Changes in weather and weed management

Climate change refers to a change in a climate system that persists for long periods of time, irrespective of the cause. Montana has such an interesting history of environmental changes over the years.

At one time western Montana lay beneath a lake approximately 2,000 feet deep. Ice dams 4,000 feet tall that blocked the Clark Fork River burst causing the 3,000 square mile Glacial Lake Missoula to drain in less than a week into the Pacific Ocean. Much of the rock formations and other geology we experience today are a result of that huge, rapid flooding event.

Mammoth bones with tusks six to seven feet long were unearthed on a ranch in Eastern Montana and mastodon bones were found in a gravel pit in Deer Lodge. Thousands of acres of petrified redwoods and other trees that grew during a warmer and wetter climate system in that area at the time can be found in the Gallatin.

Long time Montana residents have told me stories of their winters growing up with a lot more snow. Changes in the weather patterns during all seasons even in my relatively short 15 plus years in southwest Montana have caused me to adjust management tactics when it comes to controlling noxious weeds.

Three years ago, after a deep freeze on the tenth of October froze all of our spray equipment, we moved winterization closer to the end of September versus late October. When temps climbed up into the 50's in November and December this year we lamented the fact that we didn't have access to our equipment to spray. Spray contractors treating the roadsides have had shorter treatment windows due to a combination of unusually hotter, dryer and colder weather conditions than was customary during growing seasons in the past. About three or four years ago I started receiving reports of plants never seen before and plants in places never seen before from folks that have spent their whole lives, often upwards of 40 years, on the same land. Many are experiencing unexpected and inexplicable changes in trees, shrubs and ornamentals in their landscape.

Weather patterns affect both native and non-native plant growth and recent studies are showing that the chemical properties of certain herbicides can also change, altering their performance on weeds and therefore control efficacy. Indications are that under warmer conditions herbicides may be less effective as certain invasive weeds grow faster and are more resistant to chemicals under higher levels of carbon dioxide.

Changing climates may also disrupt the effectiveness of biological control agents by creating a mismatch between previously synced phenology of the invader's life stage and the insect used to help control it. Shorter and milder winters may lead invaders previously limited by long cold winters to be able to survive farther north and expand their populations. Additionally, many studies have documented invasive species that are able to shift their phenology more rapidly than native species to take greater advantage of an extended growing season by greening up earlier and staying green later. We've seen this happen with spotted knapweed in our area this season. Determining the response of weeds and herbicides to changes in climate variables is important as we look to optimize weed management strategies.

During an international invasive species conference I attended in January, a plenary speaker introduced the idea that "adaptation does not equal mitigation." This approach leans on being proactive versus reactive with the additive effect of being very intentional in response to the changes we are seeing on the landscape.

At the local level, we can concentrate our efforts on adapting to changes to ensure that we are meeting if not exceeding our vegetation management goals. Some examples include vigilance in following herbicide label recommendations for temperature and timing of applications. Most labels suggest that temperatures must reach an overnight low of no less than the mid to upper 50's and temps should not exceed the lower 80's during application. Increasing the amount of water applied while maintaining the correct chemical rate per area is a good strategy during excessively dry times. Limiting disturbance and other preventative approaches as well as reseeding and other revegetation practices can go a long way toward restoring and maintaining a weed resistant native and desirable plant community.

I'd like to encourage you to pay attention to the ways in which the plants around you are responding to weather conditions and make adjustments versus relying on potentially outdated practices of the past and old habits. As always, please reach out to myself and the other land management agencies and practitioners for available resources and support.