Ohio State University Extension

Horticulture and Crop Sciences

Asparagus Production Management and Marketing

Bulletin 826

Varieties and Plant Characteristics

In recent years, much plant breeding research has developed new high-yielding hybrid varieties that surpass the older open-pollinated types.

The asparagus plant is a long-lived monocotyledonous, herbaceous perennial that can be grown in the same area for 20 to 30 years. The plant is grown for its succulent fleshy shoots (spears), which appear after a prolonged winter rest period. The crown of the plant is the critical growth center. The plant consists of underground stems (rhizomes), fleshy roots, and fibrous roots. The fleshy roots serve as storage organs and the fibrous roots as absorption organs. The fibrous roots die after each year's growth. Each successive generation of fleshy roots originates from the crown of the plant and so, over time, it moves closer (up or down) to the optimum depth for growth in a particular soil. Thus, the asparagus plant moves upward or downward as it ages (36).

Loughton (27) planted asparagus at five different depths ranging from 6 to 12 inches. After 7 harvest seasons, the final depth of all crowns, regardless of the original planting depth, had "floated" to about 4.5 inches of the soil surface.

As each growing season progresses, the rhizomes develop buds that generate the spears for the following spring and summer months. From the center of the asparagus plant there is a progressive lateral growth of rhizomes and roots of several centimeters per year. This growth is more rapid when the plant is young. A fairly old plant (10 to 15 years) will have a rhizome radius of 2 feet or more. In addition to their lateral growth, the fleshy and fibrous roots penetrate the soil very deeply over time (36).

If not cut for food, the shoots that give rise to the spears each year eventually become the ferns of the asparagus plant. As the spear ages, lignin accumulates in the pericycle region to provide support for the ferns. The usual location for the initial buildup of lignin is the butt end or the white basal stem area. The young, tender green tip or bud of the spear does not possess lignin (36).

Traditional asparagus varieties are dioecious, meaning that they have male and female reproductive structures on different plants. Female plants produce spears and also produce seed when the plant is in the fern stage. The production of seed diverts photosynthetic energy from spear production and reduces the yield of spears. The seeds fall to the ground and germinate, creating an undesirable asparagus-seedling weed problem in the field (Figure 1). Male plants yield more asparagus with no seeds produced while in the fern stage of growth.

In the late 1800s, Professor William J. Green (18), horticulturist at the Ohio Experiment Station found that male asparagus plants are about 50% more productive than female plants (Table 1).

Today, we have new male hybrid varieties developed to enable the plant to expend its energy solely on spear production, without the production of seeds, thus increasing yields. Yields of these male hybrids have been 2 to 3 times that of the standard open-pollinated Mary Washington types as reported by several researchers across the U.S. (Table 2).

New Jersey Male Hybrids

Dr. Howard Ellison, asparagus breeder, and Dr. Stephen Garrison, Extension Specialist in Vegetables at Rutgers University in New Brunswick, New Jersey, have devoted their lives to asparagus breeding and cultural practice studies. Ellison observed that although asparagus produces both male and female plants, about one in 500 male plants will produce male flowers and a few flowers with functional male and female parts. By selfing flowers on one of these plants, called "hermaphrodites," Ellison produced his first male hybrid. These male hybrids have tolerance to asparagus rust and Fusarium crown rot, two major fungus diseases that affect asparagus.

The New Jersey male hybrids have wide geographic adaptability and have yielded well in several variety trials across the U.S. (Table 2). Some of the more popular New Jersey male hybrids include Jersey Giant, Jersey Knight, and Jersey Gem.

Yields of the New Jersey male hybrids depend on the soil type and whether the asparagus is planted in virgin soil (free of Fusarium) or in Fusarium infested soil. Growers should establish trial plantings of the newer hybrids to see which ones grow best under local conditions.

California Hybrids

The California hybrids, which do not have a super male as a parent, have both male and female plants (dioecious). The attributes of the California hybrids enable the grower in a warmer climate to harvest a taller spear (8-9") without the tip of the spear opening up or "ferning out" under warm weather conditions and still be tender. The New Jersey male hybrids and open-pollinated varieties will fern out at a much shorter spear height (5-6") under warm temperatures (above 70 degrees F). However, the New Jersey male hybrids still seem to yield more than the California hybrids in warmer climates.

The California hybrids have been bred for the warm, arid climates and do their best in those areas, such as California and Oklahoma. But in colder climates having cold winters, the yields of UC 157, a California release, start decreasing after the third or fourth year because of winter injury to the crown.

Some of the newer California hybrids include Atlas, Apollo, Grande, and a new purple asparagus called Viola or Purple Passion. Viola has a beautiful deep purple color when raw. It is also sweeter than green asparagus. When Viola is cooked, the purple color is destroyed and the spears are green.

Apollo and Atlas will keep their tips tight in the heat but they also outyield UC 157 by as much as 25%. The parentage of these varieties have a New Jersey supermale and UC 157 for the female. This combines the heat tolerance of UC 157 with the higher productivity of the New Jersey males.

Atlas and Apollo seem to be yielding better than UC 157 in northern growing areas such as Michigan and Washington state, probably because of their New Jersey supermale parent.

Open-Pollinated Varieties

Other common or "open-pollinated" varieties include 'Mary Washington,' 'Martha Washington,' and 'Waltham Washington.' These Washington varieties were developed by breeding and selection work done by the U.S. Department of Agriculture, in cooperation with the Massachusetts Agricultural Experiment Station and other cooperators, for the purpose of obtaining varieties resistant to asparagus rust (43). These varieties are no longer recommended for commercial production.

The 'Viking KB3' is an open-pollinated selection of 'Mary Washington' developed in Ontario, Canada. It yields higher than 'Mary Washington.' In general, the open-pollinated varieties yield less than the California and New Jersey hybrids.

The New Jersey male hybrids are recommended for commercial planting in the colder, northern areas of the U.S. as well as the warmer areas of the south and west. The California hybrids generally do better in the warmer, arid climates of the U.S. where winter injury from cold winter temperatures is not a problem. However, some of the newer California hybrids such as Atlas and Apollo may have better survivability in the northern growing areas.