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Q: One of my friends tells me that they put coffee grounds in their compost bin. When I talked about this to another friend, she said that she carries the coffee grounds outside every morning and scatters them around her rose bushes. I hesitate to do either, because I have read that coffee grounds make the soil acid. What do you think?

A: As long as you don't put your coffee grounds in the garbage, you can't lose. First of all, you can forget the idea that coffee grounds are acidic and will make the soil acid. Yes, there is acid in ground coffee, but it is in a form that dissolves in water. All the acid goes into the cups of coffee. Used coffee grounds are not acidic; they have been tested and found neutral.

The used grounds do contain some nitrogen, although it is not in a form that is available to plants. The coffee grounds must break down in soil or a compost pile in order to become fertilizer. That is accomplished by the microbes in the dirt or the pile. Coffee grounds actually speed up the fertilizer making process because soil micro-organisms love coffee grounds. These creatures return the favor by changing nitrogen into ammonium nitrate, which plants can use.

When added to a compost pile, coffee grounds are part of the "green stuff," even though their color is brown. They can be balanced in the pile with additions of "brown stuff" in the form of dead leaves or paper (even though the paper is white). A recent experiment at Oregon State University made high quality compost with equal parts of dead leaves, coffee grounds, and grass clippings.

Coffee grounds probably are most useful to plants not by providing fertilizer but by improving soil texture. The grounds create both air spaces and water drainage. Added to poor quality soil, coffee grounds will gradually improve the soil's ability to grow plants; added to good soil, grounds will make it even better.

Like any soil additive, coffee grounds should be used with common sense. A pile of unmixed coffee grounds will not grow plants. A thick layer of grounds on top of garden soil will actually shed water instead of slowing its drainage; the grounds will keep water from penetrating to the dirt in the same way as will a thick layer of leaves.

Mixed with a little dirt and perhaps some kitchen garbage, the grounds are the makings of excellent soil. In other words, scattering coffee grounds around the roses leads to better roses. Burying the roses in unadulterated coffee grounds would lead to problems. An advocate of coffee grounds even tried growing lettuce seeds in a mix of half grounds and half soil. It was not a success. The seeds germinated poorly and grew slowly. As usual, moderation is a good principle.

Gardeners have experimented with coffee grounds as a slug repellant and as a way to keep cats out of the garden. The jury is still out on these uses; some gardeners have had good results and some have not. If coffee grounds repel anything, no one understands why. On the other hand, coffee grounds definitely attract earthworms. Gardeners with worm composting bins know that their worms love to eat the grounds.

If tossing coffee grounds into a compost bin, be sure to throw in the paper filter as well. If you know of a coffee business which is not giving away its used coffee grounds, the owners might be glad to have you take their supply.

Q: What about using vinegar to kill weeds? Does it work?

A: Yes, vinegar does kill weeds, but that does not mean that it will become the only weedkiller you will ever use, or even your favorite one. Sometimes gardeners feel that vinegar will be both effective and safe simply because it is not chemical but natural. But what does "natural" mean? Vinegar can be an ingredient in foods, of course, but the ingredient of vinegar that kills weeds is acetic acid, which definitely is a chemical.

Is vinegar a safe weedkiller? It will not harm plants which are not sprayed. It will not harm anything planted on sprayed ground after sprayed weeds have died. It will not damage the soil or the ground water. Vinegar will kill the leaves on every green plant it touches, whether the plant is a weed or a garden flower. And vinegar should be handled with care because it is irritating to skin, and even more to eyes.

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All of this cautionary information applies to vinegar from a grocery store. It is distilled to a standard five percent acetic acid. Farm and garden supply stores may carry products with higher concentrations of acetic acid, up to 20 percent. Handling instructions for these products are on the container and must be followed carefully. At this concentration, acetic acid can be damaging.

Most gardeners who kill weeds with vinegar use the standard five percent strength. Different varieties (cider, white, rice, wine) taste different in foods, but all are equal in weed killing ability. Do not dilute them with water. Because vinegar kills weeds by burning their leaves, it must be sprayed on dry leaves, and results will be quickest if the vinegar is sprayed on a sunny day. Damage to leaves is visible within an hour.

Because vinegar does not spare garden plants, it is easiest to use in places where there are only weeds to kill, not in flower or vegetable beds. Rather than try to protect neighboring plants from spray, some gardeners choose to paint vinegar on weeds with a brush.

There are recipes online which add dish soap or salt to a vinegar spray. A few drops of dish soap act as a "spreader sticker" to keep the vinegar from running off leaves before they are damaged. Personally, I have never found it necessary. I use plain vinegar. Do not add salt unless you plan to have no plants grow in the sprayed ground in the future. Salt in the soil will prevent all plant growth, and there is no easy way to remove the salt.

Vinegar usually kills annual weeds, like chickweed, with a single generous spraying. Because only the leaves but not the roots are burned, perennial weeds may require a second or even a third spraying before they die. The roots will generate new leaves; if the new growth also is killed, the roots finally starve to death. For example, dandelions in spring have big leaves but only small roots. One vinegar spray may do them in. By late summer a dandelion root has a quantity of food stored; more than one time of spraying will be required. Vinegar will not kill grasses.

To sum up, vinegar is most useful for killing annual weeds on sunny days, especially when the weed is not growing next to a garden plant. It will kill weeds quickly, not two weeks later when the weed seeds have matured and blown away. Like all weedkillers, it must be handled with care.