Every winter, Washington state experiences one extreme end of daylight hours, bottoming out around 8.5 hours of daylight on the winter solstice (Dec. 21). A stark contrast to summer solstice (June 21) which peaks around 16 hours of daylight. It’s no surprise that our energy, routines, and behaviors reflect the world around us, and plants are no stranger to this experience.
Dormancy 101
Like humans, plants have their own lifecycles and seasonal behaviors/patterns. Some of these patterns are easier to notice: new leaves in spring, flowers blooming in summer, leaves falling in autumn. But a less observable, and just as reliable plant behavior, is a plant’s dormancy in the winter.
In plants, dormancy is a period of protective rest that is triggered as a survival tactic in extreme conditions (freezing temperatures, drought, extreme heat, etc.). A pause on visible growth. In this state of hibernation, plants slow their energy production (photosynthesis) and protect stored energy by sending it away from the elements (to the roots) until conditions feel safe.
Winter temperatures trigger this state, making it a seasonal occurrence.
Dormancy in different Lifecycles
Dormancy as we are recognizing here mostly pertains to perennial plants (those with a 2+ year life cycle). But biennial plants (2-year life cycle) also experience dormancy once in their lifetime. Biennial plants typically germinate in spring (or fall), grow into rosette form (low growing circular arrangement of leaves), then go dormant with the others in fall/winter. Their 2nd spring they grow tall, produce flowers and seeds, and then die. Lastly, annual plants (1-year lifecycle) do not experience true dormancy as they just die after seed production; but their seeds often rely on a cold cycle to prepare to germinate in the spring.
How it Looks (Dormancy)
Dormancy is obvious in some plants, and completely imperceptible in others.
See the different ways perennial plants (2+ year life cycle) exist through the seasons in the examples below.
Invasive knotweed, or Fallopia sp., (Class B Noxious Weed) is often mistaken as dead in the wintertime, but it is actually dormant. All of its fleshy plant parts die back completely at the surface, leaving dead hollow stems, but below ground the root system is full of energy waiting for mild enough conditions to regrow come spring. Winter knotweed photo credit: knotweedremoval.tips.
Cherry laurel, or Prunus laurocerasus, (Weed of Concern) makes no obvious change to the naked eye, keeping its leaves and woody parts throughout the winter, but it too goes dormant in the cold season and pauses growth aboveground.
Common hawthorn, or Crataegus monogyna, (Class C Noxious Weed) loses its leaves in the fall as a sign of its dormancy, while its woody parts remain. Fallen leaves have a wintering purpose, this routine release allows the plant to take an energy break (from making the green pigment chlorophyll) and returns the nutrients to its roots below (nature’s mulch, leave the leaves people!).
What does this mean for weed control?
Weed control at any time is better than not at all! Do not let the preferred timing mentioned below stop you from weed control if now is the best time for you.
Gardeners know that the time to prune (cut back) plants is in the cold season. By contrast, the ideal time to control a weed is right before it produces its flowers/seeds, often spring/summertime.
This preferred timing of pruning vs. weeding has everything to do with dormancy.
When a plant is not dormant, it is in a state of “active growth”. These states ebb and flow with the plant’s seasonal growth (budding, flowering, die back, etc.) and can further tell us where its energy is concentrated within the plant itself, this is key.
Pink/white sparkles in the two images above indicate where the energy is concentrated within the plant at these two opposing stages. Click arrows on either side of the image or wait (images automatically alternate every 5 seconds).
During dormancy, plants are protecting their energy from external conditions such as wind, rain, and cooler temperatures. The most extreme changes occur above ground; thus, the energy of our plants flows below ground into the roots at this stage. The most common illustration of this is when tree leaves change color and drop in the fall. As days get shorter and colder, photosynthesis (energy production) takes more work than it’s worth, and holding onto leaves costs more energy than it creates. During winter, the tree relies on its underground stored energy supply and needs more before it can be ready to make seeds come spring. This is when active growth begins and energy shifts back upward from the roots and pushes out new leaves so that the plant can create new energy using photosynthesis. Energy continues to flow upwards to encourage flower and seed production and peaks around this time. Once seeds are released, the energy starts to make its way back to the roots, and the cycle begins again.
And…what does this have to do with weeds?
Weed control at any time is better than no weed control at all! The importance of dormancy timing varies from species to species and on the infestation itself. In an ideal world, weeds would all be controlled in their early stages before they get the chance to establish robust root systems and plant mass. But the reality is, we often don’t notice weeds until they’re well established. In recognizing this, it’s helpful to know that in established plants weed control can be most effective if we are working with their energy instead of against it.
Weed Control Timing: Dormancy versus Active Growth
When weeds are dormant, most of their energy is stored in their roots. At this time, weed control will potentially be less effective and require more follow up. When we dig or pull our weeds during dormancy, the root fragments that get left in the soil have more energy to re-grow and a better chance of coming back as new weeds. Particularly in plants that can spread via fragments (usually 2+ year plants with rhizomes [sideways spreading root systems]). Additionally, at this time other methods such as herbicide are less effective because the plant has paused its processes and will not absorb as much of the chemical.
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When weeds are actively growing (budding/flowering/early seeds), most of their energy has shifted from their roots to the above ground parts. This is the best time to control our weeds, though we should still always anticipate follow up maintenance. When we dig or pull our weeds in this stage, the root fragments that get left in the soil have less energy and will have a lower chance of survival. Additionally, by getting to the plants before they can flower, we reduce the amount of seeds that will be made and able to spread. At this time, methods such as systemic herbicide (plant pesticides that move through the plant systems) are at their peak effectiveness because in an effort to make seeds, plants are taking in as many resources as they can get and thus will best absorb the chemical. Note: always read the herbicide label first as it takes dormancy into account on a species-to-species basis, and some species still absorb herbicide during this time. When we understand the growth process of plants, we can deploy our tools most efficiently and effectively, using less energy on our part and using only as much herbicide as needed.
Winter Weed Control
By timing our weed control work with the plant’s annual cycles, we can save time, resources, and have less of a negative impact on our environment. Winter weed control is not a bad idea, it just may not be as effective for the reasons mentioned above.
Winter weed management may focus less on the removal and direct control of the plants themselves, but can include options such as:
- Sheet mulching (see our fall blog post for detailed guidance).
- Planting native (dormancy = good time to transplant + rainy season = less upkeep).
- Planning for next year’s weed control strategy.
- Updating your observations in iNaturalist so you have a plant inventory for next season.
- Reading about weeds in your area so you’re ready for the spring!
- If you really want to do winter removal work, you can opt to focus on species that rely less on their root systems to spread (no rhizomes) such as scotch broom or biennials.
- Go dormant yourself and rest! Remember, winter is mild in the Pacific Northwest, and our weeds will be in active growth soon enough.
If you have any questions about any weeds or weedy activities, email us at noxious.weeds@kingcounty.gov. Happy weeding!
References
Agronomy | A Conceptual Framework for Winter Dormancy in Deciduous Trees (mdpi.com)
The Art of Dying: Senescence in Leaves | Roundglass Sustain
Nonstructural Carbon in Woody Plants | Annual Review of Plant Biology (annualreviews.org)
I think the only weed I know of that is more sensitive to pesticides is English ivy on a Sunday day in January.