Choosing the right organic mulch

By Melanie Mathieson
Gardening Guru

Mulches influence moisture penetration in few ways. Bulky materials such as wood chips, sawdust and straw temporarily hold a fair amount volume of water, and thus prevent loss by runoff when the rate of application — natural or artificial — is too rapid for soil penetration. This may be more important with a heavy silt soil than with a porous sandy soil. However, maintaining the soil structure loose and open may be the most important factor involved. Rain beating on an exposed soil, compacts it and subsequent baking in the sun almost completely gets rid of its capacity to absorb water rapidly. The open soil structure found under mulch is also favourable to rapid air exchange. Roots require oxygen for the respiration process through which energy for growth is released. Choosing the best mulch for your application has to take this and other factors into consideration. Mulch can offer all the right features except for one factor like its appearance or cost to purchase and suddenly is isn’t the right product for you anymore. I have summarized some of the types of mulches that we have easy access to in our region and have listed the determining characteristics for each type. I hope this column will assist you determining the right organic mulch product for your needs.
Bark and Wood Chips
Appearance: Good – Bark stays a nice reddish brown colour but wood chips will fade from a fresh light wood colour to a weathered grey over time.
Insulating value in summer: Good
Insulating value in winter: Good
Need to remove in spring: No, can stay on the garden year round.
Nourishment and aeration afforded to underlying soil by decomposition: Fair
Lets water and oxygen move freely into soil: Good
Ease of application and maintenance: Good but needs to be replenished every couple of growing seasons as it naturally decomposes.
Highly recommend either product to be used as mulch. If you need a large amount buy in bulk to save on cost. You may even know someone who can deliver you a truckload directly from the bush if you are lucky.
I have used poplar bark right as well as pine bark, either works fine.
Sawdust
Appearance: Good, but will fade from a fresh light wood colour to a weathered grey over time.
Insulating value in summer: Good
Insulating value in winter: Good
Need to remove in spring: Yes, as will get compacted from the weight of the snow in winter.
Nourishment and aeration afforded to underlying soil by decomposition: Fair
Lets water and oxygen move freely into soil: Fair
Ease of application and maintenance: Good but needs to be replenished each season as can easily become compacted and can develop patches of mould poor fungus (mushrooms)
Because of its tendency to compact or develop mould or fungus, sawdust isn’t high on my list as a mulch product unless you want to use it as a winter protection mulch.
Pine Needles
Appearance: Pine needle mulches provide the reddish-brown color (although less vivid) that store-bought bark mulch offers -- for free
Insulating value in summer: Good
Insulating value in winter: Good
Need to remove in spring: Yes
Nourishment and aeration afforded to underlying soil by decomposition: Fair
Lets water and oxygen move freely into the soil: Excellent
Ease of application and maintenance: Good
Note: Can be hard to get enough pine needles to use as a mulch unless you have a lot of pine trees planted on your property.
Best used as mulch on gardens with plants that require acidic soils (evergreen trees or shrubs, rhododendrons).
A good choice if you require a garden with acidic soil.
This is also something to consider if you have a cat problem in your flowerbeds. Cats do not like to walk on the prickly needles as the sharp needles can also help to deter snails.
Wear long sleeves and gloves when installing to protect your skin.
Leaves
Appearance: Fair
Insulating value in summer: Excellent
Insulating value in winter: Excellent
Need to remove in spring: Yes, when used as winter protection and mounded on top of plants, but can also be laid directly on soil as a mulch for the growing season. Make sure to shred first to ensure good water and air penetration through the layer.
Nourishment and aeration afforded to underlying soil by decomposition: Excellent
Lets water and oxygen move freely into soil: Fair (unless leaves are very finely shredded).
Ease of application and maintenance: Fair – make sure to shred leaves first.
This is a good choice in the sense that leaves are free and your neighbours may be very happy to supply you too. Leaves will decay and breakdown very quickly depending on the seasonal conditions. This can leave you without mulch near the end of the season but will provide your garden with a good supply of nutrients.
Straw
Appearance: Straw brightens your area nicely but then will fade to a weathered grey if used the entire growing season.
Insulating value in summer: Excellent
Insulating value in winter: Excellent
Need to remove in spring: Yes, when used as winter protection and mounded on top of plants, but can also be laid directly on soil as a mulch for the growing season.
Nourishment and aeration afforded to underlying soil by decomposition: Excellent
Lets water and oxygen move freely into soil: Excellent
Ease of application and maintenance: Fair
Note: never use hay because it has too many seeds in it and you’ll soon have a hay field growing. Straw will also sprout but not as prolifically as hay.
This is a great mulch for the vegetable garden and one of the best for mounding onto plants for winter protection. If you are able to get flax straw in the District or from Manitoba this is the best straw to use as it contains few weed seeds and doesn’t biodegrade very rapidly. Flax straw can often be used for a few seasons if dried out and stored properly.
Grass Clippings
Appearance: Bright green when freshly applied but turns brown very quickly.
Insulating value in summer: Fair to moderate – decays very quickly so adequate thickness needs to be maintained.
Insulating value in winter: Poor.
Need to remove in spring: Not recommended for use as winter protection mulch.
Nourishment and aeration afforded to underlying soil by decomposition: Excellent because of rapid decay but can tie up nitrogen in decomposition process.
Lets water and oxygen move freely into soil: Poor to fair. Can become compacted in a tight mat if not properly installed and maintained, preventing flow of water and oxygen.
Ease of application and maintenance: Poor to fair. If you have enough grass clippings to use as mulch you must let the clippings dry thoroughly before application. Clippings are light weight and susceptible to being blown and/or washed away. So this can be a cumbersome process if you do not have a place in your yard to do this. Wet grass can very quickly become slimy and produce foul odours.
Note: never use clippings from lawns that have been treated with herbicides, or other pesticides, for at least 4 weeks after application.
Grass clippings are the best used in vegetable and annual flower beds that receive annual cultivation to prepare a seedbed. Around leafy vegetables (such as lettuce, spinach, chard) carefully place the mulch at the base of the plant. But also keep in mind that you can make a mixture with shredded leaves, sawdust, shredded paper, wood shavings, etc.
As you can see from this handy reference list there are many options to choose from when determining which mulch product suits your needs. Take your time and analyse the pros and cons of each organic mulch listed, then make your decision on what best suits your individual needs. Next week we will take a look at inorganic mulches.