How do I mulch flower beds the right way?

To mulch a bed properly, clear out the weeds, spread a fresh organic mulch 2 to 3 inches deep across the whole bed, and keep it pulled back from stems, trunks and the house. Most mulching problems come from piling it too deep or against plants, not from choosing the wrong type.

Pick a mulch that suits the bed

Virginia Cooperative Extension's home ornamentals guide lists pine bark, hardwood bark, pine straw and wood chips as good mulches,^[1] and its shrub guide adds straw and slightly decomposed or shredded leaves.^[2] They behave differently once they are down:

• Pine needles interlock and stay in place better than most mulches, especially on a slope.^[5]
• Shredded hardwood does not wash away easily and decomposes relatively slowly.^[5]
• Large pine bark nuggets float and may not stay put in a heavy rain.^[5]
• Small wood chips decompose quickly and use nitrogen from the soil as they do.^[5]

Texture matters for weeds too: shredded mulches encourage weed growth more than larger particle mulches.^[1] On very acid soil, note that hardwood barks may contain high levels of manganese, which can be toxic to plants in soils with a pH below 5.0.^[6] A soil test tells you where you stand.

Check it before you spread it

Smell a handful. Extension's advice is to use mulch that is free of weed seed and has no rotten egg or ammonia odor, because improperly composted mulch can have a low pH and contain chemicals that injure plants.^[1] Sour mulch smells like vinegar, ammonia, sulfur or silage,^[5] and bedding plants and low-growing woody plants are the most easily damaged.^[5] Good mulch smells like fresh-cut wood or garden soil.

If you mulch with grass clippings, check where they came from. Do not use clippings from a lawn treated that season with herbicides or a weed-and-feed product.^[6]

Work out how much you need

There are 27 cubic feet in a cubic yard, and one cubic yard covers 324 square feet an inch deep.^[5] So multiply the bed's length by its width, multiply that by the depth you want in inches, and divide by 324. A 10-by-20-foot bed at 3 inches comes to 200 × 3 ÷ 324, or just under 2 cubic yards. Bulk mulch is sold by the yard; bags are labeled in cubic feet, so check the bag size before you count bags.

Prepare the bed

Weed first. Organic mulches suppress annual weeds but not perennial ones,^[1] so dig out anything with runners or deep roots before you cover it. Extension also notes that newspaper on the soil surface before mulching helps suppress weeds;^[1] lay it on bare soil and cover it completely.

A clean, cut edge between the lawn and the bed holds the mulch in place and makes creeping grass easier to spot. For keeping beds clean over the season, see how to keep weeds out of garden beds.

Spread it at the right depth

Extension's shrub guide says to place mulch 2 to 3 inches deep,^[2] and its ornamentals guide says two to four inches deep and to avoid over mulching.^[1] Texture decides where you land in that range: fine-textured wood and bark mulches should not be more than 2 to 3 inches deep, while coarse pine bark nuggets can go to 4 inches.^[5] Grass clippings and shredded leaves should never be deeper than 2 inches, because they mat together.^[5] Shallow-rooted plants like boxwood should have no more than an inch of mulch over their roots.^[7]

Deeper is not better. In Extension's words, overly deep mulch, or mulch piled against the trunk, promotes shallow roots, disease and pest injury.^[2] Excessive mulch reduces soil oxygen for roots.^[7] If you can still see last year's mulch, you may only need to rake it loose and add a thin layer; see how often to replace mulch.

Keep it off stems, trunks and the house

The cone of mulch in the photo is the most common mistake. Clemson Extension says to keep mulch 2 to 3 inches away from the stems of woody plants to prevent decay and rodent damage.^[5] Around trees, Virginia Tech recommends a thin layer no more than two inches deep, in a donut-shaped ring with little or no mulch touching the trunk.^[1] It explains why: mulch piled against a trunk keeps the bark moist, inviting decay organisms, insects and voles that feed below the mulch surface, which may girdle and kill the tree.^[1]

Along the house, Extension's termite advice is to pull all mulch and landscaping back at least 6 inches from the foundation.^[3] Its termite publication explains that mulch spread against the foundation holds moisture at the base of the house, and moist soil is prime termite habitat.^[4]

Timing and watering

Clemson suggests you mulch new plantings right after you plant them, and established plants in early spring, before weed seeds germinate.^[5] Newly installed mulch should be watered^[5] so it settles and does not dry into a mat that sheds rain.

About landscape fabric

Fabric under organic mulch rarely stays weed-free. Extension notes that weeds may germinate in organic mulch over landscape fabric and send roots through it,^[1] and advises a shallow mulch layer (1 inch) above the fabric.^[1] It calls landscape fabrics more appropriate for tree and shrub beds^[1] and says not to use them when planting groundcovers or bulbs.^[1] In a flower bed you replant often, plain mulch is easier.

Stop and call a pro if

• Soil washes out of the bed on a slope and holding it means regrading or building a wall.
• You find mud tubes, soft wood or swarming insects where the bed meets the house.
• Mushrooms are growing from the base of a tree, or a trunk has been buried in mulch for years and roots now circle it.
• Water stands in the bed after rain and the fix is drainage, not mulch.
• The job is several cubic yards and moving it by wheelbarrow is more than you can do safely.

If the mulch grows mushrooms or slime after rain, see why mold or fungus grows in mulch. Browse the lawn and landscaping answers, see mulch and bed services, or look up lawn and landscaping businesses around Blacksburg. To have your beds weeded, edged and mulched, describe the job once and local businesses quote it.