When Plants Take Over

Unlike a rambunctious vine such as kudzu, plant invasiveness is not always immediately apparent. This is the time of year that the wild spread of ornamental pears (Pyrus calleryana) in Georgia, however, is especially apparent. White-blooming trees on the side of the road this time of year are not native; we don’t have trees (in North Georgia anyway) that bloom this early.

Ornamental pear (Pyrus calleryana) wildlings

People might think this is a bonus. “It’s nice to have these blooming trees so early!” And it would be if the plants weren't invasive, spreading into areas and crowding out what native plants might have been there. These trees create dense foliage and replace native trees like maples, redbuds, sassafras, and serviceberries that would naturally grow in the part-shade woodland edges. These pears only provide one kind of support to insects: pollen for bees which are often non-native honey bees. Unlike native trees, they provide no services to insects the rest of the year, such as for butterfly and moth larvae. Insect diversity suffers when one plant takes over.

When do we know that something has become invasive? How does it start? It usually starts with cultivation which introduces the plant into ornamental landscapes (yards, parks, etc.). Sometimes the cultivated plant (developed by nurseries for traits like better flowers or fruits) is actually promoted as ‘sterile,’ meaning that it won’t set fruit (really they are just self-incompatible, not actually sterile as has been proven since then). This was true when ‘Bradford’ pears were sold beginning in the mid-1960’s (the nurserymen must have realized that the tiny fruits were a problem if they felt that declaring it ‘sterile’ was a plus?!).

Leftover fruit on callery pear tree

Once enough of them are in the broader landscape (enough of them for pollinators to fly between), plants exchange generic material (pollen) which makes many of them have a better fruit set.

Japanese honeysuckle (Lonicera japonica) is an example with many years of history. Collected from Asia in 1804, cultivated in the US by 1842, 'improved' in 1862 ... the first documentation of its escape to the wild was in 1882. By then, it had been planted in thousands of gardens. It is now considered to have invaded more acres in the southeastern US than any other non-native plant (including kudzu which is more noticeable but on far fewer acres).

It doesn’t take a math major to figure out how things multiply into increasingly large numbers. My neighborhood has a large amount of non-native mahonia (Mahonia bealei). In the 15 years that I’ve lived here, I have removed on average about 3 seedlings a year. That’s not much, right? If I hadn’t, however, I’d now have 45 plants in my two-acre yard. Each one would be flowering and creating more. I have neighbors with large amounts of this plant (one is even proud of how many he has). Each year, they spread further down the road, as birds eat the fruits and fly just a little bit further.

Instead of having mahonia, my woods have more blueberries, more native azaleas, more viburnums and other native shrubs. I have more diversity than if I’d let this plant grow as often as it showed up.

Same with the callery pear: every year, fruit gets carried just a little bit further by a bird, a squirrel or even a human. We should absolutely stop planting it and stop selling it. Just as important: we should be removing these escaped plants, because every one of them adds to the multiplier effect. These wild pear plants are especially fruitful. Please remove any that you find. Support our native insects by making room for native plants.

This blueberry (Vaccinium) is native to my yard and it supports insect herbivores