Weeds are opportunistic, competitive and invasive. These are the qualities that make them so successful in nature and so frustrating for gardeners. Those who garden in the Northwest and the Western Mountain States share many of the same weed pests. Knowing a bit about how and when different weeds germinate and how local conditions affect this, can help when combating unwanted weeds in garden and landscape beds.
1. Western WA & Western OR: Weeds here are typified by species that prefer cool summers and mild winters.
Winter Annual Weeds. Many of the most problematic species in ornamental gardens are winter annuals, plants that germinate in the fall when autumn rains and cooling soils tell the seeds it’s time to germinate. Winters are rarely cold enough to eliminate these fall-germinators, and because soil rarely freezes for very long or very deep, these weeds don’t ever really stop growing. Instead, they grow slowly through the rainy winter months and then take off in spring when lengthening days start to provide enough sunlight to warm the soil and increase photosynthesis.
Summer Annual Weeds. Wet springs, longer days, and warming soils bring on summer annual weeds that don’t mind moderate summer temperatures—don’t look for heat-loving species here! Summers are rarely completely without precipitation, although rainfall is considerably reduced by late summer. Shady sites, or locations near surface water, usually remain moist all summer long, even without supplemental irrigation. Fall rains start the cycle again.
Perennial Weeds. Many perennial weed species, like many ornamental plants in this region, remain green all winter long, photosynthesizing whenever temperatures are above freezing. Those perennials whose stems die back to the soil in the winter often come on with a vengeance as temperatures warm in the spring, with growth rates that can be truly astounding. By late summer, these perennials generally slow down, although many experience a fall green-up that helps them prepare their roots for a long winter’s nap.
2. Central WA, Central and Southeastern OR & Southern ID: Weeds here are typified by species that thrive in hot summers and cold winters. Soils in this region are often alkaline, which is in a rarity in other parts of the Pacific Northwest. Precipitation is scant, and usually only falls in late autumn through late winter.
Winter Annual Weeds. Fall rain may not be adequate to germinate winter annual weed seeds. The soil reliably freezes every winter, although snow may keep temperatures high enough to allow some fall-germinating annuals to overwinter.
Summer Annual Weeds. Summer annual weed seeds here begin germination following exposure to winter’s cold and when soils are warming and days are lengthening in spring. Most annual weeds require supplemental irrigation to survive. The growing season is relatively short at higher elevations, although summer temperatures are usually high throughout.
Perennial Weeds. Perennial weed species are predominantly herbaceous, with stems and foliage dying back to a perennial crown and roots in the fall, and they must be able to withstand sub-zero temperatures in winter. After beginning growth, these perennials grow rapidly, but will require supplemental irrigation to survive. Fall re-growth of perennial weeds is common, provided there is sufficient soil water present. Such plants produce carbohydrates in their leaves which are predominantly moved to their roots, improving their ability to survive winter’s cold and to rapidly grow come spring.
3. Eastern WA, Northeastern OR, Northern ID, CO, UT, MT & WY: Weeds here are typified by species that prefer warm, dry summers and cold winters, often with appreciable snow cover.
Winter Annual Weeds. Winter annual species are not as common here as they are west of the Cascade Mountains, since many weeds that germinate in fall are too small at the onset of winter to survive in the frozen soil.
Summer Annual Weeds. Summer annual weed species predominate, particularly those that favor cool and moist springs, conditions allowing them to germinate early, grow rapidly while soil moisture levels are good, and to then be deeply rooted enough to flower and seed in a warm and dry summer. Most summer annual weed species produce seeds that must first go through a cold treatment before they will germinate, all but guaranteeing that conditions will be optimal for the seedling to establish itself and reproduce after winter melts away. Precipitation nearly ceases in early summer, often not returning until fall. Weeds will often persist only in gardens receiving supplemental irrigation.
Perennial Weeds. Perennial weeds are species that can withstand sub-zero temperatures, almost all of which are herbaceous perennials that die back to a perennial root each fall. These species grow rapidly in the spring, often growing remarkably thickly to the exclusion of all other plants. Perennials generally have finished growing here by late summer, although many experience a flush of growth if fall rains arrive while days are still warm, enabling these plants to enter into winter in fine condition.
Weed Control Notes