Take steps to improve future asparagus productivity

2012 was an exceptionally difficult year for asparagus growers in the Midwest and Northeast, as well as in some parts of Canada.

Because of unusually warm weather in the spring, bud dormancy on the crowns was lifted in early March, causing spears to emerge much earlier than normal. For example, the first report of spear emergence in Michigan was March 19 in Benton Harbor. In Hart, Mich., most commercial fields had about 50 percent spear emergence by April 8-10 – a month earlier than the 2011 growing season. Early spear emergence has many disadvantages for the industry, including lack of labor for early harvests, premature sprouting of crowns slated for new production fields and processor facilities not being ready to receive asparagus.

Furthermore, early emerging spears are at high risk for frost damage, a major cause of low yields in asparagus production. A damaging frost can easily suppress two to three good pickings, and a series of frost events early this season caused serious losses to the asparagus industry in Michigan and other states. Despite the reports of unusually early spear emergence in Hart, most commercial growers there didn’t have their first pickings until April 19-20, as a result of the multiple frost injuries. Since larger spears tend to emerge early in the season, losses were sustained to both quantity and quality.

It is critical for growers to adopt management strategies that will help improve productivity in the future. Asparagus is a perennial crop, and because of its physiology and growth cycle, management practices in any given year have significant implications for spear production and yield in subsequent years. A key factor contributing to yield in asparagus is the amount of carbohydrates stored in the crown (root system) at the time the crop goes dormant – sometimes referred to as the “energy of the crown.” During the harvest period, these carbohydrates stored in the crown are depleted to sustain continuous spear production. Therefore, a crown fully charged with carbohydrates is necessary for optimum yield.

Following the harvest period, asparagus spears are allowed to develop into fern. During the initial phase of fern development, there is further decline in the carbohydrate content of the crown; however, once the fern has reached maturity and photosynthesis is fully operational, the crown is rapidly recharged with carbohydrates. The tiny “leaves” on the fern, called cladophylls, are responsible for most of the photosynthesis in asparagus, and the amount of “carbohydrates” produced during photosynthesis depends on light and temperature and varies among cultivars.

However, there are several management factors that can easily be manipulated by growers to improve photosynthesis and the amount of carbohydrates stored in the crown, including (1) the length of the harvest period, (2) the length of the fern development period, (3) the total photosynthetic area (fern) of the crop, and (4) the health of the fern.

The length of the harvest period determines the amount of carbohydrate left in the crown and the time left for crown recharge. In regions with a temperate climate like Michigan, there should be an adequate balance between the length of the harvest period and that of fern development. The length of the fern development period should allow the crop to fully recharge the crown with carbohydrates before the dormancy period that follows fern kill by frost.

Our studies over the last two years show that under Michigan conditions, asparagus fern development stops around the first week of September. After that time, few new spears are formed and nutrient and carbohydrates begin to move in the crown. In a normal year, about 75 days prior to the first week of September may be required for full fern development and adequate crown recharge.

After a tough harvest season like 2012, it is critical to adopt management practices that will ensure adequate yield in the future. Growers should avoid over-picking, which may deplete carbohydrates in the crown, shorten the recharge time and compromise future yields. They should avoid shoots damage during the fern development phase, in order to enhance the total photosynthetic area of the crop. Finally, they should keep the fern healthy and vigorous by reducing biotic (diseases, insects, weeds) and abiotic (drought, fertility) stresses.

– Mathieu Ngouajio

Refer to the following resources for information on best management practices to promote healthy fern growth:

• 2012 Insect, Disease and Nematode Control for Commercial Vegetables
• 2012 Weed Control Guide for Vegetable Crops
• Nutrient Recommendations for Vegetable Crops in Michigan