A couple of years ago I added a garden torch to my arsenal
of tools to maintain the landscape. Fire
can be a natural and effective tool in controlling invasive species and is one
of the better ways to maintain a prairie ecosystem. But, fire can also get out of control and its
effectiveness is diminished if it is not timed properly or its intensity is not
sufficient to destroy the target species.
On the garden scale a garden torch is a good way to kill weeds growing
in cracks in a drive or walkway. It is
also effective at killing young seedlings, particularly annuals. I found it particularly useful for for clearling plants away from the partially buried wire fence around the vegetable garden. Perennials and established plants require
much more heat to kill them. For
established weeds repeated burning 2-3 weeks apart may be necessary. Though it seems counter-intuitive, flaming moistened soil is
more effective at killing young plants and seeds than dry soil because the
moisture helps conduct the heat through
the soil.
Here's my Benzomatic garden torch. It is light weight and a good size to fit into tight spaces. The one pound tank lasts an hour or so, depending on how big a flame you use. |
I originally got my torch to use on Japanese
stiltgrass. While spring burning does
kill the sprouts effectively, other plants are actively growing at this time
and the fire will set those back as well.
Also stiltgrass germination occurs over an extended time period, so
multiple burns would be necessary.
For stiltgrass in a cool season lawn, burning in late summer, while the
perennial grasses are in summer dormancy, is very effective at preventing the
stiltgrass from setting seed.
This success with stiltgrass got me to thinking about using
the torch on garlic mustard, Alliaria
petiolata. Garlic mustard is a
monocarpic biennial. It spends its first year as a rosette of deeply reticulated, reniform
(kidney-shaped) leaves that builds up energy reserves in its fleshy white
roots. The following year it sends up a
2-4 foot flowering stalk in mid- to late-spring. It seems that winter time might be a good time to take out the garlic mustard.
In the past when I have tried to burn garlic mustard I found
I had to hold the flame on the plant for a long time before I could see much
damage. Before burning through a lot of
propane, I decided to do a little research on using fire against garlic
mustard. The U.S. Forest Service has
reports that document the effects of fire on a number of important plant
species, including invasives. Follow
this link to the Forest Service report on garlic mustard. A study authored by Victora Nuzzo for the Illinois
Department of Conservation in 1990 compared the effectiveness of fire,
herbicide and cutting for the control of garlic mustard. They found that for fire to be effective it
needed to be moderately intense and carried out in early spring. Ideal burning conditions were often not
available. Herbicidal spray treatments
were most effective in early fall and spring, timed for when native vegetation
was dormant and thus less susceptible to the herbicidal effects. One study out of Ohio showed that winter-time application of
glyphosate when temperatures were between 25-45 F was very effective at killing
garlic mustard rosettes.
Cutting plants after seed formation had begun in mid- to
late-summer was very effective at reducing the number of plants in subsequent
years. Cutting at ground level resulted
in nearly complete mortality, while cutting a couple of inches above the soil
surface was only about 70% effective at killing the plant. Since flowers and seeds continue to develop
on cut stems, it is important to bag and dispose of all the garlic mustard
cuttings.
Pulling mature garlic mustard is also very effective, and
can be done anytime. As with cuttings,
if there are any flowers present the pulled plants should be bagged and
disposed of. While completely non-toxic,
pulling garlic mustard is labor intensive (pulling and clean-up) and disturbs
the soil. This allows for the
germination of additional seeds buried in the soil.
Here are some of the interesting garlic mustard facts that I
came across while researching this post:
- Seeds lie dormant for a year before germinating. This means you may not see the effect of control measures on new seedlings until the second growing season.
- Flowers are self pollinating; many are already pollinated before the flower opens.
- Flowers can photosynthesize. They can continue to mature to form seed even after they have been cut or pulled.
- Plants are monocarpic. They will continue to live until they set seed, even multiple years.
- Seedling mortality is high, 80-90%, during their first year. More effort should be expended on removing maturing plants particularly in late fall to early spring.
- Of the common herbicides, glyphosate and triclopyr are very effective at controlling garlic mustard, 2,4-D is not.
So, Burn or Not?
So to answer the original question, should I use fire to control my garlic mustard? The answer, for me, is no. I will need to use a combination of tactics appropriate to the situation and season. Winter spraying with glyphosate will be easier and more effective than fire at my location. In the spring, pulling or carefully targeted spraying would be most effective. And for those plants remaining in the late summer, cutting close to the ground or continued pulling will be in order.
To test this I will target spray with 3% glyphosate in a section of the woods in February if (when) we get a stretch of days in the 40’s with no rain and see
how that compares to untreated areas.
(February seems a little early, but we are in the 60’s in early January,
so plants may be getting woken up a few months before normal.) Rather than blanket spraying I will try to
target the spray onto existing rosettes.
One of the reasons to use target application is to avoid those few
natives that are still in leaf through the winter.
Some native plants that may look similar to garlic mustard
rosettes include violets, Heuchera, Tiarella, and white avens (Geum canadense). Ground ivy (Glechoma hederacea) and common
mallow (Malva neglecta) are some weeds that look similar, but are not currently on my hit list. (I found no studies that indicated that ground ivy is susceptible to herbicides in winter. Based on that I wouldn't waste time and materials treating it in the winter.)
Compare the palmate lobed leaves with the deeply veined (reticulated), bright green leaves of the garlic mustard. Here, pulling is the only option for removing the garlic mustard. |
The leaves of foamflower are distinctly lobed. The vein pattern is different from that of garlic mustard. |
Golden ragwort has regularly toothed leaf margins and a more linear vein pattern. |
White avens over winters as a loose rosette of leaves. Though most winter leaves have 3 lobes, some appear roundish and vaguely similar to garlic mustard. |
The leaf shape of common mallow is similar to that of garlic mustard, but the vein pattern is different. |
In areas where I am more actively managing the garlic mustard I will continue pulling plants through late spring (before flowering). In late summer I will see if there is a practical way to cut the plants close to the ground without leveling the surrounding vegetation. Otherwise, keep pulling!
This table summarizes the garlic mustard control methods mentioned in this post.
Summary of Garlic Mustard control measures
Method
|
Timing
|
Pros
|
Cons
|
Effectiveness
|
Fire
|
Late Fall to Early Spring
|
Non-toxic
|
Difficult to achieve ideal conditions; Need to keep under control
|
A good moderately hot fire is effective, but difficult to achieve.
|
Herbicide Spray
|
Dormant season
|
High kill rate; can be targeted; no soil disturbance
|
Spraying toxic materials; may affect non-target species
|
Dormant season spraying reduces damage to non-target species.
|
Cutting
|
Summer, after flowering
|
Non-toxic; cutting at ground level nearly 100% effective; minimal
soil disturbance
|
Labor intensive; must dispose of stems/flower stalks; use of weed
whacker causes collateral damage.
|
Very effective when done right.
|
Pulling
|
Anytime ground is soft and easily worked
|
Non-toxic; very effective as long as most of the root is removed
|
Labor intensive; disposal of pulled plants required once flowers are
present; soil disturbance
|
Very effective
|