Protect perennial asparagus plants from cold temperature damage by mulching the asparagus bed with compost, aged manure, or straw before freezing weather or snow arrives.
Asparagus crowns can produce tasty spears for 20 years or longer if given the right care and nutrients.
The ferny stalks and branching growth of asparagus plants capture energy through photosynthesis during the growing season strengthening asparagus crowns (the collection of fleshy roots just below the soil surface) for spear production the following year.
Leave asparagus stems on plants as long as they remain green—well into autumn. When stalks turn brown and brittle cut them off at ground level and top dress the bed with compost or manure. (Place cut stalks and ferns in the trash—not in the compost pile; asparagus-beetle eggs can overwinter in cut stalks.)
Again, cutting stalks back too soon means sacrificing photosynthesis and will mean less spear production next year. The exception: cut down female asparagus plants before they set seed (red berries); seed production diverts energy from crowns and lowers future yield. (Picking off berries is a solution, but highly time consuming. Growing only male plants may be the best course—as well as the source of thicker spears.)
Where heavy snow covers asparagus beds in the winter let stalks turn brown and fall across the planting bed to form their own mulch to protect plant crowns. Then add 6-inches of straw, pine needles, or well-rotted or chopped leaves to the top of fallen stalks to give crowns extra protection from freezing temperatures.
A planting bed of well-drained garden loam is the best home for asparagus plants. Adding organic matter—well-aged compost or well-aged manure and a layer of straw or chopped leaves—to the asparagus bed each autumn will ensure crowns and plants have the nutrients they need to produce ample spears for years and years.
Autumn is also a good time to check the soil pH in asparagus beds. Asparagus plants prefer a sweet soil—a pH of at least 7.0. Add lime to raise the pH or sulfur to lower it.