Mulch – pluses and minuses

(Photo:  Mulch experiment in Annapolis Royal, NS)

Mulches, The Pluses and The Minuses

Mulch saves soil, moisture, and labour in all garden pots and plots. But not all mulches are equal. All mulches limit evaporation; reduce erosion; suppress weeds to a degree; reduce soil splash on leaves; keep leaves, vines, and fruit from contacting soil (limiting bacteria & fungus transfer); prevent some insects from traveling and egg laying; and discourage cat toileting and squirrel gardening. Mulches are excellent, but they are not 100% effective.

(Photo: this straw mulch did not prevent the greenery growing through it)

In choosing a mulch, it is important to consider what else mulches will do: some subtract nitrogen as they decompose (e.g. leaves, straw, bark); some have chemical colourants (e.g. bark, wood chips); some add heat (e.g. stones, brick chips, glass beads); some change the soil pH over time (e.g. pine needles), some shelter insects, disease, and rodents (e.g. cut & dropped seedless weeds, straw); some complicate weeding when the weeds grow through them (e.g. straw, brick chips); some will grow and have to be dug out (e.g. grass clippings, buckwheat); some may disintegrate when you try to remove them (e.g. some UV sensitive plastics, landscape fabrics); some are allelopathic and produce toxins in-ground that inhibit other plants from germinating (e.g. corn gluten, black walnut, sumac), some mulches are very rich in certain nutrients (and possible pathogens) and can burn young plants (e.g. fresh mushroom mulch, fresh manures), and some prevent water and air from reaching the soil to nourish plants (e.g. plastics, old floor coverings).

(Photo:  Buckwheat at Montreal Botanical — I was warned it could grow)

(Photo: Brick Chips and Weeds — weeding now quite difficult)

(Photo:  Geotextile stops water and air penetration; forms mosquito pools and run-off canals)

To save labour in weeding, most mulches need an underlay that deprives weed seeds of sun (e.g. newsprint, cardboard — do not use landscape fabric, plastic, old floor coverings). Carpets and carpet underlays leach chemicals as they weather and some release unwanted fibres and bits of rubber, glue, and plastic. A newspaper or cardboard underlay will eventually decompose (cardboard more slowly) but landscape fabrics and plastics have to be removed later in the fall or the spring. If you must use landscape fabric or plastic, try bands or strips rather than total coverage.

Mulch placed on top of the underlay has to be removed and stored also. If the top layer is leaves, dry grass clippings, pine needles, buckwheat hulls, or pulled seedless weeds, these can be folded into the garden. Stones, brick chips, wood chips, bark, pine needles, and straw will need to be stored in the fall or raked off in the spring and put back after seed germination.  Landscape fabric and plastics must be removed in the fall or the spring; however, by late fall or early spring, they may be in brittle pieces if they are sensitive to UV rays and the ravages of weather.

(Photo:  our Newsprint Underlay to stop grass & weeds at the fence line)

(Photo:  newspaper underlay covered over with a mulch of shredded leaves — 95% effective)

If no underlay is used, stones, brick chips, wood chips, pine needles, and bark chunks will be embedded in the soil and become harder to remove and separate from the soil; these will make next season’s soil clumpy and much more challenging to rake and shape. Grass, buckwheat, and some pulled, seedless weeds may grow and have to be removed or covered over next spring with an overlay of cardboard, wood (boards or plywood sheets), or plastic to kill the green growth.

Brick chips, stones, clear plastic, and glass pebbles can act as a heat sink to accelerate the warming of spring soil and retain heat later in the day to help plants that need warm soil (e.g. sweet potato, hot peppers).

(Photo: Temporary Clear plastic “solarization” for 3 kinds of  Sweet Potato – Chinese, Kona, & Georgia Jet)

“Mulching” with compost or manure is not really adding a mulch at all. Compost and manure are soil amendments and fertilizers. Placing these on the garden is excellent, but then put a “organic mulch” over these for all the reasons listed in the front of this article.

(Photo:  I use stones as a mulch on hot peppers – also discourages squirrel gardeners)

Glass beads, brick chips, or stones on top of a container of potting mix (no underlay) helps as a heat sink in pots of heat loving plants (e.g., peppers), but they also hasten drying.

Mulch materials (alphabetical order)

Note: all organic mulches use nitrogen as they decompose

• Bark (decomposes over time, remove in spring to prevent soil clumping or use on permanent walkways or shrub beds, or add inorganic underlay and make permanent). Re-use as mulch next season.
• Brick chips (heat sink, remove in fall, or add inorganic underlay and make permanent).
• Buckwheat (decomposes, fertilizes, fold into soil, may grow unless underlay added).
• Cardboard (decomposes, helps soil structure, fold in when possible, makes good organic underlay).
• Corn gluten (decomposes, fertilizes, fold in in fall, prevents seed germination; keep away from planted seeds until after they germinate).
• Glass pellets (heat sink; remove in fall, or add inorganic underlay and make permanent).
• Grass clippings (decompose, fertilize, fold in, will grow grass unless underlay added).
• Landscape fabric as mulch or underlay (remove in fall; if UV-sensitive, may disintegrate into annoying pieces — not recommended but better than plastic).
• Leaves -shredded are best- (decompose, fertilize, fold into soil, – excellent, organic, renewable resource)
• Newspaper underlay (decomposes, fold in when possible, 5+ layers makes good underlay).
• Pine needles (decompose, fold in when possible, acidic, affects pH balance over time).
• Plastic film as mulch or underlay- not recommended-  (prevents moisture and air getting to plant roots; remove in fall; if UV-sensitive, may disintegrate into annoying pieces).
• Pulled/cut weeds minus seeds (decompose, fertilize, fold in).
• Sawdust from untreated wood (decomposes over time but slowly, adds to soil structure; fold in).
• Stones (heat sink; remove in fall, or add inorganic underlay and make permanent).
• Straw (decomposes, adds to soil nutrients & structure, fold in if possible or remove in fall or spring to prevent soil clumping, or move to walkways for next season).  Re-use it as mulch next season.
• Wood chips (decompose over time, remove in fall, use on walkways next season, or add inorganic underlay and make permanent).  Re-use as mulch next season.

Please take a closer look at the mulch experiment pictured at the front of this article:  pine needles, red plastic carpet, black plastic, clear plastic, and bare soil.   I would not recommend any of them as a mulch!  Best, most convenient, and least expensive is shredded leaves from your neighbourhood!

Happy Mulching