Apr 15, 2011
Irrigating asparagus has its advantages

Why consider irrigation in asparagus? Asparagus is not traditionally irrigated in rain-fed production areas of the Midwest and Northeast. However, drought stress during fern growth limits production of the root carbohydrates necessary for high yields the following spring.

Drought-stressed plants are also likely to be more susceptible to fungal diseases including phytophothora and fusarium, which increasingly plague the asparagus industry. Additional potential benefits of irrigation include: 1) reduced risks associated with planting of higher-yielding but more drought-sensitive varieties (e.g. Millenium); 2) more efficient and effective application of pesticides and fertilizers, especially when sub-surface drip systems are used; 3) improved spear quality through cooling during harvest; 4) reduced risks associated with higher density plantings and more aggressive picking in early years; and 5) increased opportunities for integration of cover crops with minimal risk of competition between asparagus and the cover crop.

Will irrigation pay? Although irrigation can provide important benefits, the costs of installation and maintenance run between $1,000-$2,000 per acre, depending on the system used. To justify this type of expense, Michigan State University (MSU) agricultural economist Stephen Harsh estimates that yield benefits from irrigation for a typical asparagus producer in western Michigan may need to average 10 percent or more. This type of yield boost is most likely in micro-climates where hot-dry periods are common in July and August, when asparagus fern has greatest demand for water; for relatively drought-sensitive varieties including Millenium; on light, sandy soils. In other words, if you are growing Jersey varieties on heavier ground, irrigation is unlikely to pay off.

It is also important to realize that irrigation will not increase yields every year. Rather, irrigation systems are an insurance policy against severe drought stresses that may only occur every three or four years. For example, in western Michigan, irrigation during the very dry 2005-2007 growing seasons would have been very helpful for preventing yield losses that were due in part to dry conditions. On the other hand, during the 2009 and 2010 growing seasons, rainfall was plentiful, so irrigation would have had little or no benefit. Once an irrigation system is installed, it is tempting to use it, but irrigation in wet years may do more harm than good. For example, overhead irrigation can increase leaf wetness and hence susceptibility to disease. Likewise, excessive irrigation in the late summer and fall may also promote fern growth at the expense of carbohydrate storage, thereby reducing subsequent yields.

Living mulches

Irrigation creates opportunities for greater integration of cover crops into asparagus production, with reduced risk of competition for water between asparagus and the cover crop. Living mulches are simply cover crops grown beneath the asparagus crop. While many growers plant winter rye in late summer, experiments with living mulch have also included perennial cover crops (e.g. dutch white clover) or annual cover crops planted immediately after harvest and allowed to grow below the fern throughout the summer and fall. These living mulches may provide several benefits. During extreme rainfall events, they can protect the soil from degradation and reduce the risks of nutrient and pesticide run-off. Living mulches may also mitigate soil compaction, increase soil organic matter, improve nutrient cycling and suppress weeds. However, without irrigation, these cover crops also compete directly with the crop for soil moisture. Experiments are currently underway to evaluate the potential benefits of living mulches where irrigation is available.

Central-pivot irrigation is an attractive option since it is a proven technology that many asparagus growers already use in other crops. Because of their durability, central-pivot systems can be used after the asparagus crop is gone, in either new asparagus plantings or in other crops like carrots. By wetting the entire soil surface, central-pivot systems are more effective than drip irrigation at cooling the soil, activating certain herbicides and establishing and maintaining cover crops.

On the other hand, sub-surface drip irrigation has several critical advantages relative to overhead systems. Drip tubing can be placed in the trench directly below asparagus crowns at the time of planting and left in place for the life of the asparagus. Like all drip systems, it delivers water more efficiently than overhead irrigation and reduces costs associated with well construction and pumping. In drip systems, water is delivered directly to asparagus roots, weeds are not watered and the potential for exacerbating foliar diseases through leaf wetting is minimized. Drip systems can also be used to deliver fertilizers and pesticides efficiently to the crop. For example, in ongoing studies, MSU entomologist Zsofia Szendrei has found that application of systemic insecticides through a drip tape was more effective at managing asparagus miner than foliar applications.

Although irrigation is likely to be an important tool for asparagus production systems of the future, many questions remain about how best to optimize these systems. At MSU, with support from the Michigan Asparagus Research Board and a USDA-Michigan Department of Agriculture Specialty Crop Block Grant, we have established a long-term asparagus trial comparing the effects of sub-surface drip and overhead irrigation on asparagus yield and profitability. In addition, trials on growers’ fields will examine the effects of irrigation in combination with various cover crops on soil moisture, weed suppression and fern health.

For more information about these trials, email me at [email protected].

By Daniel Brainard, Michigan State University

Click here to watch a video on asparagus irrigation

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