Garden Detective

Tis the season, and I've been getting a lot of emails asking if and why mulch is necessary. In a nutshell, mulch keeps the soil moist and its temperature even, suppresses weeds and keeps things looking nice and tidy. It helps keep soil from eroding and reduces soil compaction around delicate roots.

But what is mulch? It's simply a generic term that applies to any ground covering that serves the above purposes. It can be made of a solid fabric or plastic sheet, shredded bark, leaves (except oak), wood chips, pine needles, grass clippings, pebbles, even recycled tires. Of all the organic and inorganic types of mulch available, the most common applied in spring is shredded bark.

But you shouldn't just toss it about willy-nilly -- now there's a word I don't use often enough -- and there's a method to the mulching.

First, wait until the soil warms up. I saw landscapers applying the stuff to my neighbors' gardens as early as St. Patrick's Day. If applied too soon, it will be counterproductive, keeping the coolness in the soil instead of the warmth.

Secondly, too much of a good thing can cause problems. Keep mulch to no more than 3 inches deep. If applied too thickly, it will attract rodents and become a breeding ground for fungus. Large, uncovered piles of mulch left sitting around too long will decompose anaerobically and become toxic to plants once applied. Plan deliveries accordingly.

You've probably passed a common suburban travesty dozens of times this week alone. And you might not even have been aware of it. Trees are dying slow deaths because of a common practice we'll refer to as "volcano mulching." Sure, it might look cool to have a 2-foot-high mound of mulch surrounding your trees, but if you can't see the natural flair that extends from the bottom of the trunk to the soil line, that means the tree is, in effect, suffocating.

The buried portion of the trunk eventually will decay, and the rotting will make the tree susceptible to disease. Certain trees will even send girdling roots into the mulch, restricting the trunk and eventually strangling the tree. It makes me crazy to see people doing this.

Applying mulch 2 to 3 inches deep over the roots in a circle around the trunk - but not touching it - will protect the tree from lawn mower injuries. More than that is harmful.

Shredded bark mulch should be applied in late fall and mid-spring. Wood chips usually need to be replaced every three years, while inorganic mulches last much longer. Pine needles can be used around ericaceous (acid-loving) plants such as blueberries and rhododendrons. They also make a nice winter mulch, as do discarded Christmas tree branches.

Sheet mulches can be rolled out under pebble walkways or mulched areas to suppress weed growth. Dark plastic or landscaping fabric (geotextile mulch) can be applied to vegetable gardens before seedlings are planted. Simply cut an X into the mulch and gently insert the root end of the plant. If the plastic isn't pre-scored or porous, poke holes in it to allow water to reach roots.