Invasive Perrenials

One disadvantage of some perennials is that they are invasive, that is, they  spread where they aren’t wanted and they are difficult to control and keep in gardens. They may spread by roots, as in the case of mints, or by seeds, as in the case of purple loosestrife. Those spreading by roots can be useful if in a confined situation, or when used in rough areas, such as slopes, to control erosion. In perennial beds, however, the root-spreaders can kill less vigorous species and take over most of the bed. Those spreading by seeds may be useful, as is the case with many field wildflowers, yet some, such as purple loosestrife, colonize wet areas, eventually destroying them and the wetland wildlife habitats. Purple loosestrife and other similar invasive perennials are banned from sale in many states for this reason.

The invasive nature of perennials is relative. Many on the list below may not even be hardy in certain areas, therefore acting neither as perennials nor as problems in these areas. Or they may be hardy, though not vigorous or a problem, in many areas. Others, especially the root-spreaders, may be less invasive or not invasive at all depending on factors such as culture and soil types. Those invading by seed may not be a problem in northern areas if the short growing season keeps them from going to seed (such as some silver grasses).

The term "invasive" is itself relative. To some this means any plant spreading.  To others an invasive perennial is one that not only spreads but is also quite vigorous and difficult to control. Those that spread but can be controlled by yearly cultivating or dividing are termed either "spreading" or "aggressive," depending on how fast they spread.

Other than cultivating, dividing, and weeding out seedlings, root-spreading perennials may be controlled by planting them in containers either in or on the ground. If planted in the ground, make sure roots do not exit the drain holes or go over the top. To keep these perennials from year to year they may need to be divided and repotted annually to keep them from dying out. Systemic herbicides may also be used to control root-spreading perennials; several applications may be necessary for effective control. Herbicides that act by merely burning back the foliage are not very effective with vigorous perennials, which merely resprout from the roots.

The following list is only a beginning of some of the more common perennials listed as invasive by some, in some areas of the world. Others, especially those that "self sow" once they go to seed, may be in this group as well. Some species or cultivars of a perennial may be invasive while others are not; the listing of a genus below does not mean that all of its members are invasive. This list should be used only as a "red flag" to check closer into a particular plant or genus of perennials before planting.

Adapted from University of Vermont Extension, 2000