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Weed Management Options for Asparagus

Photo: Annie Klodd

As we reach the end of asparagus harvest, growers are probably thinking, “Oh great, now I get to get rid of all these weeds.” Late June is a good time to wrap up harvest and clean up the field to make room for fern growth.

This article will discuss weed management from the framework of IWM (Integrated weed management). It will describe cultural and mechanical methods of weed management followed by chemical options.

Because asparagus is a perennial crop that has green tissue from April to December, one of the biggest challenges is managing weeds within the asparagus rows. Growers must look for methods and windows of time for weed management that do not harm the growth of the plant. They also must develop a strategy for perennial weeds such as Canada thistle and quack grass, which thrive in asparagus beds due to the open soil and lack of tillage.

Managing weeds between the rows is much easier in comparison to the rows themselves. Aisles can be kept weed free without the use of herbicides via cultivation, flaming, mulching, or cover crops.

Non-chemical weed management

Cultivation: Cultivation can be used cautiously and at specific times in the rows, when the spears and ferns are not present. Additionally, cultivation can be used throughout the season to eliminate small weeds in the row aisles. Some asparagus producers cultivate the rows in the early spring, after snow melt but before spear emergence. If all spears are harvested below the soil surface at the end of the harvest season, cultivation may also be possible in the rows immediately following the last harvest. At both timings, the cultivation must be very shallow, less than 3 inches, and should be done before new spears start emerging. Understandably, this presents challenges.

Before spear emergence, the main role of spring cultivation is to incorporate fertilizer. However, it can also eliminate winter annual weeds that have started to resume growth. The action of cultivating in the spring can also stimulate growth of weed seeds that were previously buried. Therefore, growers should consider whether it is really necessary to cultivate each spring.

Post-harvest cultivation should be done immediately following the final harvest, and only if spears were harvested below the soil surface. Post-harvest cultivation can uproot small emerged weeds to keep the soil relatively weed free until the ferns become established enough to outcompete weeds. Shallow cultivation is not likely to control thistles or other weeds with taproots and rhizomes, or large established annual weeds.

Cover crops: Rather than cultivating between the rows, asparagus growers can avoid tillage and increase their soil health by planting cover crops in the aisles. Usually, these are perennial cover crop mixes such as fescues, perennial ryegrass, and clover. Annual cover crops may be a good choice for newly planted fields, as they are quicker to establish. More details on cover cropping asparagus will be provided in a future article.
An asparagus field with a perennial cover crop. The spears are easier to see in person! Photo: Annie Klodd

Flaming (propane weeding): Flame weeders, also called propane weeders, emit heat from propane-powered torches that kill weeds by heat. Despite the name “flame weeder” this tool is not meant to burn the weeds. Flame weeds come in many different shapes and sizes, from individual torches on a backpack model, to pull-behind implements for tractors. Flaming can be used carefully within the rows, or in the aisles.

Hand-removal: Despite our best efforts, asparagus production sometimes requires removing weeds by hand. Hand removal comes in when large annual weeds and perennial weeds are too well established to control via shallow cultivation or flaming. It is also necessary to remove weeds in the rows during the peak growing season, when the presence of spears precludes the use of cultivation, flaming, or herbicides. Hand-removal can also be used to eliminate any weeds that escape through cover crops and mulches.

In the absence of herbicides, hand removal is the most effective choice for managing Canada thistle. While thistles can be mowed and cultivated, this cannot easily be done in the rows with enough frequency to control an established thistle population. Flaming has not been shown as an effective tool against thistles or quack grass, as it only burns the tops and does not impact the belowground rhizomes (spreading roots) that these plants use for reproduction. Therefore, growers resort to hand-removal for stubborn perennial weeds as well as large, established annuals.
Canada thistle can be a very aggressive weed in asparagus stands and usually requires either hand removal or herbicides. Photo: Annie Klodd

Mulching: While not a commonly used practice, the aisles between the rows can be mulched using wood chips, straw, or landscape fabric in order to smother weeds between rows. Mulch is not recommended in the rows, and it can lead to high pest pressure from asparagus beetle and asparagus aphid.

Herbicide Options for Asparagus 

A number of herbicides are available for use on asparagus. Depending on the type of asparagus beds (newly planted crowns or established beds), choosing the most effective herbicide and rate represents a challenge to the grower. The desired herbicide should produce long-term weed control, be safe to use on the asparagus fern, and be legal to use. As outlined, new and established asparagus present different sets of requirements.

Herbicide Types and Application Timings

There are legitimate reasons why many growers choose to use herbicides (either synthetic or organic) in asparagus. First, many Minnesota asparagus growers are also growing strawberries, and therefore are too busy in late June to spend a considerable amount of time with weed management after asparagus harvest is complete. Just a couple of well-timed herbicide applications drastically reduces the amount of time spent on good weed management. Secondly, using herbicides reduces the need to cultivate, thereby presenting an opportunity to move the system toward no-till or minimal-till and improving soil health.

A wide selection of herbicides can be applied before, during, or after the harvest. There are multiple options for pre-emergent and post-emergent herbicides labeled for asparagus. While post-emergent herbicides target actively growing weeds, pre-emergent herbicides have soil activity that prevents new weeds from emerging. The problems facing the grower are choosing an application method that complements the operation and choosing a chemical and rate that will control weeds after harvest for the duration of the growing season.

Herbicide application timings include:
Before planting crowns
Early spring before spears emerge
After final harvest but before ferns grow
During harvest after cutting all emerged spears*
To ferns during the post-harvest period

*Because asparagus spears continually emerge and are harvested frequently during the harvest period, herbicide application during this time is mostly not possible. The exception to this is that select products can be applied during harvest if all spears are harvested first, regardless of size.

When to use pre-emergent herbicides: The use of a pre-emergent herbicide in the spring is beneficial for this crop, because it reduces the amount of weeds emerging during the busy harvest period. A second pre-emergent application may be done after harvest, to keep weeds down until the ferns are large enough to outcompete them.

When to use post-emergent herbicides: Post-emergent herbicides can be used in the early spring to kill winter annual weeds, and after harvest to kill weeds that came up during harvest. In general, weeds are more susceptible to herbicide control when they are less than six inches tall. Larger weeds will still require hand removal, as described above.

Organic Herbicides:

Ammonium nonanoate: an organic, non-selective post-emergent herbicide. It can be used in the rows before planting new crowns, or before spear emergence in mature stands. It has effectiveness on many annual weed species, and will suppress growth of some perennial weeds. More concentrated solutions should be used for larger weeds. See the label for more information.

Caprylic or capric acid: an organic, non-selective post-emergent herbicide. Like ammonium nonanoate, it can be used in the rows before planting new crowns, or before spear emergence in mature stands to kill actively growing weeds. It has some activity on most annual weeds and suppresses growth of perennials. For best results, leaves should be thoroughly wetted. See the label for more information.

Below is a table of herbicides labeled for use in asparagus. The table shows when each product can be used, whether it is a pre-emergent or post-emergent herbicide, what types of weeds it targets, and whether it can be used on new plantings. This table is found in the Midwest Vegetable Production Guide for Commercial Growers, page 92.

New Crown Plantings

Good weed management is critical for establishing high yielding, healthy new asparagus beds. The newly planted crowns have very small root systems, so just a few weeds around each plant can drastically decrease fern growth and subsequent yields.

As described previously, most new asparagus production fields are established by planting one-year-old nursery grown asparagus crowns into deep furrows. Since the first new shoots take several weeks to emerge and grow to a size that can be safely cultivated, weeds in the furrow may become too large to cultivate. Therefore, a pre-emergence herbicide with long residual activity will reduce early season weed populations and decrease the number of cultivations needed to keep fields weed-free.

Preventing Herbicide Injury

Herbicide injury can occur to asparagus stands if herbicides are applied against the label guidelines. Common causes of herbicide injury include exceeding labeled rates, failing to calibrate the sprayer before spraying, and spraying at a time in the season that is not permitted by the label.

For example, while halsulfuron (i.e. Sandea) can be applied after harvest, it should not be broadcast over the rows; it must be applied using a directed or shielded sprayer to prevent direct damage to ferns.  Additionally, if broadleaf herbicides such as dicamba or 2,4-D are applied in the spring slightly too late, once spears start to emerge, that can cause spear damage.

Another common scenario for herbicide injury is when part of the field finishes harvest and starts producing ferns before the whole field is ready for the final harvest. In this case, growers may mow the entire field a few inches above the soil surface, and then apply a labeled herbicide before ferns begin to grow back.

Herbicide damage in one year can decrease yield the following year by injuring the ferns and crowns. If herbicide damage is severe, such as if it killed a significant portion of the ferns, the grower may consider doing a reduced harvest the following year in order to allow the stand to recover. Monitor rate of spear emergence the following spring to determine if this is necessary.

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