|Lantana 'Gold Mound'|
Here in the Coachella Valley, Pennisetum setaceum, the common fountain grass spreads far too rapidly and as a garden escape is threatening to colonise many of the desert areas. To those of us from the UK it may be surprising to know that Ailanthus, the tree of heaven, Arundo donax, the giant reed, Colocasia esculenta and Paulownia tomentosa, the foxglove tree are also regarded as invasive species in the USA. The exoticists amongst you will be shocked by the last three - stalwart constituents of an exotic border. Amazingly Agapanthus is on the invasive species list for New Zealand as is Acanthus and horror of horrors - Cortaderia - pampas! Maybe this blog needs an invasive species warning!
|Colocasia esculenta - taro|
Many of us will also have admired the beautiful blue flowers and curious inflated green bladders on the water hyacinth, Eichornia crassipes, growing in tropical pools at botanic gardens. This too can be invasive in warm countries and has almost totally colonised Lake Victoria in East Africa. As well as generally blocking waterways it creates an ideal climate for breeding mosquitos. There has been some success with controlling it using insects which eat it. But its still beautiful!
|Eichhornia crassipes - water hyacinth|
Back in the UK we have our own monster weeds. Most gardeners will have struggled with bindweed at some stage but it always comes as a surprise that it was first imported as an ornamental in the 19C. I have an old book back home with a pretty little picture of it in a hanging basket. The same is true of ground elder, Japanese knotweed, giant hogweed and Himalayan balsam - all imported by those most illustrious of gardeners, the Victorians! (Correction - ground elder actually introduced by Romans.) Little did they know of the problems they would cause for future generations of gardeners. Eradication of Japanese knotweed remains an almost impossible problem and with the rapidly dwindling arsenal of herbicides available for use, its not going to get any easier.
|Impatiens glanduliferum - Himalayan balsam|
Sycamore, Acer pseudoplatanus is regarded by many as a weed tree. It spreads rapidly and produces a dense canopy, again preventing the growth of other species. Each mature tree is capable of producing many thousands of seedlings each year which perpetuate the problem It still amazes me that local authorities will place Tree Preservation Orders on this most undesirable of species. A few years back I decimated a whole sycamore woodland to the shock of the local University community. Check out these Flickr pics to see what we found underneath.
|Sycamore seedling - looks harmless!|
Years ago I was taught that a weed is a plant in the wrong place but in some cases its not just chickweed in your chicory but aliens in the landscape!