Hotline (Questions?)

What makes a plant invasive?According to the national invasive species information website, www. invasivespeciesinfo.gov, “an invasive plant is 1) non-native and 2) it causes or is likely to cause economic or environmental harm…”.The California Invasive Plant Councils’s website, wwwcal-ipa.org, Plants A to Z, identifies invasive plants as those that spread widely and easily and that out-compete native species.

In researching invasive plants used in landscapes for this column, I was surprised at how many of those listed were common in Central Valley gardens.Bamboo (running) has been know as an invasive plant for decades, but the list of the “16 most invasive species you’ll find at a nursery”, given at www.epicgardening.com, also includes landscapers’ favorites such as Japanese barberry, Chinese privet (Ligustrum sinesi), Japanese spirea and nandina.

A quick look around your garden might just turn up several common but invasive species.And, following the descriptions of invasive plants given above, you might be able to add a few invaders you’ve identified to the lists.

In my garden, former owners (meaning, I didn’t do it) planted Japanese spirea, nandina, and Chinese privet as well as Mexican feather grass (Stipa tenuissima) and Euphorbia characias. The last two plants were in a separate drought-tolerant landscape zone.Mexican feather grass has been identified as an invasive plants, but Euphorbia characias has not yet been added to the lists and is still available in nurseries and garden centers.Although the feather grass and the characias were pulled out four years ago, seeds of both still sprout every spring.The characias is particularly invasive; hundreds of tiny seedling reappear every year, the wiry stems are tough to cut with a wiggle hoe and the roots easily penetrate through the mulch into the soil making them hard to pull out if more than two or three inches tall.

I’d add California bay trees (Umbellularia california) to the invasive species list as well.There are six large mature California bay trees that surround my backyard. They produce enormous numbers of bay berries that root easily on the soil or mulch surface and, if not pulled out when very small, will quickly establish long tap roots.The berries are big enough to be slippery.The trees also produce numerous suckers on their roots which are tough to cut and which must be cut off precisely at the base in order to prevent regrowth.Lots of extra work keeping the bay trees at bay.

Xylosma was planted for many years as a hedge or filler plant.Xylosma is also a very invasive plant; the seeds remain viable in the soil for many years and unless the roots are completely removed they will continue to produce wiry, tough new branches.Many of the gardens in my older, established neighborhood have branches of long-removed xylosma shooting up through more recent plantings.

As the list of species and cultivars of species of invasive plants grows, it’s important to keep up with the most current research in order to prevent inadvertently adding invasive species to our gardens.