Invasive ? plant lists
Diane Whitehead (Wed, 03 Sep 2008 18:47:01 PDT)

Why do plants become invasive? And if they do, does it harm the
environment?

Our noxious weed lists until recently were concerned with field weeds,
often ones that are poisonous to farm animals. Many were imported
accidentally. I saw European weeds in one area of South Africa that
had been present as seeds in the hay brought in to feed the cavalry
horses during a war. There were prairie weeds growing here at the
terminus of the railroad bringing boxcars full of wheat.

Some are colonizers of disturbed ground -like alders that spring up in
a clearing and, with their nitrogen-fixing root nodules, enrich the
soil for the slower-growing trees that follow them. If the area is
not disturbed again, a natural progression to a climax forest will
occur.

Some ground always remains disturbed, and so we have roadside weeds,
like California poppies. A plus, I think.

Some plants are perfectly suited to an area and thrive there, even
when the gardener would rather be growing something else. These
invasives are often native plants - almost every plant I battle in my
garden is a native. Well, I have to admit there is one import: a
purple leaved violet that people blame incorrectly on Labrador.

So, let's look at sweet shy violets. Violets can spread, even if you
pick off all the spring flowers before they make seeds, because the
plants have sneaky little flowers in midsummer that don't open, but
still produce seeds. I know gardeners who won't allow a violet in
their garden. Would violets be a suitable plant to ban from
importation? But, from what I have experienced, there are two kinds
of violets: the ones that spread, and the ones that are impossible to
grow. There don't seem to be many in the middle. However, even the
ones that spread don't seem to cause trouble. The whole plant is very
nutritious, and deer browse them in midwinter. Considering how many
deer we now have, this is probably not advantageous, but I guess it is
better to have healthy deer than malnourished ones.

Why have other plants become invasive?

Many of the ones here have berries that attract birds. Garden
magazines would point out which berries would not be eaten, so that
they would decorate the garden for a long time. Nature magazines, on
the other hand, would encourage us to plant berrying shrubs, vines and
trees to feed the birds. It is only recently that there have been
complaints because birds fly into the woods, perch on a branch, and
excrete the berry seeds.

But would the presence of berries be enough to produce a potential
invasive? Holly farms here have been shipping Christmas holly East
for a century and many gardens include holly trees. Birds eat the
berries, and so we have holly trees growing in our forests. They
don't seem to do any harm - the ones I see are usually not as big as
the native understory shrubs, all of which have spreading roots and
form huge colonies. The woodland hollies I've seen never have berries
because male and female holly trees never seem to get together, and
our woods are too shady for holly flowers anyway.

One berrying plant that can cause problems is ivy, which can cover
large areas of ground, and grow vigorously to the tops of trees.
Maybe the important thing to notice here is that it is a vine with
berries that are spread by birds, and so has two methods of invading.

Quite a few plants of current concern are ones that grow in or at the
edge of water. Maybe that could be a warning sign for potential
harmful invasiveness.

Diane Whitehead
Victoria, British Columbia, Canada
maritime zone 8, cool Mediterranean climate
mild rainy winters, mild dry summers