Mulch For Cannabis: Best Types, Depth And When To Apply
Quick answer: Mulch is any material spread over the soil around plants. For outdoor cannabis, organic mulches such as straw, shredded leaves or compost conserve soil moisture, help control weeds and moderate soil temperature. Apply it after the soil has warmed, 2 to 4 inches deep (4 to 6 for straw), and keep it 2 to 3 inches away from the stem.
What Does Mulch Do for Cannabis Plants?
University of New Hampshire Extension defines a garden mulch as any material spread on the soil surface to modify the environment where the plant is growing. Iowa State University lists what mulches help with: they control annual weeds, conserve soil moisture, moderate soil temperatures and reduce soil erosion.
University of Wisconsin–Madison Extension goes further. Mulch reduces soil erosion and crusting, because rainwater strikes the mulch rather than splashing directly on the soil, and it helps protect plants from pathogen inoculum that can be splashed up from the soil.
Water is the big one for an outdoor plant. Mulch retains soil moisture by decreasing evaporation from the soil surface, which pairs well with the basics in watering cannabis plants.
How Organic Mulch Changes Soil Temperature
In summer, organic mulches tend to buffer soil temperature by keeping the ground cooler in the daytime and warmer at night. They also add organic matter to the soil and don’t have to be removed at the end of the growing season.
Best Mulch for Outdoor Cannabis
Organic mulches come from plant material, and extension guides agree on a few that suit vegetable-style beds.
| Mulch | Depth | Notes |
|---|---|---|
| Straw | 4 to 6 inches (Wisconsin) | Apply deeper than other organic mulches, since they pack down |
| Shredded leaves | 3 to 4 inches (New Hampshire) | Shred or compost leaves first to prevent matting |
| Grass clippings | 2 to 3 inches (New Hampshire) | Let them dry first; fresh clippings may form a dense mat |
| Compost | 2 to 3 inches (New Hampshire) | Improves soil structure, but doesn’t suppress weeds as well as other mulches |
| Shredded hardwood or pine bark | 2 to 3 inches (New Hampshire) | Extra nitrogen needed |
| Pine needles | 3 to 4 inches (New Hampshire) | Easy to apply and will not compact |
Sources for the table: University of Wisconsin–Madison Extension, University of New Hampshire Extension and Iowa State University. For the soil underneath, see the best soil for growing marijuana outdoors.
Straw
Iowa State University calls straw an excellent mulch for the vegetable garden. It’s less likely to mat and smother crowns, but it may provide a winter habitat for mice and other rodents. If rodents are a worry, see how to protect weed plants from mice and rats.
Leaves, Clippings and Compost
Shred or compost leaves before using them as mulch to prevent matting and speed up decomposition. Let grass clippings dry before using them, as fresh clippings may form a dense mat or produce an odor. Grass clippings also decompose quickly. Compost, used alone or with other mulches, helps improve soil structure, drainage and fertility, though it doesn’t suppress weeds as well as other mulches. More on that in best fertilizer and compost for growing outdoors.
Wood Chips and Bark
Bark and wood chips are slow to decompose and are best used in landscape plantings, around trees, shrubs and roses, and in perennial beds. Mulches with lots of carbon compared with nitrogen, such as hardwood bark, straw and sawdust, stimulate microbial growth that depletes available nitrogen in the soil, and the effect is greater on herbaceous plants. UNH Extension notes that shredded hardwood and pine bark mulches need extra nitrogen.
How Deep to Mulch Cannabis
Generally, 2 to 4 inches is effective, according to Iowa State University, with coarser mulches such as wood chips at the deeper end and finer mulches such as grass clippings at the shallower end.
Depth works both ways. A thin mulch layer is less effective at weed suppression and moisture conservation and needs frequent replenishment. An excessively deep layer can lead to waterlogging, reduced soil oxygen levels and shallow rooting.
Young transplants get less at first. University of Wisconsin–Madison Extension says vegetables grown from transplants should be mulched sparingly, about 1 inch, when they’re young, then gradually increase the mulch depth to 3 inches as the plants grow taller.
When to Mulch Outdoor Cannabis
Place organic mulches around annual plantings only after the soil has warmed up. Applying organic mulches too early in spring will keep the soil cold longer and slow plant growth. UNH Extension agrees: natural mulches should go down after the crop has begun to grow and after the soil has warmed.
Sandy soils warm up more quickly than heavy clay soils, so mulch can often go down sooner on sandy ground. See transplanting marijuana plants for the planting step.
How to Mulch Around Cannabis Plants
- Weed the area well before applying mulch.
- Check the soil. The soil should also be weed-free and moist when natural mulches go down.
- Then spread mulch over the entire planting area.
- Keep mulch 2 to 3 inches away from the base of plants to prevent conditions that encourage diseases or attract rodents.
Don’t pile mulch against the stem. UNH Extension warns not to let mulch materials touch plant stems, as this increases the risk of disease, and not to form a mulch volcano at the bottom of the plant.
Plastic Mulch and Landscape Fabric
Unlike organic mulches, synthetic mulches keep the soil warmer than bare soil in the summer. Clear plastic warms the soil the most, with soil temperatures averaging 9°F above uncovered soil, while black plastic warms the soil about 5°F above bare soil.
Black plastic is excellent for conserving moisture, warming the soil and providing weed control, but since it won’t decompose, it must be removed after each crop. For essential oil hemp, Penn State Extension’s quick-start guide prefers raised beds with plastic and drip irrigation for weed control, and suggests laying plastic and drip weeks before the planting window.
Landscape cloth, woven of fabric, plastic or paper, lets water and air move through it. Adding several inches of another organic mulching material on top gives better moisture, temperature and weed control.
Mulch Problems to Watch For
One drawback of organic mulches is that you may see increased slug problems, especially in damp weather. Mulches also shelter many insects, most of them beneficial or harmless recyclers, but they can attract pests such as slugs, snails, sowbugs and earwigs. For prevention basics, see protect marijuana plants from pests and bugs.
Watch for sour mulch. Mulch stored in very large or saturated piles can ferment and produce compounds that harm plants. Sour mulch has a strong vinegar or silage odor, while properly stored mulch smells like freshly cut bark or fertile garden compost.
Indoors, mulch has one more catch. Colorado State University researchers note that live-soil beds kept in continuous indoor use often carry a surface mulch, and that surface mulches favor the establishment of rice root aphid. The root aphids on cannabis guide covers that pest.
Frequently Asked Questions
Is Mulch Good for Cannabis Plants?
For outdoor beds, usually yes. Iowa State University says mulches help control annual weeds, conserve soil moisture, moderate soil temperatures and reduce soil erosion. The catch is how you use it: too deep, too early or piled against the stem, mulch can cause problems, so keep it 2 to 4 inches deep and off the stem.
What Is the Best Mulch for Outdoor Cannabis?
Straw, shredded leaves, dried grass clippings and compost all suit vegetable-style beds, according to Iowa State University and University of Wisconsin–Madison Extension. Straw is an excellent vegetable garden mulch that’s less likely to mat. Bark and wood chips are better suited to landscape plantings and can tie up nitrogen in the soil.
How Thick Should Mulch Be Around Cannabis?
Generally 2 to 4 inches, says Iowa State University, with finer mulches at the shallower end and coarser ones at the deeper end. Straw packs down, so Wisconsin suggests about 4 to 6 inches of it. For young transplants, start with about 1 inch and work up to 3 inches as the plants grow taller.
When Should You Mulch Cannabis Plants?
After the soil has warmed up. University of Wisconsin–Madison Extension says applying organic mulches too early in spring will keep the soil cold longer and slow plant growth. UNH Extension adds that natural mulches should go down after the crop has begun to grow, on weed-free, moist soil.
Should Mulch Touch the Stem?
No. UNH Extension says to keep mulch 2 to 3 inches away from the base of plants to prevent conditions that encourage diseases or attract rodents. Letting mulch materials touch plant stems increases the risk of disease. Avoid piling it into a mound, sometimes called a mulch volcano, at the bottom of the plant.
Is Plastic Mulch Good for Cannabis?
It can work. Black plastic conserves moisture, warms the soil and controls weeds, but it has to be removed after each crop, says UNH Extension. University of Wisconsin–Madison Extension says black plastic warms the soil about 5°F above bare soil. Penn State prefers a raised bed with plastic and drip irrigation for essential oil hemp.
Sources
- Iowa State University Extension, Using Mulch in the Garden
- UNH Extension, Garden Mulches (fact sheet)
- University of Wisconsin–Madison Extension, Mulches for Home Gardens and Plantings
- Penn State Extension, Mitigating Production Risks in Hemp for Fiber, Grain, and Essential Oils
- Cranshaw et al., Colorado State University, Developing Insect Pest Management Systems for Hemp in the United States (Journal of Integrated Pest Management, 2019)
