Invasive ? plant lists

Diane Whitehead
Wed, 03 Sep 2008 18:47:01 PDT
> On 3-Sep-08, at 3:28 PM, William Aley wrote:
>> . Is there any model or
>> method that will predict when a plant species, introduced as a
>> horticultural interest  will convert to becoming the next ecological
>> invasive disaster?

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

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  

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


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