What is an invasive plant?

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An invasive plant is one that is harmful to native plants and ecosystems.  An exotic plant is a non-native plant that has been brought in by natural (birds, mammals, wind, water) or human (cars, trucks, shoes, garden ornamentals) means.  Not all exotic plants are invasive.  Some are benign.

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Are all invasive plants equally harmful?

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No.  There is a ranking system to determine how harmful an invasive plant can be.  The first question is: can the plant live in this new environment?  Many plants  that are highly invasive in a warmer climate cannot survive in Alaska.  The plant is then rated on four more characteristics: ecological impact, dispersal ability, distribution, and feasibility of control.  The plant is then assigned an invasiveness ranking between 1 and 100.  Pineapple weed has a ranking of 32 which makes it of little concern.  It has not been observed in undisturbed natural areas and it doesn't compete with native species..  It is easily pulled out and is a weed of farmyards and roadsides.  On the other hand, spotted knapweed has a rank of 86.  Research has shown that spotted knapweed can increase soil erosion leading to sediments in fish streams, grow so densely that reduces native plant populations, is dispersed by wind so can travel far and fast, inhibits growth of other plants, and seeds can remain viable in the soil for 8 years.  Plants ranked 80 or more are considered extremely Invasive.  Those ranked 70-79 are highly invasive, 60-69 moderately invasive, 50-59 modestly invasive, 49 and lower weakly invasive.