June joy: use the sun
With intense heat and long sunny days, June is the best time to solarize. Solarizing is a powerful way to make wholesale, non-chemical progress on tough invaders like mugwort and other not wanted plants.
Solarizing uses clear plastic sheeting to tightly cover large, mostly sunny areas for at least a month. This increases the temperature of the underlying soil to over 100 degrees F, killing almost everything. It’s best used where there is a large, dense concentration of invasive plants and little or no desirable native plant life.
Mugwort, with its dense rhizomes and ability to completely dominate an area in just a few years, is the perfect candidate for solarizing. Other targets include poison ivy (native but often undesirable) and grass (it’s an efficient way to reduce lawns and replace with more useful, pollinator-friendly plantings). Here’s how to do it.
1. Dig up desirable plants in the target area; pot or transplant them. Plan replacement plantings, either seeds or mature plants.
2. Cut or mow the target area as low as possible. Trample sharp stems that could puncture the plastic.
3. Buy (or find used) common polyethylene plastic sheeting, 4 to 6 mils in thickness. For large areas, use 10- or 20-foot widths, 100 feet long, at a cost of $50-$100.
4. Pick a day after a rain, or water the target area thoroughly, because moist soil retains more heat. Spread the plastic sheeting tightly across the area, anchoring it well. Weigh down the perimeter with soil, stones, boards, staples – anything that will keep it firmly in place over time despite wind, animals, earthquakes, etc. Weigh down the interior as well.
5. Walk away and let the sun roast. Check the area regularly to police escapees, especially plants that travel via vines and rhizomes. Make sure the plastic stays weighed down flat. Patch any holes that allow heat to escape with tape and clear plastic.
6. After at least a month, pull back some of the plastic and check for signs of life. Leave plastic in place if there’s any doubt. When there’s no evident life, remove the plastic and prepare the ground for replacement plantings or seeds. Or remove some of the plastic and leave the rest in place to re-plant one section at a time. Fold up and store the plastic—re-use it for at least a couple years before it starts to deteriorate.
A year ago, an Invasive Action team solarized a dense patch of mugwort on the far side of Salmon Brook at the main bridge at Holcomb Farm (see the June 2021 Not Wanted column at GranbyInvasivePlants.weebly.com). In two months, no mugwort survived.
The other target invasive, Japanese knotweed, got the same treatment but with mixed results. Younger knotweed plants were immolated, but the mature, very deep-rooted plants survived, aggressively pushing the plastic up and aside.
There are two effective non-chemical knotweed strategies. One, occultation, involves cutting to the ground, then covering with heavy black plastic for many months. Black plastic does not warm the soil as much as clear, but it deprives the plants of light. The other, successive cuttings three times a year before the end of August for three years, forces the roots to repeatedly squander resources on plant growth with minimal nutrient replenishment. This method allows for gradual replacement plantings during the three years. For success in southeastern Connecticut with this method, see the Facebook group NixtheKnotweed.
Dr. Fred Moffa of Granby Family Eyecare has launched an inspiring solarizing project (below) to control a dense, 100-foot infestation of mugwort on his business’s border.
What else to do in June
Solarizing can be effective, but may be more difficult, on invasive shrubs and small trees that have been cut to the ground, including Asiatic bittersweet, multiflora rose, Japanese barberry, autumn and Russian olive, winged euonymus and wineberry. Black plastic may work better on larger, more mature versions because it blocks light and prevents resprouting.
June is peak flowering time for most of our worst invasive plants. Cutting them to the ground may not kill them, but it stops them from flowering, prevents them from setting seeds, deprives their roots of the fruits of leaf photosynthesis, and makes them easier to approach for uprooting, continued cutting, or careful cut-and-paint application of herbicide later in the season. We humans brought most of these plants to our shores many decades ago and they are well-established; it takes sustained effort over a long period of time to gain control over them.
Join the fun!
Invasive Action takes place the second Saturday morning of the month. Join neighbors, learn more and have a powerful impact June 11. To sign up, or for more information on invasive plants in Granby, use the form at GranbyInvasivePlants.weebly.com