Aug 19, 2011
Greenhouse transplants for asparagus production

Asparagus production in the U.S. was valued at over $90 million in 2010. Although almost every state produces some fresh market asparagus, California, Michigan, and Washington dominate large-scale commercial production of both fresh and processed asparagus.

Asparagus is one of the few perennial vegetables and fields are usually established for decades of production. Unfortunately long-term monoculture systems contribute to the buildup of pathogens and toxic compounds in the soil. For asparagus, this has resulted in replant suppression problems.

Replant suppression is a phenomenon that prevents new plantings of asparagus from establishing successfully in a field that has previously been planted with asparagus. This can be avoided by moving the production to virgin fields, with no prior history of asparagus; that is currently the case for countries like Peru, where the industry is young and most of the production is on virgin grounds. In the U.S., however, commercial production of asparagus dates back to the mid 1800s. Therefore, virgin land is scarce and growers are forced to go back to old fields (replant situation) for both crown production and establishment of new crops. As a consequence, there is damping-off of seedlings in crown nurseries; poor stand establishment in young fields, and a slow decline in productivity of mature fields. The life span of a productive field is easily reduced from 15-20 years to 5-10 years. In addition to marketing and regulatory issues, like the free trade agreement, current asparagus production systems require improvements to enhance its long-term sustainability. The long-term survival of the industry relies on the use of clean planting materials in clean production fields.
Currently crowns are the standard planting materials for establishment of production fields. In Michigan, for example, current production practice involves two steps. In year one, seeds are planted in crown nurseries. In early spring of year two, the young crowns are dug mechanically and transplanted into production fields.
To produce clean crowns, growers have fumigated crown nursery beds, and have treated crowns with fungicides prior to planting in production fields. This approach has been championed by researchers like Mary Hausbeck from Michigan State University and has helped improve asparagus production. Unfortunately, some of the fumigants (e.g. methyl bromide) are no longer available to growers, and alternative fumigants require either a long plant-back period or their efficacy is inconsistent due to environmental conditions.

In addition to chemical strategies cultural approaches including direct seeding and the use of transplants have been evaluated for asparagus field establishment. Early studies in collaboration with John Bakker, executive director of the Michigan Asparagus Advisory Board, showed that direct seeding was not a viable option for Michigan asparagus industry because of poor stand establishment.

Greenhouse grown transplants on the other hand, have shown great potentials as alternative clean planting materials for establishment of asparagus fields. Studies conducted by our team from 2007 to 2011 have shown that healthy and vigorous asparagus transplants can be produced in the greenhouse in 10 to 12 weeks using trays with 38 to 84 cells. Preliminary data on field evaluations have shown that fields established with transplants had yields equal to or higher than yields obtained from fields established with crowns. We have also observed an increased number of asparagus shoots with plants established from transplants, compared to those established from crown.

Growers considering using transplants should be aware that the new production system is a completely different ball game, especially during the first year of field establishment. Like annual vegetables, asparagus seedlings are subject to transplant shock. Therefore, appropriate plant hardening-off is critical prior to transplanting. Adequate soil moisture is essential for good transplant establishment, and young transplants should be irrigated immediately if climatic or soil conditions are not optimum. Herbicide injury, weed competition, and disease and insect damage during the first year are detrimental to the young transplants.

Best results have been achieved on fumigated field with low weed pressure or in fields where hand or mechanical weeding took place during the first year. When fields are established with crowns, growers usually have a few harvests during the second year of asparagus production; with transplants, it is recommended to avoid picking the second year, unless there is a risk of frost that will kill the spears if not harvested anyway. Whether the field is established with crowns or transplants, crop management practices are similar after the second year.

The use of greenhouse grown transplants offers to asparagus growers an additional tool for production of clean planting materials for establishment of new production fields. In addition to the high yields obtained in fields established with transplants, transplants may also help reduce seed cost. This is especially beneficial, since seed of elite asparagus cultivars is very expensive and difficult to source.

By Mathieu Ngouajio, Michigan State University

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