Asparagus (Asparagus officinalis) is a long-lived herbaceous perennial plant cultivated for its tender young shoots. It is one of the first crops to emerge in spring, signaling the start of the growing season in temperate regions. Asparagus belongs to the family Asparagaceae, though it was historically grouped with the lily family (Liliaceae). Once established, plants are capable of producing a harvestable crop for 15 years or longer due to their robust underground structure.
Anatomy of the Edible Spear
The familiar edible portion of the plant is a tightly compressed young stem, or shoot, commonly referred to as a spear. These spears emerge directly from the soil in the spring, typically reaching a diameter between 8 and 24 millimeters. They are harvested when they are approximately 6 to 8 inches tall, just before the tip begins to loosen or “fern out.”
The spear’s cylindrical stalk is enveloped by small, scale-like structures, which are actually highly reduced leaves. These scale leaves are triangular in shape and protect the rapidly growing tissue of the stem. The tip of the spear is pointed and tightly furled, resembling an unopened pinecone, where the immature branches are all bunched together.
While the most common variety is green, the color of the spear can vary depending on the cultivation method and cultivar. White asparagus, popular in Europe, is achieved through blanching, where the spears are covered with soil or plastic to prevent exposure to sunlight. A deep purple variety also exists, which contains a higher concentration of anthocyanins, a type of antioxidant.
If left unharvested, the spear rapidly develops a tough, woody exterior as supportive cells multiply and take on lignin. This hardening is a natural process that prepares the stem to support the dense foliage of the mature plant.
Identifying the Mature Fern Foliage
If the young shoot is not harvested, it develops into the mature structure known as the fern. This mature plant is significantly taller than the edible spear, growing into a bushy, feathery structure that can reach heights of 3 to 7 feet. The fully grown stalk has a stout, multi-branched habit, creating a canopy that shades the ground.
The soft, feathery appearance of the foliage is due to masses of fine, green, needle-like structures called cladodes. These cladodes are modified stems that perform photosynthesis for the plant, as the true leaves remain as the small, protective scales on the main stem. Cladodes are clustered together, giving the mature plant its characteristic airy texture.
Asparagus is a dioecious plant, meaning individual plants are either male or female. Both sexes produce small, bell-shaped, greenish-white to yellowish blooms. Female plants follow flowering with the production of small, round, bright red berries in late summer, which contain the seeds and are considered mildly toxic to humans.
Male plants focus energy on producing more spears, often resulting in a higher yield and longer life because they do not expend energy on seed production. Female plants divert energy into developing berries and seeds, which can slightly reduce spear production and plant vigor. The presence of red berries is the easiest visual clue to identify a female plant.
The Perennial Crown and Root System
The ability of asparagus to return year after year is rooted in its extensive underground structure, composed of the crown and its associated roots. The crown is a horizontal, thick, woody rootstock, or rhizome, that lies a few inches below the soil surface. This structure forms the buds for the following year’s spears.
Radiating outward from the crown are two types of roots with distinct functions. The most visible are the thick, fleshy, cord-like storage roots, which can spread horizontally up to 6 feet or more. These roots are packed with stored carbohydrates, primarily starch, which acts as the energy reserve fueling the rapid growth of the spears in early spring.
Interspersed with these storage roots are a network of finer, more fibrous roots. These are the absorptive roots responsible for taking up water and nutrients from the soil to support the above-ground growth. The entire root system is extensive, expanding in length and number each year.
This underground system defines the plant’s perennial nature and its cultivation requirements. The growth of the fern foliage during the summer is necessary to photosynthesize and replenish the carbohydrate stores in the fleshy roots. If the spears are harvested too heavily, the root system is depleted, leading to weak, spindly spears in subsequent seasons.