A QUICK PRIMER ON MULCH Gardener's Best Friend

Fri, 09/22/2017 - 17:00 -- Admin

Plants feed from the soil, and mulch develops the kind of soil that makes healthy plants.  When we force our plants to rely on fertilizer rather than healthy soil for food, they become stressed from forced growing, incorrect balance of nutrients, and drought at the air-soil barrier.  Relying on fertilizer is like relying on candy for nutrition: a short-term buzz, but a long-term problem.

There are four aspects in gardening.  Mulch addresses the improvement of all of them.

1.  The SOIL

  • Improves structure (i.e. balance of sand, organic materials, bio-available minerals)
  • Controls moisture levels.  Studies show moisture retention at 6, 18, and 36 inches is higher with mulch than without.  Highest moisture is at 6 inches, where most of feeding and delicate roots are found.
  • Improves drainage as well as movement of water
  • Prevents erosion and compaction of soil
  • Moderates temperature

2.  The PLANTS

  • Provides nutrients as it slowly composts, giving your plants a season-long feeding.


  • Directly suppresses pathogens and pests
  • Enhances and feeds beneficial organisms
  • Neutralizes pollutants (from studies on land reclamation in Washington State, and Stanley Park's Lost Lagoon in Vancouver)


  • Economic: recycles organic materials so it is less costly to maintain landscape, conserves water, decreases need for pesticides
  • Aesthetic: provides a uniform, clean, tended look to your landscape
  • Ease of application: throw it down, walk away
  • Ease of weeding: mulch kills many weeds.  The more vigorous ones that sometimes grow through it are easy to pull because they have little in the soil below.  Weeds that propagate by root runnners move up into the mulch so are easily pulled out to their full length.

Mulch is classified into three main types:

1.  Landscape mulch: also called arborist wood chips, is composed of freshly chipped or shredded bark, woody chops, leaves, needles.  A study in 1990 tested 15 different landscape mulches, including bark, sawdust, compost, grass clippings, leaves.  Arborist wood chips performed best in every category.  Contrary to some opinions, cedar tree and hedge trimmings make excellent arborist mulch.  Cedar kills many common garden pests and pathogens.

2.  Food crop mulch: 'feed and doctor mulch' for the home gardener.  The main one in this type is compost (combo of kitchen waste, grass, leaves, very small twigs, etc).  Other crop mulches are composted manure, dug-in cover crop.  Top this type of mulch with arborist mulch or coconut fibre (coir fibre) for moisture and pest control.

3.  Specialized mulches:

  • Rock mulch - sometimes combined with oyster shell.  Enhances reflection and ground heat for cactus and succulents. Turkey grit is used to create lime leaching for specialty plants like hardy cyclamen.  This is used in some types of alpine and French intensive gardening.  #10 size is most used, and is available in feed stores.
  • Acid mulch - for rhododendrons, pieris, hydrangea, camellia etc.  This will include grass, evergreen needles, coffee grounds.  Acid mulch works well on vegetables needing higher nitrogen, like oriental greens and lettuces.
  • Feed and doctor mulch - often combined with manure for spring time feeding beneath arborist mulch.  Eg. coffee grounds, pine needles, grass clippings, alfalfa pellets (very good on roses), rotted kitchen compost, and leaf mould.

How do the others compare?  Studies showed other mulches used alone in tests behaved as follows:

  • Sawdust - too fine, so it compacts and creates a barrier to nutrients.  Use only with other materials, as part of an arborist mulch mixture.  NB: NEVER use wood shop scraps, as they are often treated.
  • Cosmetic bark mulch - it provides very few nutrients, and encourages certain 'wood eaters' like sow bugs.  It does compost, however, so can make a top decoration for areas that look better with a coarse groundcover.
  • Landscape cloth - this mulch creates several problems while not really solving any.  It creates a barrier, often allowing a film of water to sit around tender roots, causing rot or air pockets.  Small pests can nest in this film.  It breaks down and rips within 3 to 4 years, the remnants soon becoming a nuisance.  Like cardboard and newsprint mulch, it creates a barrier that discourages the transfer of food and water.  It does block some types of weeds for the first year that are already beneath the barrier but, after that, weed seeds that drop from the surrounding areas simply take hold on top of the cloth.  Heavy-duty, industrial thickness landscape matting designed to be a base for sand under walkways lasts much longer, and suppresses weeds caught beneath for longer.  But the result in the landscape is the same.  In addition, landscape cloth encourages many noxious weeds, like English ivy, wild sorrel, Himalayan blackberry and morning glory, all of which put out runners.  Their runners travel much further unimpeded, and spread through you garden rapidly wherever the cloth is broken.  Use it only when using a rock/gravel mulch, so you can roll it back and feed the soil underneath.
  • Straw and straw-based rotted manures - straw is often a base for chicken manure, mushroom manure, and some horse manure.  Alone it is good as insulation, and breaks down, but is too expensive.  Cow manure is usually sawdust-based, so can add tilth to a soil that needs it.
  • Newspaper & cardboard sheets - good to blanch the stems of your leeks, chicory or cardoon; however they may become pest havens.  These materials create a barrier to water and air exchange when too wet and, when they dry out, stop water from getting into the soil.  However, they are useful in path creation and as sheet mulching/lasagne gardening.

How best to apply mulch around a plant?

Remember Tim Horton's and Iceland: donuts good, volcano bad!  Apply the mulch around each plant in a donut shape, allowing a little (no more than an inch) of bare soil around the stems or stalks of the plant.  A mounded shape (the volcano) holds dampness against the plant right at ground level.  This is a vulnerable area for many plants, and constant moisture attracts moulds, pill bugs, and damping-off rot.

What NOT to use:

  1. Recycled rubber.  This was once fashionable as it was touted as being permanent, but it actually breaks down like any other organic material.  Unfortunately it releases toxins, and is flammable when it gets dry in summer.
  2. Coffee grounds alone.  It is a great combo with other materials, but will compact as it is too fine, so creates a barrier.  Use 20% maximum in compost, or a 1/2 inch layer scratched into the top of the soil to suppress mould and mildew diseases, topped with 4 inches coarser mulch.
  3. Sawdust from wood shop/carpentry shops, unless you know the source wood!
  4. Landscape cloth alone.

When and How to Apply Mulch

SPRING:  when you trim all the trees, perennnials, and hedges, chop up the trimmings and put them around your plants immediately.  Studies have shown that 'aging' or composting your landscape mulch is not only unnecessary, but removes nutrients the plants will use as the mulch composts in place.  The old myth that 'raw' mulch steals nutrients and nitrogen from a plant or soil in the composting process is just that - a myth.  When you are ready to add your compost and seasonal amendments, pull back the mulch, mix the amendments into the soil, and pull the old mulch back on top.

AUTUMN: rake up the leaves, mix into the mulch around the plants.  For areas with evergreen shrubs, be sure to include the needles and cones.  If you have more leaves that you can use, bag them and let them rot over winter.  A bag next to the compost bin makes it easy to layer your compost over the winter when you throw out your kitchen greens.  By spring the bagged leaves will be partially rotted, and contain worms and wonderful leaf mould that is perfect to mix back into the mulch that will be slightly depleted from the heavy winter rains.

How Much Mulch is Good?

  • 6 inches for ornamental areas
  • 2 - 4 inches on top of compost for small food gardens
  • 8 - 12 inches for restoration sites
  • 12 inches for restoration sites where weeds are a major problem


  • Washington State University, Puyallup Extension Centre publications
  • Washington State University, Master Gardener Magazine, Summer 2007
  • A-Z Encyclopaedia of Garden Plants, Canadian Edition
  • UBC Garden forum (on-line)

Author: Jo Canning, Master Gardener


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