Do worm castings work in cold climates?
Written by
Olivia Mitchell
Reviewed by
Prof. Samuel Fitzgerald, Ph.D.Worm castings or worm manure is effective in cold climates with appropriate timing. Microbial activity ceases in soil temperatures below 40°F (4°C), but is initiated with warmer weather. I garden in Minnesota and have developed specialized methods for the cold-adapted. They should be used during brief warming or for special methods of season extension. Monitoring of soil temperatures will be a crucial factor in actions taken in zones 4-6.
Season Extension Methods
- Use cold frames to maintain soil above 40°F for early spring applications
- Apply under row covers during late fall to extend activity by 4-6 weeks
- Greenhouse applications allow year-round casting benefits
Perennial Preparation
- Top-dress berry bushes and roses before first hard frost
- Mulch applied over castings insulates microbial colonies
- Spring growth emerges 10-14 days earlier with fall applications
Container Gardening
- Move pots to south-facing walls capturing winter sun heat
- Apply castings monthly to indoor plants during cold months
- Use thermal mass like water jugs to regulate root zone temps
Monitor soil temperatures by using a probe thermometer, and take recordings at 9 a.m. when the temperature is at a stable point. I accept daily measurements of my soil temperature throughout the changing seasons of the year. Applications should be made in the spring when the soil temperature is consistently at 40°F for three consecutive days. Applications should be made in the fall before the ground freezes solidly.
Insulate application areas with the use of organic mulches. Straw or shredded leaves will protect the microbial colonies from the freeze-thaw cycle. My berry patch is mulched with a layer of castings six inches thick. This maintains biological activity during short periods of warm weather in winter. Nutrients are made available to the plants during the early thawing.
Modify application forms for cold conditions. Granular castings are better than teas in cold weather, pre-mix with compost for slow-release nutrients. I mix castings in fall compost piles. The result is a nutrient-rich soil ready for use by spring. Avoid liquid applications under 45ºF.
Read the full article: Unlocking the Benefits of Worm Castings
