Pampas grass, blue gum eucalyptus, giant reed, broom and Acacia dealbata are some of the best examples of the worst naturalized exotic species. They were imported for a variety of reasons or by accident, and now proliferate aggressively in the wild. With few of the pathogens they contend with in their respective native homelands, they have unfair advantages over locally native flora.
Such naturalization is a serious problem for native fauna as well. Monarch butterflies that swarm to the bloom of blue gum eucalyptus are amazing to observe, but are being distracted from native flora that rely on them for pollination. Both native and exotic rodents proliferate unnaturally within the protection of thickets of naturalized English ivy, and consume too many seed from other plants.
Fortunately, there is a difference between naturalization and sustainability. Many exotic yuccas can survive quite nicely in chaparral regions without irrigation or other intervention, but are unable to disperse seed and truly naturalize without the particular species of moth that are their exclusive pollinators in their respective native homelands. Cultivars of pampas grass are ‘supposedly’ sterile.
Some plants that seem to naturalize do not proliferate or migrate enough to become aggressively invasive or truly naturalized. That is why daffodil can be planted on roadsides to bloom annually, and hopefully multiply somewhat, but does not spread far from where initially planted. In fact, it is unfortunately less likely to naturalize, and more likely to slowly diminish through several years here.
Many plants that proliferate within the cultivation of our home gardens and landscapes will not migrate far from where they they get regular watering. Even after fancier and more colorful varieties revert to their most basic feral forms, they are delightful weeds that are more often left to bloom wherever they appear. Those that appear where they are not wanted are easy enough to eradicate.
These include sweet alyssum, forget-me-not, four-o’-clock, campion, cosmos and nasturtium.