Exotic Pest Plants

Friday, September 16th, 2011 by Jenny Watts
    • Plant Beets now for fall harvest. They will have a deeper red color than beets planted for spring harvest, and tend to have higher sugar levels too.
    • Fall vegetables can be planted now for a fall harvest of broccoli, Brussels sprouts, cabbage, chard and lettuce.
    • Trim grapevines to allow more sun to reach the fruit and sweeten the grapes, if they are being shaded heavily by the foliage.
    • Feed fuchsias, begonias, summer annuals and container plants to keep them green and blooming right up until frost.
    • Divide Oriental poppies and bearded iris now. Add some bone meal in the bottom of the hole when you replant them.

Exotic Pest Plants

For many decades the nursery trade has been introducing plants gathered from around the world into our gardens. Any plant that is not native to an area is called an “exotic”. Most exotics are great additions to our gardens, and many tolerate garden conditions much better than the native shrubs.

But when an exotic plant becomes a weed, reproducing on its own in the wild, it is a pest plant. Pampas grass is a typical example. Introduced from South America, it now grows wild along the coastal areas of California. Scotch broom, which takes hold in disturbed soil such as along roadsides, is also a big problem in this county.

Why are these plants a problem? Because when they are so successful at reproducing in non-native areas, they crowd out the native plants that are critical for the survival of native birds and other wildlife.

Many of these pest plants are indeed weeds even in their native environment. Yellow star thistle, one of the most widespread of these weeds, came from Russia in the 19th century in alfalfa hay. It now covers countless acres of California’s hillsides, giving the “green and gold” a bluish cast. Others have intentionally been sold by nurseries for ornamental purposes but have turned out to be weeds.

An exotic plant’s ability to reproduce requires optimal conditions. So a particular pest plant will only be a pest in certain regions.

Many plants are particularly a problem in wet areas or around ponds. Cattails and umbrella plant can be very invasive, spreading by both roots and seeds. Yellow water iris has very aggressive roots which can fill up a small pond quickly.

If you have invasive plants on your property, try suffocating small seedlings and annual plants. Place double layers of thick UV-stabilized plastic sheeting, either clear or black, over the infestation and secure the plastic with stakes or weights. Make sure the plastic extends at least five feet past the edge of the infestation on all sides. Leave the plastic in place for at least two years. This technique will kill everything beneath the plastic—invasive and non-invasive plants alike. Once the plastic is removed, sow a cover crop such as annual rye to prevent new invasions.

Mowing or cutting plants down to the ground is also effective but only if you are committed to it. You will need to mow the area three or four times a year for up to five years. This will eventually exhaust the root system and kill the plants.

Herbicides are another option but must be used with caution.

Be careful about choosing plants for wild areas of your property so that you don’t contribute to the problem.