October 28, 2017
Dot Zanotti Ingels
Plants that are touted as “readily reseeding” can be such a blessing. In my garden, I love that I planted a package of nasturtiums many years ago and I continue to enjoy the gift of an abundance of free nasturtiums year after year. I planted two varieties of Nicotiana alata from 4-inch pots a long time ago and they continue to come up all over the garden to the delight of the hummingbirds. Santa Barbara daisy, Erigeron karvinskianus, readily reseeds around my yard where nothing else wants to grow. It blooms all year and the deer do not like it.
Our native California poppy, Eschscholzia californica, reseeds itself prolifically all over our golden state. Wildflowers are the recurring gifts of their predecessors, much to our total delight.
One of the gifts these plants share is that we are happy to see them and they are easy to dig up if they are not where we want them or too many appear. Conversely, I planted Geranium pyrenaicum ‘Bill Wallis’ from one 4-inch pot a long time ago, but, while it readily reseeds, it is difficult to dig up and control. I live next to an open space. The weed grass seeds that blow or are personally delivered by birds into my yard are a constant headache.
The real problems come with non-native species that readily reseed and invade our waterways, valleys and hillsides with reckless abandon. They inhibit native species of flora and fauna with their rampant spread. They can lead to a reduction in the diversity of desirable plants and change the relationships between plants and the organisms of the area that depend on them.
One glaring example is Scotch broom (Cytisus scoparius). Broom was introduced to California as an ornamental plant. We recognize it all over Marin in spring when it is covered in bright yellow flowers. Now it has become a widespread naturalized invasive weed. Because of its aggressive seed dispersal, broom removal has become extremely difficult. It crowds out the native vegetation and grows most prolifically in some of the least accessible areas. Mechanical removal (by hand or machine) is costly and prohibitive. The use of chemical removal with glyphosate is an issue that inflames the community with concerns about its safety to the environment.
Other plants ubiquitous to our area and destructively invasive are pampas grass (Cortaderia selloana), thistle and giant reeds (Arundo donax). It is important to note that a species that is invasive in one part of the state may not be a problem somewhere else. For example, the cold of the mountains takes care of plants that are invasive on the beach. It is important to know what is growing around you and what you introduce into your garden.
As gardeners, we are always looking for something new to add to our landscapes. Sometimes we choose a plant to bring home as a souvenir from our travels. We need to be aware that moving a plant to a new habitat may turn a beautiful native from somewhere else into an invasive species here as its seeds spread. Scientists at the University of California Davis have developed a 20-question plant risk evaluation tool that can help you accurately predict whether a newly introduced plant has a high potential for escape from cultivation. The Davis integrated pest management website also lists plants that are invasive in our area and can help you with suggestions for weed management. The California Invasive Plant Council website also is an excellent resource to help us recognize invasive plants in our wildlands.