, if you can get any seeds at all to germinate!

This is nature’s way of protecting asparagus seeds from sprouting at a time when they will be unable to survive.

The maximum temperature for asparagus seed germination is 95 degrees Fahrenheit (35 degrees Celsius).  If the soil is any warmer than this, germination rates will decrease.

Combined with high humidity, high temperatures can encourage the growth of mold, which is another threat to your plants.

According to the University of Illinois Extension, asparagus should be planted as soon as the ground can be worked in the spring (after the soil thaws).  The University of Maryland suggests transplanting asparagus plants 12 weeks after planting from seed.

The ideal (optimal) temperature for asparagus seed germination is between 65 degrees Fahrenheit (18.3 degrees Celsius) and 85 degrees Fahrenheit (29.4 degrees Celsius).

When transplanting established asparagus crowns, put them out in early spring when they are still dormant.  To avoid setting out asparagus plants too early, Check out this calculator from the Old Farmer’s Almanac to find the last spring frost date in your area.

Keep in mind that these temperatures refer to soil temperature, not air temperature.  If you want to find out the soil temperature, use a probe-type thermometer to check.

If the thermometer reads a temperature that is too low, then you have some options.  One option is to wait until the sun warms up the soil.

To speed up this process, clear away any debris, such as leaves or grass clippings, from the soil surface.  Also make sure to choose a location for planting that gets plenty of sun, so that it can warm up the soil faster.

If you are worried about a short growing season, you can also use a cloche (a plastic or glass cover) to trap some heat and warm up the air and soil near your asparagus seeds.

A cloche can be made from a plastic water bottle to retain warmth and humidity in the soil for seeds or seedlings as they grow.

For more information, check out the table below, and check out this article from the University of California on ideal seed germination temperatures.

Seed
Germination
Temperature
Temperature
(degrees
Fahrenheit)
Temperature
(degrees
Celsius)
Minimum 50 10
Ideal 75 to 85 23.9 to 29.4
Maximum 95 35

According to Virginia Tech, asparagus plants grow best at a temperature of 60 to 5 degrees Fahrenheit (15.6 to 18.3 degrees Celsius).

Watering for Asparagus

Asparagus need regular watering, so keep the soil moist.  This is especially important in sandy soil, which drains quickly and does not hold water for long.

garden hose
Be careful not to over water or under water your asparagus plants!

According to the University of Minnesota Extension, asparagus plants will not show signs of drought stress.  For this reason, it is important to watch the soil carefully and keep it watered.

Putting mulch on top of your soil will help to retain moisture, especially during periods of hot, dry weather.  If you find that you have a problem with dry soil, check out my article on how to treat dry soil.

On the other hand, over watering your asparagus plants (or any plants for that matter) can lead to root rot and eventual death.  The best way to decide when to water is to feel the soil with your fingers.

If the soil feels dry 2 or 3 inches below the surface, then go ahead and water.  For more information, check out my article on over watering your plants.

Try to water early in the morning, rather than at night, to allow water to soak into the soil before evaporating.

Fertilizing for Asparagus

According to the Old Farmer’s Almanac, adding 2 to 4 inches (5 to 10 centimeters) of compost to your soil before planting asparagus is a good way to improve drainage for clay soil, improve water retention for sandy soil, and add nutrients to your garden.

compost bin
Compost adds nutrients to your soil and allows you to recycle yard waste and kitchen scraps.

For more information, check out my article on making compost.

You can also add a balanced 10-10-10 fertilizer at a rate 2 pounds per 100 square feet.  Avoid fresh manure or fertilizers with high nitrogen content, since both of these can burn your plants!

For more information, check out my article on over fertilizing your plants.

After adding compost and fertilizer, you can also put mulch over the top of the soil.  This will help to help to retain moisture and insulate against heat.

Spacing for Asparagus

When planting asparagus, you should start by digging a trench to plant them in a row.  You should leave 18 inches (46 centimeters) between asparagus plants.

The rows themselves should be 5 feet (1.5 meters) apart.  When you plant asparagus crowns, carefully spread the roots out in the bottom of the trench so that they are not compressed or broken.

Then, cover the asparagus crown with 2 inches of soil.  As the plant grows, add more soil gradually.

Ultimately, the tops of the crowns will be 6 inches (15 centimeters) below the surface of the soil.  For more information, check out this article on asparagus from the University of New Hampshire Extension.

Pruning for Asparagus

When you prune asparagus, do not cut back the ferns right away after harvest.  Instead, wait until the end of the season, when the ferns are completely dead (they will turn yellow or brown).

asparagus berries
Let your asparagus ferns grow after you are done harvesting. They will store energy in the roots to produce spears next year. Then, after they die, you can prune them back.

In the fall, the ferns send nutrients and energy (carbohydrates) down to the asparagus crown. If you prune the ferns too early, the crown will not get enough nutrients.  Early pruning can be one cause of skinny (thin) asparagus spears the next year.

For more information, check out this article on asparagus from the University of New Hampshire Extension.

Conclusion

By now, you have a much better idea of how big asparagus plants get.  You also know a bit more about the care that is necessary to ensure a healthy crop of asparagus in this year’s garden.

I hope you found this article helpful – if so, please share it with someone else who can use the information.  If you have any questions or advice about asparagus, please leave a comment below.

jonathon.david.madore

Hi, I'm Jonathon. I’m the gardening guy (not guru!) who is encouraging everyone to spend more time in the garden. I try to help solve common gardening problems so that you can get the best harvest every year!

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