> 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 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 >> >>