Understanding Male Hybrid Asparagus Plants

My asparagus plants were planted five years ago, but the spears grow only to the size of a pencil. It’s a male hybrid that doesn’t have seed. Why are the spears so small and how do I ensure an abundant crop?

Asparagus (Asparagus officinalis), a member of the Lily Family, is a favorite of many gardeners. Thought to be native to the eastern Mediterranean and Asia Minor, the early Greeks collected it from the wild and Romans began cultivating it around 200 BC. Asparagus has been grown in the U.S. since colonial times. It is valued as a tasty and nutritious vegetable with significant amounts of vitamins A and C and other vitamins and minerals. Once established, this early season, winter hardy vegetable is relatively easy to maintain and can thrive for 10-15 years or more. With proper care, home gardeners can have yields of three to four pounds per 10-foot row.

Image Source: Art Rachen

Older varieties of asparagus plants are dioecious, which means that male and female flowers are produced on separate plants. Only the female flowers produce berries and seeds when pollinated. Through extensive plant breeding done by Rutgers University over many years, there are now all-male hybrid asparagus plants, which are more vigorous, live longer, have more disease resistance and provide higher yields than older varieties. Another benefit of planting male hybrids is that since they do not produce seeds, there are no nuisance weeds from asparagus seedlings in the garden.

What’s Wrong with My Asparagus Plants?
Let’s review some of the most common causes for low asparagus yields. Although improper harvesting is most likely the problem, other factors can also be involved.

Harvesting Practices

  • Don’t harvest before the crop is ready. Asparagus is typically grown in the home garden by planting one- to two-year old crowns of the plant (where the plant stem meets the roots) but can also be grown from seed. Do not harvest the first year after planting. If grown from crowns, light harvest for approximately three to four weeks can occur in the second and third years. If grown from seed, a light harvest can be taken in the third year for the first four weeks of the season. Virginia Cooperative Extension research has shown that plots harvested one year after planting had a 23% smaller cumulative yield after five years.
  • Don’t over-harvest in any year. After the third year, harvest six-to eight inch spears every day or two for an eight- to ten-week period. The harvest can continue a little longer if the spear size and regrowth are maintained. Stop harvesting once new spears are about 1/4 inch in diameter.
  • After harvest, let remaining spears to grow. Allow the fern-like foliage to develop and remain until winter or early spring. Asparagus spears are actually plant stems and harvesting too many limits photosynthesis, which reduces food production in the crowns and root system and causes a decline in the plant vigor and yields in future years. If necessary, run twine down the bed to hold up the stalks so the ferns get lots of sunlight for photosynthesis.

Plant Care for Established Asparagus Plants 

  • Soil: Good plant health begins with the soil.
    • Asparagus grows best with soil pH of between pH 6.0 and 6.7. A soil test will tell you the soil pH and provide recommendations on how to adjust it.
    • Good soil drainage is important because asparagus does not do well with wet roots. Add some soil, if the plants have settled since planting. If plantings are in heavy clay soil, add organic matter to the soil.
  • Fertilizer: Asparagus is a medium to heavy feeder. The best time to fertilize is in late winter or very early spring, before spears emerge. A soil test will tell you how much fertilizer, and what kind. In most cases, a general-purpose fertilizer like 10-10-10, adding 2.5 lb. per 100 sq. ft is adequate. Plants also benefit from side dressing of fertilizer after harvest and yearly top dressing of compost.
  • Moisture: Adequate water is needed for spear and new crown development and spear quality and yield. The deep roots of asparagus supply most of the water needed but additional water may be necessary in particularly hot or dry weather. Avoid overwatering especially if your soil is clay-like and doesn’t drain quickly.
  • Weed management: Weeds will compete with asparagus plants for nutrients, and result in thinner spears. Weeds also reduce air circulation around plants, prevent foliage from drying quickly after rainfall or watering, and make the plants more susceptible to diseases and insect pests. Keep the asparagus bed weed-free throughout the season, and starting in late winter before spears emerge. Remember that fungal spores and insect eggs can overwinter in the garden. Needless to say, weed carefully – best to do it by hand (snap off spears or carefully cut spears at ground level) – to avoid damaging spears before they emerge. 

Diseases and Pests: Diseases and pests can also weaken, stunt and reduce the yield of asparagus plants. Fungal diseases affecting asparagus include:

  • purple spot (sunken purple lesions on spears, tan to brown lesions on ferns)
  • rust (spots on spears: yellow-green in the spring, darkening to brick red, then black over the growing season)
  • fusarium crown, root and lower stem rot (brown lesions on lower stems, yellowed ferns).
  • Phytophthora asparagi (spear rot, shriveled roots, yellowing of ferns)

Insect pests of asparagus include:

  • asparagus miners (small black flies)
  • common asparagus beetle (red with yellow and black stripes)
  • asparagus aphid (tiny, bluish gray; affected ferns will have a bushy, broom-like appearance)
  • Japanese beetle (metallic green head with bronze thorax and wings)

Consult this Michigan State University Extension comprehensive guide to identifying diseases and insect pests affecting asparagus plants and for recommendations on managing and treating the specific disease or pest, refer to Virginia Cooperative Extension 2021 Pest Management Guide for Home Grounds & Animals or contact the VCE Help Desk at (434) 872-4583 or albemarlevcehelpdesk@gmail.com.

Whether the problem was over-harvesting, or another issue, chances are that you can manage it. Going forward, harvest lightly for a year or two to let the asparagus crowns and roots build strength. But don’t worry – a healthy asparagus bed will last 10-15 years or even more!

Now you have the knowledge to proceed with confidence. Perhaps you are interested in preparing another asparagus bed or about to start your first one? The best advice is to plan for and prepare the soil a year ahead of planting.

References:

Asparagus”, Relf, Diane, Extension Specialist, Horticulture and McDaniel, Alan, Extension Specialist, Horticulture, Virginia Tech, Virginia Cooperative Extension, Publication 426-401), 2020.

Asparagus”, University of Illinois Extension, Illinois Vegetable Garden Guide.

Asparagus” University of Maryland Extension, updated 10 June 2021.

Asparagus”, University of Massachusetts Amherst, UMass Extension, Center for Agriculture, 2012.

Asparagus Information Bulletin”, Sandsted, R.F. et al., Cornell University, Cornell Cooperative Extension, Dept. of Plant Pathology, Information Bulletin 202, 2017.

Plant Form: An Illustrated Guide to Flowering Plant Morphology, Bell, Adrian D. and Bryan, Allen, Portland, Oregon: Timber Press, 2008.

Disease and insect pests of asparagus”, Morrison, William R III, et. al., Dept. of Entomology and Dept. of Plant, Soil and Microbial Sciences, Michigan State University, MSU Extension, Extension Bulletin E3219, Michigan State University Extension. Morrison WR, Linderman S, 2014.

Growing Asparagus in the Home Garden”, Sanders, Douglas & Bradley, Lucy, Horticultural Science, North Carolina State University, NC State Extension, Revised 9 Oct. 2019.