However I also love many alien plants including the snow drops that come up early in the spring and raise my spirits after a long, sometimes longer than others, winter. I never worry about having too many snowdrops. They do naturalize, but they do it in such a dainty controlled way. I don't think you will ever read about out of control snowdrops taking over anywhere.
The snowdrops in this picture are definitely not the aggressor! The other plant is the villain! Now we come to a plant that I consider a pernicious weed!! Garlic Mustard, Alliaria officinalis, is an alien with some serious behavior problems! I have never liked the plant and I never planted it, but while I was not watching it has grown up what seems like everywhere. Brought to this country as a potherb, the plant is a favorite of foragers like "Wildman" Steve Brill.
He would be after the tender pointed leaves and flower buds. The plant is described a spicy and hot to some and bitter to others. Frankly I would welcome a busload of foragers if they would pull up the plants and take them home!
The basal leaves are shaped like violet leaves. I have been pulling this plant, but when I discovered that the plant apparently destroys mycorrhizae fungi in the soil I declared war! Ed's trailing arbutus and other wildflowers need the fungus in the soil to survive, Even the Eastern red backed salamander is reportedly endangered by this plant. So if you are looking for a garden alien to eradicate this is one you can go for with gusto. If you want to eat it do some research first. Here I just want it gone !!!