Article and Photos by Mark Godlewski

This is the second part of a two-part blog entry. This first entry covered the types of mulch commonly available in our area and their relative advantages and disadvantages. This second entry will cover a set of flammability tests that OXA carried out on mulches in the last couple of years.

Landscape flammability is a big issue in our environment. Our climate is dry and each summer we experience forest or brush fires that result in significant property damage. There are many measures that we can take to reduce the risk of external fires. A good place to start is to review the recommendations contained in BCWildfire-homeowner-firesmart

One important component of landscape flammability is the choice of mulch. All other considerations aside, rock mulch is the best choice purely from a flammability perspective. As we discussed in the earlier blog, however, there are several good reasons to avoid rock mulch. Organic mulches are preferred for most xeriscape projects and they can be grouped into Wood Waste Mulches, and Composted Mulches.

Because organic mulches vary geographically, OXA decided it would be worthwhile to test the flammability of the local common organic mulches. We obtained representative samples of mulches and enlisted the help of Rick Euper, Fire & Life Safety Educator/ Fire Inspector at Kelowna Fire Department. Rick was instrumental in setting up these tests and conducting them in a safe and effective manner.

Fig. 1 – Mulches ready for flammability tests

We laid out the mulches in beds approximately 3” thick in an unused area behind the fire station. We conducted a set of burn tests on two different occasions. One set was in August of 2018 approximately three weeks after laying out the mulch. The second set was in August of 2020 after the mulch had weathered for two years. We had planned to do the test after one year’s weathering but that summer turned out to be unusually wet.

The mulches tested (Fig. 1) are:

Glengrow (composted green bin waste)
Ogogrow (composted wood waste and biosolids)
Natures Gold Regular (large particle sorting of Ogogrow)
Natures Gold Premium (small particle sorting of Ogogrow)
Fir Bark (medium sized)
Pine Needles

We have charted the key results of our tests in executive summary form (Fig. 2 and 3). They show the results after subjecting the two-year-old mulch to an acetylene torch for 15 seconds. For example, the area in flames for the Glengrow mulch sits at 2 units immediately after the torch is removed and the flames die out completely in 17 seconds

Fig. 2  – Flammability Chart 

Fig. 3  – Flammability Chart with photos

The pattern you see in these charts repeated itself relatively faithfully in all our tests. Natures Gold Premium is the least flammable followed by Ogogrow, Glengrow, Natures Gold Regular, Fir Bark, and finally Pine Needles which are the most flammable.

The composted mulches (Natures Gold Premium, Ogogrow, Glengrow, Natures Gold Regular) are not very flammable. Composting appears to have a strong positive influence. The volatile elements are removed, and the particles are covered in carbon. Within the composted mulches smaller particle size makes the mulch more like organic rich soil and consequently less flammable.

Fir Bark mulch supports combustion and, while it spread very slowly on our tests, all that you need is a good wind to result in a rapid spread. We were able to confirm this with a blower (Fig.4).

Not surprisingly, pine needles and cones are very flammable even without wind. Although only a few people use pine needle debris deliberately as mulch, it is often lying on the ground in many wilder landscapes. You can probably expect the same sort of problem from any other coniferous debris such as old cedar leaves.

Fig. 5 – Charcoal briquets placed on mulch

The actual dates of the mulch burn tests were August 31, 2018 and August 05, 2020. Conditions for the first burn were 25 deg C, 30% humidity and for the second burn 34 deg C, and 18% humidity. The wind was relatively light for both burns. The mulch felt slightly moist to the touch digging down two inches below the surface for the first burn and did not feel moist at all for the second.

On both dates we started by placing white hot charcoal briquets on each of the mulch types to simulate their response to a hot ember falling to the ground (Fig 5). In the case of the pine needles the briquet caused them to smoke heavily and eventually they caught fire (in the 2018 burn). The bark smoked moderately but never ignited, and the other mulches only produced a small amount of smoke.

The second flammability test consisted of exposing the mulch to open flame in the form of a torch. All the mulches caught fire quickly with the torch, especially in 2020 when an acetylene torch was used (Fig. 6). On both dates the fire started by the torch in the pine needles spread relatively quickly. The flames in the bark died down slowly in the 2018 test, but in the 2020 test they stayed lit and spread very slowly. The flames in the composted mulches all died down quickly to where they were only smoking slightly (Fig. 7).

In the 2020 test we tried using a leaf blower to simulate wind. As you might expect this had a dramatic effect on the spread of flames in both the pine needles and the fir bark.

Since we saw slightly more smoke and flames for almost all the 2020 tests relative to the 2018 tests it is tempting to claim that weathered mulch is slightly more flammable than fresh mulch. However, the conditions in 2020 were hotter and drier so that must be part of the answer.

Article and Photos by Mark Godlewski–

This is the first part of a two-part blog entry. This first entry will cover the types of mulch commonly available in our area and their relative advantages and disadvantages. The second entry will cover a set of flammability tests that OXA carried out on mulches in the last couple of years.

True xeriscape gardening generally requires the use of mulch.

Available mulches vary significantly depending on where you live, however they can be divided into four general categories:
• Recycled Synthetic Mulches (plastic, rubber, etc.)
• Rock Mulches
• Wood Waste Mulches (bark, needles, wood chips)
• Composted Mulches

We are not aware of any Recycled Synthetic Mulches being used in the Okanagan. This is a good thing because they would not fit into our “Gardening with Nature” mantra, and they are most often quite flammable.

Rock Mulches are widely used in the Okanagan. OXA does not generally recommend rock mulches. They have some advantages, but their disadvantages usually dissuade keen gardeners.

Rock Mulch advantages
• Conserves water
• Nonflammable
• Inhibits weed germination
• Comes in a wide variety of colours and textures
• Long lasting (more expensive to begin with but less expensive in the long run)
• Withstands movement by wind and rain
• Large leaves easy to remove with blower
• Withstands foot traffic well

Rock Mulch disadvantages
• Increase the soil temperature in the summer. Dark coloured rocks increase the temperature the most but light coloured rocks can also reflect light into the foliage increasing the chance of desiccation.
• May increase soil alkalinity
• In the winter, the rocks conduct the cold into the ground quite effectively.
• Tend to be more expensive to purchase and install
• Usually requires landscape cloth underneath which creates a barrier to natural aeration and transpiration of the soil
• Does not break down to feed the soil
• Although weeds are less likely to germinate in rocks, they are more difficult to remove once they mature.
• Over time the space between the rocks will fill up with dirt and debris, seeds will germinate more easily, and the weeds will be even more difficult to remove. The dirt/rock mix looks unattractive and is exceedingly difficult to separate.

Wood Waste Mulches are used fairly commonly in the Okanagan. We are surrounded by productive pine forests and there are several sawmills located in our valley. Wood waste mulches include bark, wood chips, and pine needles.

Wood Waste (bark) advantages
• Conserves water
• Natural parts of the local environment
• Easy to install
• Inhibits weed germination
• Comes in a wide variety of colours and textures
• Relatively long Lasting
• Withstands foot traffic
• Insulates the ground

Wood Waste (bark) disadvantages
• Flammable (a major issue in our part of the world)
• Bigger pieces can tend to work into the soil making digging difficult
• Does not contribute much to the soil

Excerpted from the Gardening with Nature column written by Sigrie Kendrick

As discussed in my earlier column, I am torn discussing gardening topics in the face of this unprecedented pandemic, yet gardening is, by its very nature, a ‘socially distancing’ hobby that we can all enjoy during this challenging time.

One of the many regular spring chores necessary for a healthy garden is the application of a layer of mulch.
Mulch is instrumental in maintaining a healthy garden as it acts as a weed-suppressant, moderates soil temperature, and conserves moisture. Mulch also encourages worms, insects, bacteria and fungi, all necessary for healthy soil.

Often mulch comes in the form of GlenGrow, OgoGrow, compost, or one of those big yellow bags of mulch you may have noticed around town. Although appreciative of those products I would like to suggest using a ‘living mulch’ which acts in much the same way but also allows the gardener to enjoy more plants—and who wouldn’t appreciate that? Any dense ground cover can be useful as a ‘living mulch’.

Dwarf fleeceflower makes a very interesting and hardy “living mulch” (Mark Godlewski)

If you have a shady area, consider using Lamium maculatum (Deadnettle) , Galium odoratum (Sweet woodruff), or Ajuga reptans (Bugleweed).

Once established, living mulches will provide a maintenance-saving ground cover for years without needing to be ‘topped up’ every spring, like the ‘dead’ mulches do. Any of the above xeriscape choices will allow you to conserve our most precious resource, water.

The Okanagan Xeriscape Association website features an extensive Plant Database referencing gorgeous perennial and annual plantings which will allow you to have a beautiful garden while still conserving the water necessary for drinking and for our livestock and food production. If you appreciate our database, please contribute to our charitable organization through becoming a member or make a direct donation in support of the Xeriscape Demonstration Garden in front of the H2O Centre on Gordon Drive in Kelowna.