Asparagus cannot be harvested during the year of crown planting because it must be allowed to grow and develop a strong storage root system. Using vigorous varieties, high-quality crowns, good management, and having a long growing season to permit maximum fern growth, it is possible to harvest for 2 to 4 weeks the first year after crown planting (8,33). The harvest season can be extended to 4 to 6 weeks the next year and to 6 to 8 weeks in subsequent years. Growers should make their own determinations as to when to stop harvesting in their particular location.
Buds on an asparagus crown are arranged in a “dominant hierarchy” or “pecking order” system. The first bud to emerge as a spear is the largest in diameter. When a spear from a crown is harvested, it signals another bud on that crown to send up another spear. With each successive spear that is harvested, the spear diameter decreases because the lower-order buds are smaller and produce smaller diameter spears. The largest spears occur between the second and fifth week of harvest and decrease rapidly thereafter. This is why asparagus harvesting should stop after a certain number of weeks to allow the crown to send up small diameter spears that will lignify and become ferns to manufacture carbohydrates to send down to the crowns for next year’s crop. Spear diameter during the first year after crown planting will be smaller and possibly more difficult to market than the larger spears produced after the first harvest season.
Because the length of harvest season will vary from year to year depending on air temperature, stopping the harvest when one-fourth to three-fourths of the spears have a diameter of less than 3/8 inch (about the diameter of a pencil) is a better guide than harvesting for a specified number of weeks. Experienced gained by growing the crop will make it easier for the grower to know when to discontinue the harvest. Over-harvesting will weaken the crown, reducing the amount of carbohydrates stored for the following year, and will lead to further decline of the planting, putting plants under stress and making them more susceptible to insects and diseases.
Asparagus spears can be cut or snapped to produce spears of marketable length, which is usually between 7 and 9 inches. Asparagus spears may be cut below the soil surface with a knife, or they may be hand-snapped above the soil surface. Cutting asparagus requires more labor, but increases yield 20 to 25% because spears are longer. However, cutting spears below the soil greatly increases the chance of the knife injuring a bud or emerging spear on the same crown.
When hand snapping, the spear usually breaks above the area containing fiber. In other words, the portion of the spear left in the field will be fibrous, while the harvested spear is more tender and is all edible. The small stub left in the soil after snapping dries up and disintegrates. A new spear does not come up at that spot, but comes up from another bud that enlarges on another part of the crown. Snapped, all-green asparagus has no trim-off waste and should command a higher price than cut asparagus with white butts.
To maximize quality, harvest in the morning when the spears are cooler and more easily snapped. Always harvest spears when the heads are tight and before the tips start to “fern out”. In the early, cool part of the harvest season, the heads will remain tight on 9 to 10 inch spears. Fiber content will be low in the base of the spears. In warmer weather, spears will fern out at a shorter height. Therefore, it is necessary to harvest shorter spears during warmer weather to obtain good quality.
To maximize yield, it will be necessary to harvest once or twice per day (morning and evening) when the weather is warm. During cool periods, harvest may not be necessary more than two or three times a week. Growers can estimate how long asparagus spears will grow by adding the highest and lowest daily temperatures and dividing by 2. Spear growth in asparagus, once the spear has emerged, is closely related (about 90%) to air temperature. Asparagus spears grow faster at higher temperatures, and the taller the spear, the faster it grows. Asparagus spears grow about twice as much during the day than at night because of higher day temperatures. Growers can also use the following harvesting decision examples:
Example 1: The crew is about to begin harvesting and the next planned harvest will be in 24 hours. The weather forecast is for cool and dry conditions. What instructions should be given to the crew concerning the minimum spear height selected for harvest today? In this situation, only spears that are longer than 7 inches after harvesting should be cut. All shorter spears should be left for harvesting 24 hours later.
Example 2: It is Saturday morning and the weather forecast for Saturday and Sunday is for very warm, humid, and cloudy weather with a high probability of rain showers. No labor is available to harvest on Sunday so the next harvest is nearly 48 hours away. Under these conditions, even spears that are very short (4 to 5 inches) need to be harvested on Saturday. If the 4 to 5-inch spears are not harvested, they will be 12 to 15 inch spears that have ferned-out by Monday morning. With the weekend situation described above, it would be best to harvest as late as possible on Saturday afternoon or evening and as early as possible on Monday morning to reduce the time to the next harvest and amount of ferned-out cull asparagus. Even though the ferned-out spears are not marketable, they must be cut or snapped to maintain production and to deny insects and diseases a site to get started.
The field should be cut or snapped clean with each harvest. Discard any small, spindly spears or fern growth. Ferns that are allowed to grow during harvest can harbor diseases and insects, especially asparagus beetles. This growth will also retard the emergence of new spears.
To avoid stooping, a simple harvest-aid machine is frequently used to carry workers for snapping asparagus. An average of approximately two person-hours of labor are needed to hand-cut one acre of asparagus at each harvest over the course of a picking season. Harvest-aids reduce the labor requirement by 15 to 20%, and workers are usually more content to ride a harvest-aid than to walk and stoop to cut asparagus.