Wrapping Up the Asparagus Harvest

Asparagus spears at Schmidt Farm in Preston, MN. Photo: Annie Klodd

Author: Annie Klodd, UMN Extension Educator - Fruit and Vegetable Production

Asparagus harvest is drawing to a close in most of Minnesota. The season typically ends in late June, and our dry weather seems to be causing spear growth to slow down slightly earlier this year. 

Next year's yields and profits are determined by how well the asparagus is treated in the current year. An important part of managing asparagus is deciding when to stop harvesting, and how to manage the patch after harvest.

How to tell when to stop harvesting

To wrap up the asparagus season, simply harvest any remaining spears and then call it quits. New spears that emerge after that are then left alone to develop into ferns.

Harvest of mature stands should stop about 6-8 weeks after initial spear emergence, once the spear growth and emergence slows down significantly, or when spear width is less than pencil size. In Minnesota, this is typically in late June. Far northern locations may stop harvest later, because their season began later. Harvesting for too long stresses the plants, reducing yield the following year. Remember that each time an asparagus spear is harvested, we are removing a stem. Removing too many spears (stems) can deplete the sugar reserves in the crowns and cause a gradual decline in the health of the patch.

Tips for detecting the slow-down

As the season progresses past the peak harvest time in early June, growers will notice the spears growing more slowly. Keeping daily records of how many pounds are harvested every day will help determine when spear growth slows down. Spears may also become thinner. Harvest should stop immediately once spear diameter is reduced to pencil size. Additionally, as spears grow more slowly, the heads will start to expand on shorter spears (see photo).

The head on the spear to the right is starting to expand or "fern out." As spear growth slows toward the end of the season, spears will start to fern out when they are shorter, because they're not growing as quickly. Photo: Annie Klodd.

New asparagus plantings

Plantings less than 3-5 years old should have an abbreviated harvest. Young crowns have not yet built up large enough energy reserves to withstand full harvest periods. Depending on the health and vigor of the patch, a 1-year old patch (planted one year ago) may be harvested for 0-7 days before letting the ferns grow (see photo below). While many growers in Minnesota choose not to harvest from one-year old stands, some will harvest 1-2 times within one week, and then stop. While it can feel difficult to stop harvesting after such a short time, the plants will be healthier in the long term as they establish an extensive root system and accumulate sugar reserves.

Asparagus ferns on a one-year old asparagus planting at Big River Farms in Marine on St. Croix, MN in June, 2020. Photo: Natalie Hoidal.

After Harvest

Asparagus is low maintenance after harvest, but it is not zero-maintenance. Post-harvest tasks to keep the field healthy include moderate weed management, fertilization, and in rare cases, irrigation. These tasks should not take much time. But ignoring these tasks following harvest can decrease the plants’ health and vigor the following year.

Fertilization 

Since the asparagus grows large ferns after harvest, they need nitrogen (N) to encourage maximum fern size, photosynthesis and energy storage. This N can be added in the form of fertilizer or compost.

After all remaining spears are harvested, add a nitrogen fertilizer according to the organic matter percentage (OM%) on your most recent soil test; lower OM soils should receive higher N rates (see table below). UMN soil specialists recommend 40-80 lb of actual N per acre annually. Nitrogen can be added in the form of granulated or pelleted fertilizer, or compost. Use this guide to approximate how much actual N is in your soil amendment, in order to determine how much product to apply. Broadcast these amendments over the asparagus rows. If the spears were harvested with a knife below the soil surface, then it may be possible to incorporate it at a very shallow depth.

Asparagus N recommendations based on organic matter percentage in the soil. Figure: Carl Rosen, UMN.

Phosphorus (P) and potassium (K) are typically added in the spring before harvest, rather than after harvest. It is fine to apply an N-P-K fertilizer as the nitrogen source, but keep in mind that the P and K will largely remain on or near the soil surface unless they are incorporated, and incorporation is challenging after harvest due to the emerging spears.

Weed management 

A new asparagus field with excellent weed management at the Hmong American Farmers Association in Farmington, MN. Photo: Annie Klodd

By the end of harvest, weed pressure may be heavy in the asparagus stand, especially if herbicides or regular hand-weeded were not used. After harvest is a great time to reduce weeds before the ferns grow. All growers, especially organic growers, will likely need to do some hand-weeding to eliminate large weeds and perennials. Immediately after removing all remaining spears, the rows can also be mowed once. One pass of a mower or flame weeder will not kill thistles or quackgrass, so those should be hand pulled. Cultivation, cover crops, flaming, and mowing can be used between the rows.

While the ferns will out-compete many new weeds, it is important to remove existing weeds at this time - weeds that go to seed will cause more severe weed pressure next year!

A new asparagus patch with heavy Canada thistle weed pressure. Hand weeding will be the most effective non-chemical solution in this case. Photo: Annie Klodd

For those using herbicides, a post-harvest herbicide can be applied to kill small emerged weeds (less than 6 inches) in the rows. For many weeds, herbicides become much less effective once they grow over 6 inches tall. This herbicide application must be squeezed in after removing all spears, but before more emerge to become ferns, to minimize the risk of spear injury. This is a very small time window, so growers should plan the final harvest for a time when herbicides can be applied immediately afterward. Including a pre-emergent herbicide will prevent new weeds from germinating until the ferns are well established to shade them out. The Midwest Vegetable Production Guide provides a table showing the relative effectiveness of each herbicide labeled for asparagus.

Irrigation

In times of severe drought, or if the asparagus is grown on sandy soils, growers may use irrigation to maximize fern growth and subsequent yields. It is not typical for Minnesota asparagus growers to use drip irrigation or sprinklers in their asparagus stands. On rare occasions when irrigation is needed for asparagus, it is common to pull a water tank through the field to water by hand. To learn about how asparagus is irrigated in dry regions, read this article by University of California.