This year’s high river flows deposited sediment and scoured new ground surfaces for a variety of native and non-native plants to grow. So what is growing along the Boise River? Thanks to wetland expert Chris Murphy from the Idaho Department of Fish and Game, joined by botanist Dr. Roger Rosentreter and riparian ecologist Dr. Rob Tiedemann, BREN members got an up-close look at numerous seedlings, saplings, herbs and grasses.
Black cottonwood (Populus trichocarpa) seedlings were growing in abundance along newly deposited sediment. Cottonwood seedlings are identifiable by their red stems and serrated leaves. Unfortunately, most of these seedlings will not survive because the river flows were drawn down too rapidly, lowering the water table below their rooting zone.
How can you tell between a cottonwood sucker and a sapling grown from a seed? Look at the base of the plant. The sucker will be thicker along the base where it has emerged from the root system of a mature cottonwood tree. Suckers are genetically identical to the parent tree, whereas cottonwood seedlings provide genetic diversity.
Black cottonwoods are considered a keystone species in the Boise River system, as many wildlife species rely on cottonwood for critical habitat. For an in-depth discussion on black cottonwood and river flows, please refer to Dr. Tiedemann’s blog on the subject.
As a seedling, false indigo (Amorpha fruticosa L.) looks kind of like black cottonwood – but don’t be fooled! False indigo seedlings are distinguishable by rounder, non-serrated leaves that grow opposite from each other. Unlike the cottonwood seedlings, false indigo is tolerant of drier soils. So if you see it, pull it! False indigo has become invasive along the Boise River, easily outcompeting most native woody shrub species.
A tribute to their robust rood systems, these mature false indigo plants survived through weeks of inundation and force from the river.
Another look-alike, Wood’s rose (Rosa woodsii) kind of looks like false indigo but notice the shorter, serrated leaves and of course, the thorns. Wood’s rose is an important native shrub species.
We saw numerous silver maple and catalpa seedlings. Both are non-native, so does that make them “bad”? On one hand, they take up space where native trees, such as the cottonwood, could otherwise grow. But on the other hand, they provide important habitat and are better suited for reproduction in the regulated Boise River system.
Are there any trees that should be removed? Scientists are not all in agreement, but our experts list tamarisk (Tamarix spp.), tree-of-heaven (Ailanthus altissima) and Russian olive (Elaeagnus angustifolia) as the worst offenders, as they alter the surrounding environment to their own benefit and at the expense of natives, provide marginal habitat for wildlife, and are largely left alone by insects. In short, they don’t participate in the ecosystem.
Other weedy plants
Numerous weedy plants have quickly established themselves on the newly formed and cleared surfaces, such as pigweed (Amaranthus retroflexus), pinappleweed (Matricaria discoidea), jointed goatgrass (Aegilops cylindrica) and quackgrass (Elymus repens).
So what’s growing along the Boise River? A lot of the same plants, including invasives, that were established along the river before the high water. The best thing riverfront homeowners and volunteer groups can do to help natives plants thrive is to remove noxious and invasive species. Learn more about the importance of native riparian habitat in the Boise River Enhancement Plan. If you’re interested in learning more, consider becoming a Weed Warrior!