How Big Do Asparagus Plants Get? (Plus 5 Common Questions)


If you are planning on growing asparagus in your garden this year, it helps to know how big they will get.  That way, you can plan ahead on the number of plants and the amount of space you will need.

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.

In this article, we’ll take a closer look at asparagus and how big these interesting plants will get. We’ll also discuss common questions about asparagus, ideal growing conditions, and time to maturity.

Let’s get going.

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 reach up to 7 feet (2.1 meters) as they grow to look like ferns. They are harvested for eating at 6 to 10 inches tall.

An asparagus plant can spread out over a width of 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 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.)

As the spears get larger, they become tough and fibrous. This makes them difficult to eat, so be sure to harvest before this happens!

Usually, asparagus spears are harvested at a height of 6 to 10 inches (15.2 to 25.4 centimeters). According to the University of Maryland, it is best to harvest asparagus by snapping the spears just below the soil surface.

If you wait too long, the spears will be so tough that you will need to cut them to harvest them. Even worse, they will be difficult to eat if they are harvested too late.

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 spears are 6 to 10 inches tall, with a diameter of 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 how fast 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, break off the spears just below the soil. If necessary, use a knife to cut them.

Be sure to store the harvested spears in a cool place. Eat asparagus soon after harvest, since they do not keep well.

How Often To Harvest Asparagus

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.

asparagus spear
Asparagus spears get thick and tough if left to grow too long.

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

The University of New Hampshire suggests that you should 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 Maryland suggests the following schedule for harvesting after transplanting asparagus crowns into your garden:

YearHarvest
Duration
(weeks)
0 (seeds planted)None
1 (crowns transplanted,
or 1st year after
planting seeds)
None
21
31 to 2
4+8 to 10
This table summarizes the harvest schedule
for asparagus, based on the plant’s age.

To increase your harvest every year, consider planting all-male hybrid varieties of asparagus.  These produce more (and thicker) spears and resist disease better than other varieties.

Here are a couple of all male (or mostly male) asparagus varieties to try:

  • Jersey Knight Asparagus – this asparagus variety produces medium to large spears, which are mostly male. It reaches a height of 4 to 5 feet and a width of 18 inches. It is hardy in Zones 3 to 8, and prefers a soil pH of 6.0 to 8.0. You can find Jersey Knight asparagus from Gurney’s.
  • Millenium Asparagus – this asparagus variety boasts an excellent yield of all-male spears. It was developed in Canada, so it tolerates cooler temperatures than other varieties (hardy in Zones 3 to 8). You can find Millenium asparagus from Johnny’s Selected Seeds.

Why Are My Asparagus Spears So Skinny & Thin?

Your asparagus spears may be skinny for a few different reasons:

  • The plant is still 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 in the first year after transplanting crowns.)
  • You harvested too many spears in the previous year.  The plant exhausted its energy reserves last year, and so it will produce fewer and thinner spears this year. (This is more likely in younger plants, but it is possible to over-harvest from established plants as well, especially if they lack nutrition.)
  • Last year’s ferns did not store energy. 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.)
  • It is time to stop harvesting. Asparagus spears come up thick at the beginning of the season, when they have lots of energy. As the season goes on, the spears come up thinner due to depleted energy reserves.
thin asparagus
Thin asparagus spears appear later in the season for older plants, after they use up some of their energy.

Of course, asparagus spears may end up spindly and thin due to environmental factors, such as:

  • Lack of water
  • Lack of sunlight
  • Lack of nutrients
  • Poor soil texture

(We’ll get into more detail on these factors later.)

Why Are My Asparagus Spears So Thick?

If your asparagus spears seem very thick, there are a few possible reasons:

What Does Asparagus Look Like?

Asparagus has an extensive root and crown system underground, but you will see long, green spears growing 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. This is due to a lack of chlorophyll.

In addition, asparagus spears can grow white in the absence of sunlight.  This does not happen with any specific variety.

Instead, the lack of sunlight prevents chlorophyll from forming. With no chlorophyll in the spears, there is no green color.

For more information, check out this article on asparagus from the Michigan State University Extension.

Do Asparagus Plants Multiply & Spread?

Yes, asparagus plants can multiply and spread out.  As mentioned earlier, their crown and root systems can grow to become quite extensive.

These crown and root systems will get larger in the first few years as the plant ages. This is why asparagus plants need so much space between them: the spears of adjacent won’t get tangled up, but the roots might!

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. 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”). Female asparagus plants produce these berries after they grow tall and spread out.

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 plants are not too hard to grow.  They are perennial and can live for a decade or longer with proper care.

According to the University of Minnesota Extension, asparagus prefers full sun. That means 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, it can survive in a much wider range of pH values (6.0 to 8.0).

There are many other factors that affect asparagus growth, including:

  • temperature
  • watering
  • fertilizing
  • spacing

Let’s start with temperature.

Temperature for Asparagus

According to the University of California, 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!

thermometer
Asparagus needs soil that is at least 50 degrees Fahrenheit 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.

This probe from A.M. Leonard can check soil pH, fertility, and temperature.

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 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). A cloche will 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. Just cut out the bottom of the bottle and put it over the spot where the seeds are planted.

water bottles
Cut out the bottom of a plastic bottle to make a cloche to keep seeds or plants warm.

The table below summarizes the temperature ranges for asparagus seed germination.

TempTemp
(degrees
F)
Temp
(degrees
C)
Minimum 50 10
Ideal 75 to 85 24 to 29
Maximum 95 35
This table summarizes the temperature
ranges for asparagus seed germination.

According to Virginia Tech, asparagus plants grow best at a temperature of 60 to 65 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 check 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.

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

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. Otherwise, wait to water.

Try to water early in the morning, rather than at night. This allows the 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.

The best part is, you can make your own compost from kitchen scraps and yard waste. For more information, check out my article on making compost.

The University of Minnesota Extension suggests adding a balanced 10-10-10 fertilizer at a rate 1 to 1.5 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, start by digging a trench to plant them in a row.  Leave 18 inches (46 centimeters) between plants.

asparagus plant
Plant asparagus 18 inches apart in a row. Leave 5 feet between rows.

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 planting 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 (at that point, they will turn yellow or brown).

asparagus berries
Let asparagus ferns grow after harvest. They store energy in the roots to produce spears next year.

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 in the next year.

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

Conclusion

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.

You might also want to read my article on why asparagus plants fall over.

I hope you found this article helpful – if so, please share it with someone else who can use the information.

If you want to read some of my most popular posts, check out the “Best of GreenUpSide” page here.  Enjoy!

~Jonathon

jonathon.david.madore

Hi, I'm Jon. Let's solve your gardening problems, spend more time growing, and get the best harvest every year!

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