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When Moss Goes Bad

Q. I had a truckload of topsoil brought in to fill my raised beds. The beds are now covered with moss, as are some places where the topsoil wasn't used. These areas get sun six to ten hours a day. How do I discourage the moss?

    ---Linda in Troy (Northeastern), Pa
I have a problem in my vegetable garden with spreading moss. It is a raised bed with adequate sunlight. We've tried digging up the moss to no avail. I heard something somewhere about using cider vinegar. Any suggestions?
    ---Zina in Newark, DE
I have moss covering my entire flower garden. It's shaded for about half of the day, and the ground stays moist until late summer. I rake and pull the moss off, but it just grows back. How can I get rid of this stuff? Thank you—I listen to your program on WYSO 91.3 FM from Yellow Springs.
    ---Marilyn in West Alexandria, Ohio
A. Moss doesn't come in on topsoil (which is a meaningless term; next time get compost to fill those beds!). With few exceptions, moss doesn't grow in sunny areas either; and I'm always suspicious when people use terms like "adequate" to describe how much light their plants are getting. An area with {quote} "adequate sunlight" is often adequate only in the mind of the gardener, who has never asked the poor plants how they feel about being banished to their Stygian depths.

So, the first thing to do is to take an honest look at the sunlight and airflow those beds are receiving. If you are incapable of taking an honest look, ask somebody else. Oh wait—I'll do it for you. It's dark as Hades in there! See what you can do to increase the sunlight and airflow: Cut back or remove brush and weedy growth just outside the area and thin out overhanging or air-blocking tree limbs—preferably over winter (the best and safest time to prune trees).

All of your plants (except the moss) will benefit. You'll be amazed at how dramatic the response can be when you trim back and/or remove a few plants from a crowded space.

Also test the soil. Most mosses thrive in an acidic environment and sometimes adding a little wood ash or lime to raise a highly acidic soil closer to neutral can make problems like this go away. And most plants would appreciate a friendlier soil too. Just don't use wood ash or lime near plants that need acidic soil, like azaleas, rhododendrons and blueberries. (Here's a previous Question of the Week about using wood ash to raise soil pH.)

And be sure to let the area dry out completely between watering. Shady spots don't generally need much supplemental water, and only your moss will be unhappy if you stop making it soggy in there. Here's a very important previous Question of the Week about watering wisely.

Q. Conditions have been just right for moss to grow on the shady side of our roof. Is there anything that can be done to kill the moss and prevent it from ruining the roof?

    ---Linda in Angelica NY
A. I doubt that it will 'ruin the roof', as moss has no roots to cause structural damage. I suggest you just stop looking at it—or look at it and enjoy it. I have moss on the roof over my office and it looks a heckuva lot nicer than the shingles it's covering up. Moss will keep the shingles underneath a little moister, but on a sound roof that shouldn't be a problem. If you feel you MUST do something, trim tree branches to increase the amount of sun that reaches the roof; but be aware that the rooms underneath will get hotter in the summer as a result and increase any air conditioning costs.

Q. We have moss growing between the cracks in our brick patio. Is there anything that can be applied to kill it off? Or do I just scrape it out?

    ---Thomas in Annandale, VA
A. In case you haven't noticed, we live on a pretty lively planet, Tom. Something is going to grow in those cracks; if not moss then weeds—which, unlike moss, do have roots that might eventually cause structural problems. Try and think of it as colorful green grout. Or spread some wood ash on the cracks (don't use lime here) or use one of the specialized soaps sold for natural moss and algae control.

Q. The brick walk around my garden gets very little sun and is covered in moss, which makes it slippery. I've seen suggestions on the web for spraying it with Clorox or sprinkling it with baking soda; and I also purchased some insecticidal soap.

    ---Sylvia in Philadelphia
A. Chlorine bleaches like Clorox are incredibly toxic to nearby plants, soil life, amphibians, and human lungs (chlorine was a popular and deadly trench gas during World War I). And when chlorine breaks down, it forms cancer-causing dioxins. So, that would be a 'no' to the Clorox, Sylvia. I like the baking soda, as its gritty texture would also make for surer footing; same with sand. Insecticidal soap is designed to smother insect pests, not to remove moss. You'd want to try one of the moss and algae removing soaps, which are made differently.

And, as we've been saying, increase sunlight to the area and spread some wood ash. You might also want to scrub those bricks really clean and then apply friction tape, which has a sandpaper-like consistency to its non-sticky side. I use it to keep the slipperiness factor down on the wood decking we have under some trees. It works great, and is available in rolls (like masking tape) at most hardware stores and home centers.


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