What mulch should never be used in gardens?
Written by
Liu Xiaohui
Reviewed by
Prof. Samuel Fitzgerald, Ph.D.Some mulches can actively harm your garden, even when the intentions are good. Dyed wood, walnut leaves, fresh manure, and faulty plastic lead to plant poisoning and soil toxins. I discovered this when I lost an entire bed of roses to toxic mulch.
Dyed Wood Products
- Toxins: Arsenic, chromium and copper preservatives
- Impact: Kills soil microbes and earthworms
- Signs: Bright unnatural colors leaching dye
- Alternative: Natural cedar or cypress bark
- Caution: Test dyed mulch with vinegar - if color runs, avoid
Walnut Tree Debris
- Toxin: Juglone chemical in leaves/branches
- Impact: Stunts tomatoes and kills nightshades
- Signs: Wilting within 48 hours of application
- Alternative: Oak or maple leaves
- Caution: Contamination radius extends 60 feet from walnut trees
Fresh Animal Manure
- Risk: Ammonia burns and pathogen contamination
- Impact: Scorched roots and E.coli risk
- Signs: Strong ammonia smell
- Alternative: Composted manure aged 6 months
- Caution: Never use on root vegetables
Solid Plastic Sheets
- Risk: Root rot and anaerobic soil
- Impact: Drowns plants in rainy climates
- Signs: Pooling water and foul odors
- Alternative: Permeable landscape fabric
- Caution: Requires drainage holes every 12 inches
Identify potentially harmful mulches before using them. The best "smell test" is for manure mulches. If the manure smell has ammonia, it was mistaken for composted manure. Most wood mulch will have tags with the CCA treatment codes. Plastic mulch needs drainage holes. Use questionable mulches and sow 4 bean plants in a pot. In a few days, the beans will show abnormal toxicity. This saved my garden from wood chips from having a walnut tree in a neighboring yard.
There are safe alternatives to every mulch that is harmful. It's better to use compost instead of manure. Use natural bark instead of dyed wood. Use permeable fabrics instead of solid plastic. Be cautious about where you obtain your materials! I now verify the source of mulch and age manure correctly. Your plants will be happier with these extra steps.
Read the full article: Top 10 Best Mulch for Gardens: Ultimate Guide
