Asparagus beds last 20 or more years and take quite a while to reach peak production. That creates a lot of pressure to get it right the first time! Because of the high stakes here, I've reviewed a dozen different research papers and consulted experts in order to provide you with the most accurate information.
How does asparagus actually grow?
Underground, the asparagus 'crown' contains roots and buds. The roots can grow as deep as 6 feet. The buds grow plant stems above ground, which are what we refer to as spears (the part we eat). When those spears are not harvested, they continue to grown into ferns. Those ferns photosynthesize the sun into energy for the plant roots for next growing season.
Each crown produces multiple buds and spears each season, up to 10 or more at peak productivity.
Temperature & soil requirements
Asparagus performs best between 70-85 degrees during the days. Warmer temperatures cause them to grow fast and fern out on shorter spears (which can also cause woodiness). Some varieties perform better in colder or warmer climates.
Asparagus prefers well draining soil, but some varieties like Millennium can handle heavier soils with clay. It's common advice that asparagus doesn't like wet feet, which can cause the roots to rot, however several books on rain gardens claim that asparagus can live happily and actually helps soak up water with its deep roots.
The best soil pH is neutral, around 6.5-7.0. Slightly higher and lower can be tolerated.
Asparagus plants are either male or female. Open pollinated varieties have both male and female plants, whereas hybrid varieties have been created with all-male plants.
These all-male hybrids were created because male plants produce significantly higher yields, from 50% up to 3 times more than open-pollinated varieties. Female plants produce less spears, but they tend to be slightly larger.
- Mary Washington: green, heirloom, open-pollinated variety
- Millennium: green, all-male hybrid, very high yielding, very cold hardy.
- Apollo: green, open-pollinated variety, more tolerant of heat
- Atlas: green, open-pollinated variety, more tolerant of heat
- Purple Passion: purple, open-pollinated variety. Larger spears and 20% more sugar than green varieties.
- Pacific Purple: purple, open-pollinated variety. Larger spears and 20% more sugar than green varieties.
- UC-157: green, open-pollinated variety, doesn't do as well in cold climates. Usually only sold as seed.
- Jersey Knight, Jersey Supreme, and other Jersey Series: discontinued. (All male-hybrid, green).
Read the guide on asparagus varieties for your climate.
How many crowns to buy
There are two deciding factors, how much you plan to eat, and how much space you have in your garden.
How much asparagus will you eat?
A bundle of asparagus at the grocery store is about 1 pound. How many of those bundles do you buy in a week?
Asparagus is harvested over 8 weeks during peak production. Each crown produces about 10 spears in that time frame, which is about a ½ pound. So 25 crowns would be about 12-18 pounds over 8 weeks (or 1.5 - 2 bundles / week).
How much space do you have?
Asparagus crowns can be planted quite efficiently without affecting the yield or quality. You can actually plant 25 crowns in 36 square feet. Compare that to traditional spacing guidelines (12-18 inches apart with 3-5 feet between rows), which requires 100 square feet for 25 crowns.
See "How to grow asparagus in less space" for more information.
How to plant asparagus
1. Dig the trench
Asparagus crowns are planted below the soil level in a trench, called a furrow. The crowns are placed in the trench then covered with a couple inches of soil, but the trench isn't filled to the top (yet!). After the first shoots start to come through the soil, then the rest of the trench is filled.
- Sandy soil: 8-10" deep
- Silt and loamy soil: 8" deep
- Clay soil: 5-6" deep
Consider planting the crowns in a double or triple row, where the rows are 12 inches (30cm) apart. In this case, dig 2 to 3 trenches, 12" apart on center. See "How to grow asparagus is less space" for more information."
2. Soak crowns
Right before planting, so the crowns in water for 30 minutes to an hour. Any longer and you might be risking some rot damage.
3. Plant crowns
Crowns are placed in the furrows head to toe (bud to root) 12" apart. There is no need to fan the roots out. According to the University of Minnesota, "Some older growing guides recommend this practice, which is often referred to as an “octopus formation,” but it is time-consuming, and research does not support the claim that it leads to improved plant health or vigor."
If you are only planting a single row, you can space the crowns as close as 6 inches (15cm) without lowering the yield.
For more details on these high-density plantings, see "How to grow asparagus in less space."
4. Partially fill trench & water
After you plant the crowns, quickly cover them with a couple inches of soil before they dry out. Water the plants and soil in the trench.
As spears start to emerge from the ground about 2-3 weeks later, you will add another couple inches of soil. Continue until the entire trench is filled over the season.
5. Keep the bed weed-free
Keep weeds out of the asparagus bed. The roots need to get established and competition from weeds can significantly slow that down.
6. Lets plants 'fern out' (do not harvest this year!)
The first summer you plant asparagus, don't cut any of the spears. Let them 'fern out.' This lets the plant soak up the sun and convert it to energy for the roots, creating a strong, healthy plant.
7. Cut back dead ferns
Asparagus plants go dormant in winter and the above-ground ferns dry out and die back. Cut them down anytime after they have turned brown. Here are some considerations on the timing:
- Cut down in fall, as soon as they dry out: this can help remove overwintering asparagus beetles and diseases.
- Cut down in early spring: beneficial bugs that hang out in the ferns over winter are great food for migratory birds returning in spring.
Do not remove green ferns - as they are still alive and sending energy to the roots.
How to harvest
You can harvest asparagus spears the second summer after you planted. Some people wait until the third year, but research shows harvesting for a short period in the second year increases yields in subsequent years.
Harvest while the tips are still tight and compact, before they fern out.
There are generally two methods to harvest asparagus spears:
- Use a knife and cut the spear below the soil
- Snap the spear off by hand, near ground level
Cutting below the soil
Many farmers use a knife and cut below the ground because the spears end up 20% longer. Since asparagus is sold by weight, this makes their harvest more profitable. In addition the University of Minnesota notes that, "the advantage to cutting spears below the soil is that the white, woody base restricts water loss, which preserves spear quality."
However, the portion of the spear that is below the dirt is usually very fibrous and tough - so it ends up getting trimmed off anyway. You'll know it when you see the white butt ends (they are white because the sunlight couldn't reach that part, so it wasn't able to turn green).
Snapping above the soil
The best way for most home gardeners to harvest asparagus is snap it off near the base. It should break before it hits any fibrous portion. The leftover stump will wither, and a new spear will grown from a bud underground.
The above-ground method is also better, because when using a knife below the soil, you might accidentally cut a bud or spear that has already started to grow, but isn't visible yet.
Ideally, you harvest spears right before you use them. Once harvested, they very quickly convert their sugars to starch, losing flavor and that super fresh quality. This happens very quickly over the first 24 hours, and then at a slower rate after that.
Storing harvested spears in the fridge slows that sugar conversion, preserving freshness. Asparagus can also be canned (in a pressure cooker), and frozen.
Asparagus crowns need time to establish - the stronger they are, the heavier the yields. Harvesting too much in their early years weakens the plant and your overall yield will suffer. The timeline below assumes you are planting 1-year old crowns.
- Year of planting (first summer): do not cut any spears
- 2nd summer: harvest spears for up to 2 weeks.
- 3rd summer: harvest spears for 4 - 6 weeks
- 4th year and beyond: harvest spears for 8-12 weeks
During harvest season, each crown shoots out a new spear that's ready to harvest in 2-4 days (depending on the weather and other growing conditions). If you haven't grown asparagus before, it's unreal to watch how quickly spears grow.
Yield in your garden
During peak production year (starting around year 7), you can expect:
- ½ - ¾ pound per crown per season (about 10-15 spears)
- 25 crowns yields 12-18 pounds per season
- That's about 2-3 pounds per week (over 8 weeks)
Each crown will produce about 10 spears during its most productive years, which is about half a pound. You may even get up to ¾ pound per crown, according to some sources, like Utah State.
While I haven't found any actual data from farms or test fields backing up the claims of higher ¾ lb yields, it is possible that home gardening outperforms farms with our higher level of pampering.
Early years produce less while the plants get established. How much less? Based on data from several research trials at various universities, there appears to be a trend:
- Year 2: (first time you can harvest), has minimal yields
- Year 3: produces about 33% of peak (4-5 lbs per 25 crowns)
- Year 4-5: production continues to increase to 45-55% of peak
- Year 6: production is at about 66% of peak (8-10 lbs per 25 crowns)
- Year 7-8: peak production is reached
- Year 10 and later: production declines about 5% per year, according to farmers
Many farmers stop harvesting their asparagus fields after 15 years because product declines lose profitability. But the home gardener can continue to enjoy the harvest.
When to stop harvesting & post harvest care
At any point during your harvest season, stop cutting spears if they get pencil thin (smaller than ⅜") or growth slows considerably. This means the plant is running out of energy. At this point, harvest all spears, regardless of size and height.
Now is a good time to apply fertilizer, specifically nitrogen. Nitrogen helps plants produce leafy growth, which will help the asparagus plants 'fern out' nicely.
The ferns absorb energy from the sun and transfer it to the roots for storage over winter. This helps produce a good crop spears the following year.
More asparagus guides
- Carl Cantaluppi, Replicated Asparagus Cultivar Evaluation and Ranking
- Growing asparagus in Minnesota, University of Minnesota
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