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When Is the Right Time to Put Down Moss Killer?

By Jolene Hansen ; Updated September 21, 2017
Healthy lawns resist unwanted mosses.
Ryan McVay/Photodisc/Getty Images

Some gardeners cultivate green carpets of moss on purpose. When those same plants appear uninvited in a lawn or other area, however, they may look decidedly less attractive. Mosses, like grasses, span many plant genera and thousands of species. They thrive from U.S. Department of Agriculture plant hardiness zones 1 through 13. Understanding why mosses appear and how to reclaim your territory from them will help you keep areas moss-free.

Why Mosses Invade

Successfully battling mosses in your lawn grass requires understanding that mosses don't kill lawn grass. When poor growing conditions stress and weaken grass, it falters. Mosses often move in and take over available ground. They can survive inhospitable conditions, and they even produce their own food. A thick, healthy lawn keeps mosses and other unwanted plants at bay. Applying chemicals to counteract mosses is a short-term solution. Underlying problems must be addressed to keep areas free from mosses long term.

Which Weapons to Use

Mosses are tough, resilient plants that resist common grass killers and weedkillers. Effective, iron-based chemicals target mosses, causing them to dry out and die. Repeat applications may be necessary. If you wish to kill moss in a lawn, then apply ferrous sulfate, also called iron sulfate, mixed with water. Mix 5 ounces of ferrous sulfate with 4 gallons of water for each 1,000 square feet of lawn you want to cover. Liquid applications completely cover mosses and act more quickly than granular products. Because iron products stain concrete, take care when spraying the ferrous sulfate-water mixture near sidewalks and foundations. Some ready-to-use products combine moss killers with lawn fertilizers to dehydrate moss and stimulate grass growth. Follow manufacturers' labels on products precisely.

When to Attack

Moss killers must be applied during moss' active growth period to be effective. Mosses grow very little during summer months, and most of their growth occurs during mid-spring and early fall. In mild climates, they continue to grow through winter. After treating mosses with a product, keep them dry for at least 48 hours. Wearing protective eye-wear, gloves and other protective clothing when you use a spray is important. Clear an area of children and pets before you apply a moss-killing product, and, if rain is expected, postpone spraying. Changing irrigation schedules may be necessary to keep a treated area dry. When 48 hours pass, water the treated area thoroughly. Within seven to 10 days, mosses should turn black. Rake the treated area to collect the dead mosses, and reseed the site with grass seeds if desired.

How to Hold Your Ground

Restoring your turf to good health will help you retain the upper hand over lawn mosses. If areas are too shady for grass, consider using shade-tolerant ground-covers there instead. Mosses thrive in acidic soil while grasses do best in soil with a higher pH level. A soil test can confirm whether or not adjustments to your turf areas are needed and what nutrients your lawn may lack. Compacted soil creates poor drainage that gives mosses a boost and weakens turf. Regular aeration of a lawn eliminates compaction and improves the lawn's health. Taking simple steps to create healthy turf is the most effective way to keep lawn mosses in check.


About the Author


Jolene Hansen is a lifelong gardening enthusiast and former horticulture professional. She is passionate about reshaping the way people experience gardens and gardening. Hansen's work appears regularly in consumer and trade publications, as well as numerous internet gardening and lifestyle channels.