When to cut back asparagus is a common question among vegetable gardeners. While growing asparagus isn’t difficult, knowing when and how to prune these perennial plants can mean the difference between a prolific harvest of healthy spears and poor production from pest-ridden plants. In this article, I’ll share important insight into how asparagus grows and then offer information on the best time to cut back asparagus plants for optimum plant health and production.
Why it’s important to know when to cut back asparagus
Properly timed asparagus pruning is essential for two primary reasons.
- First, spear production can be negatively affected if the mature ferns are cut off at the wrong time. In the next section, I’ll dive into the different phases of asparagus growth. I’ll also let you know why it’s important to leave the ferns stand for as long as possible and how improperly timed pruning can cause issues with production.
- The second reason that properly timed asparagus pruning is important is because of the lifecycle of the primary pest of asparagus plants: the asparagus beetle. If you want to reduce the number of these beetles damaging your plants next year, knowing when to cut back asparagus is key. I’ll cover the topic of asparagus beetles and the effect pruning has on them in a later section, too.
The overall vigor of your asparagus plants is influenced by many factors, including whether they are planted in full sun (yes!) or full shade (no!), how the plants are maintained, and, yes, when the plants are cut back each season.
Let’s start our exploration into when to cut back asparagus by examining how this vegetable grows.
A quick look at how asparagus grows
A spring harvest of tender asparagus spears is a true delight. There are three phases to asparagus growth, with each being an equally important step in the lifecycle.
Phase 1: Spears
This phase occurs in the early spring when new shoots emerge from asparagus crowns. These tender spears are tapered at the top, and this is the phase of plant growth that humans eat. The spears are harvested for 6-8 weeks in the spring, but not until the third year of the plant’s existence.
During the first year and second year after planting, do not harvest any spears to enable the crown to grow and reach a healthy size. The only cutting back you’ll do during this phase is to make your harvest.
For more on how to harvest asparagus and how long you can do it for, please visit this article which has advice on how to plant, grow, and harvest asparagus.
Phase 2: Ferns
This phase of asparagus growth occurs from late spring through fall. This is known as the fern phase. The start of this phase begins when the harvest of spears is stopped and the stems are allowed to mature into asparagus ferns. Asparagus ferns on established plants grow 4 to 6 feet tall, have tiny, needle-like leaves, and are very upright and rigid in growth, though the tips of the ferns are soft and flexible.
Each asparagus plant is either male or female. Female plants in the fern phase produce small, red berries if there are male plants nearby to fertilize their small, nondescript flowers.
This critical phase is when the plant makes food for itself. It spends this phase photosynthesizing and transferring the resulting carbohydrates back down into the roots to store energy and fuel next year’s spear production. If you cut asparagus back while it is in its fern phase, you will negatively affect production. This is a big no-no.
Phase 3: Dormancy
The third phase of asparagus growth is dormancy. Asparagus is a perennial plant that requires a dormant period during the cycle of each year. Dormancy occurs in the winter, after the ferns have been killed by a frost. While not much is going on above ground, the dormant phase is important for root and crown growth, at least until the ground freezes solid.
Dormancy is a critical period for asparagus plants. It is also the phase during which asparagus pruning takes place. A quick answer to the question of when to cut back asparagus is: During dormancy. But is it better to cut asparagus back at the start of dormancy or at the end?
Let’s dive into that next.
The most critical factor affected by the timing of asparagus pruning
Now that you know when to cut back asparagus (during dormancy), we’ll look at whether the start of dormancy or the end of dormancy is the best time to do so. The answer to this conundrum depends on one primary thing: Pest pressure.
As far as the plant is concerned, it doesn’t matter whether you cut the ferns back in late fall or very early spring, but when to cut back asparagus becomes a much more critical factor when you have asparagus beetles in your asparagus beds feasting on your plants.
How asparagus pruning affects asparagus beetles
Asparagus beetles (both spotted and striped) are Public Enemy #1 in the asparagus patch.
Striped beetles (also called common asparagus beetles – Crioceris asparagi) emerge in early spring and feed on the new spears and maturing asparagus fronds (they are the most damaging species), while spotted asparagus beetles (Crioceris duodecimpunctata) emerge later in the season and feed primarily on the berries of female plants.
Striped asparagus beetles overwinter as adults in garden debris, including on and around old, dead asparagus stems. They emerge in the spring, mate, and lay tiny, dark, oval-shaped eggs on new asparagus spears through early summer. The Army green, grub-like larvae feast on the ferns all summer long, affecting their ability to photosynthesize and therefore limiting their ability to fuel next season’s growth. Sometime between early summer and August, each larva drops to the ground, burrows in, and pupates. A new generation of adults emerge in September or October and take shelter in debris for the winter.
Since the ferns are so critical to spear production the following year, you can see why limiting these fern-munching pests is important. So, the answer to the question of when to cut back asparagus plants if you have beetles is in the fall, just after the ferns have been killed by frost. This limits overwintering sites for the adult beetles.
If you don’t have asparagus beetles in your garden, timing is less critical and may be more of a personal preference. Some gardeners have a problem with the way the dead ferns look in the winter. If that’s you, then do your pruning in the late fall. I personally like how they look covered in snow, so I leave them stand and cut them back in the early spring.
When to cut back asparagus – Option 1: in the early spring
You may be wondering what I mean when I say “early spring.” Depending on your climate, this can mean early March, late March, or early April. But you don’t have to base it on the calendar. You can base it on the soil temperature.
New asparagus spears begin to emerge when the soil temperature reaches around 50°F. Ideally, you’ll want to cut the dead foliage off before any new spears emerge if you choose to do your pruning in the spring. You don’t have to monitor the soil temperature every day or anything, but it pays to keep an eye on it. Waiting too long to prune asparagus could lead to accidentally damaging the new spears as they break through the soil. It’s better to err on the side of too early than too late.
When to cut back asparagus – Option 2: in the late fall
If you opt to prune in late fall, know that this can also mean early winter, depending on your climate. Always wait until the first frost to prune asparagus if you’re opting to do the job in the fall. This gives the ferns as long as possible to continue making carbohydrates and feeding the crowns and roots of the plants. Once frost strikes, photosynthesis slows down and eventually stops, so any time after that is a good answer to the “when to cut back asparagus” question.
In reality, any time during the winter months is okay to prune back asparagus. But, since most of us don’t want to head to our garden and prune when the snow is flying and temperatures are frigid, we opt for late fall or early spring instead.
The best way to cut back asparagus
While there’s no one right way to cut back asparagus plants, there are some techniques and tools that are more efficient. For large asparagus patches, I like to cut back the dead foliage using a long-bladed hedge clipper. For small stands of asparagus plants, a sharp hand pruner will do.
If you are cutting the plants back in the fall, they will be “juicier” and heavier and slightly harder to cut down. If you want until the early spring, the fern stalks will be drier, lighter, and a little easier to cut down.
Cut the plants all the way back to within 0 to 1 inch of the ground. Leaving longer “stumps” behind can sometimes lead to crown rot as they decay.
What to do after pruning asparagus plants
If you opt for springtime pruning, after you’ve cut back your asparagus and before any new spears emerge, add 1-2 inches of straw or a layer of an organic mulch such as compost, shredded leaves, or untreated lawn clippings onto the bed. This helps limit weeds and stabilize the soil. You can also add a general organic granular fertilizer to asparagus plants at this time. Choose one that contains an equal percentage of nitrogen, phosphorus, and potassium (N-P-K).
If you cut back your asparagus in the late fall instead, you can also do these same things in the spring, before the emergence of new spears.
To watch an asparagus plant being cut down and learn more about how to do it, check out this video taken in my garden:
Asparagus health and longevity
Asparagus plants can live for 25 or more years. Treat them right, and they’ll offer a harvest of delicious spears each and every season. Knowing when to cut back asparagus is an important step in their care. For more on how to choose the best varieties, how to plant asparagus, and harvesting tips, please read our article on asparagus growing secrets from the pros.
For more on perennial vegetables, check out the following posts:
- Perennial onion types
- 15 perennial vegetable choices
- Growing artichokes at home
- When and how to harvest rhubarb
- How to grow strawberries in raised beds
Pin this article to your Vegetable Gardening board for future reference.