Climate change is happening, and it hurts good things and makes bad things worse. I’ve been covering that theme a lot, it seems.

The specific topic this week is weeds. And yes, there are going to be more of them, and they will be harder to eradicate, Sonja K. Birthisel told an attentive audience of farmers last month at the Maine State Agricultural Show in Augusta.

Maine is getting warmer and wetter, said Birthisel, who is doing post-doctoral research in weed ecology and pest management at the University of Maine. The average temperature has risen three degrees Fahrenheit from 1890 to 2019, and we are also getting an average of six inches more rain annually, according to data that runs from 1890 to 2015. Those are major changes that affect weed growth. The amount of carbon dioxide in the atmosphere also affects weed growth, and that too is increasing (and causing climate change).

In these conditions, weeds grow faster and the roots grow deeper, making them harder to eradicate – both for conventional and organic growers. For farmers who use weedkillers — the talk was coordinated by the Maine Board of Pesticides Control — that means that more poison will be required to produce the desired result.

Organic farmers, who remove weeds by physical methods, have it even worse. With the increased rainfall and — because the rain tends to come in heavier storms it has been accompanied, counter-intuitively, by more periods of drought — on average there are .4 fewer days per week suitable for working the soil just since 1995, Birthisel said. Translated into hours, that’s 9.6 hours per week, and considering that farmers work from sunrise to sunset during the growing season, some tasks will not get done.

But farmers and home gardeners can fight back.

One way to do so is with mulch, which comes in many varieties. A method that has yet to come to Maine (but is used in the mid-Atlantic) is to plant a high biomass cover crop, such as rye or oats, roll the cover crop to kill it (a machine bends down the stems at ground level) and then use no-till planting methods. Like all cover crops, these add organic matter to the soil and prevent weed seeds from germinating. Research has shown that this method works best with crops that grow until late in the season, such as pumpkins.

A simpler method is using plastic. One innovation is called solarization. After allowing a batch of weeds to just barely sprout in a vegetable garden, the grower covers the area with clear plastic. The area under the plastic heats up and kills the weeds. This method works best if done in late May through the first week of June, Birthisel said, or if doing a late crop, in mid-summer. Old plastic (like pieces coming off from hoop houses) works better than new plastic, she said.

In response to a question, Birthisel said that clear plastic works better than the black plastic many gardeners use between rows of vegetables. Black plastic absorbs the sun’s energy and radiates it back into the atmosphere, she explained, while clear plastic lets the sunlight reach the soil. Research has shown that only minimal damage is done to beneficial microbes in the soil with this method, and the plot needs to rest only about a day before it can be planted.

A leek moth, aka onion leaf miner. The larvae feed on Allium plants by mining into the leaves or the bulbs. Tomasz Klejdysz/Shutterstock

Pest report

As always, gardeners in the upcoming season could face new pests. Several speakers at the Maine State Agricultural Show talked about new pests — make that new to Maine — that we should be on the lookout for next summer.

The leek moth really likes leeks but will feed on any type of onion including ornamental alliums. It was first discovered in Ontario but has now made it to Jackman and Rangeley. The moth is about 1/4-inch long. To ward them off, put out pheromone traps, said Dave Fuller, an extension educator based in Farmington. And if you find them on your onions, protect the alliums with floating row covers, he said. There is a parasitic  that might provide biological control of the pest eventually, he added.

The Swede midge attacks brassicas, such as broccoli and Brussels sprouts. It has been found in Aroostook County and Farmington. Caleb Goossen, an organic crop specialist with the Maine Organic Farmers and Gardeners, said crop rotation works against this midge because it isn’t highly mobile. Floating row covers can also prevent trouble if farmers and gardeners know the midge has arrived in the area.

Spotted lantern fly has yet to be found in Maine, but it is moving up from Pennsylvania. It attacks many trees, including hardwoods and fruit trees. Its favorite host is the tree of heaven, an invasive plant (originally from China) that is rare in Maine. So with any luck, we’ll escape the spotted lantern fly.

Frankly, we could use a break.

Tom Atwell is a freelance writer gardening in Cape Elizabeth. He can be contacted at: [email protected]


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