Alien attack… Invasive plant species and how to deal with them. Part 3

Previously I blogged about gunnera spreading along canals and streams… well another riverbank monster is Himalayan balsam (Impatiens glandulifera)… But while the trend is to bash it, it has some good edible applications; The peppery seeds are wonderful in salads and baked on breads. From the same family as busy lizzies this very busy plant escaped from gardens to naturalise in the wild. Towering sometimes to 3 meters, its seed capsules explode with tremendous force and fling their shot as far as 7 meters from the parent. They form dense stands and crowd everything else out. Rapid along riverbanks and canals, streams, ponds and lake margins and any dampish hill, they die away completely in winter and leave the surface soil open to erosion which in many instances causes sediment to be deposited into waterways, burning up tax revenues on clearance and detrimentally affecting fish spawning.

The Himalayan plant is everywhere nowadays, easily recognisable by its purplish–pink to pale pink flowers that bloom abundantly from  June to August in the shape of a English police helmet (attested to by one of its common names), and at other times by their translucent, fleshy stems. They bear large pointed leaves, ovalish, edged with obvious teeth each one with a small globular ‘gland’.  So floriferous that they do steal all the bees.  Often reducing the native populations by 25% in a given year, achieved by detracting from native pollination. So floriferous that seed falls to the ground at a recorded density of between 5000 – 6000 seeds per square meter of surface soil. Prolific is the word.  Herbicide is effective at home. Wild colonies are subject to balsam bashing days by local conservation groups; this entails whipping the plants with sticks before they flower or set seed – if in seed you are only spreading the populations. Repeated cutting back will help eradicate.

Giant hogweed (Heracleum mantegazzianum) is far from edible,  in fact it would blister your mouth to a hospitalization degree. A taller (3-5m) and more robust cousin of hogweed and cow parsley, it is phototoxic; meaning its juice will burn your skin in daylight. So hogweed bashing or even strimming is not an option. The lesions the plant cause are slow to heal and what is known as post-inflammatory hyper-pigmentation can persist for as long as 6 years after initial contact with the sap. The giant hogweed also contains  furanocoumarins which are detrimental to birdlife and is a danger to pets.

This biennial plant was introduced from south west Asia in the 19th century, intended as as an ornamental garden plant. Today it is more commonly found in escaped colonies along river banks, road sides and waste ground, occasionally along walkways and park paths, triggering closure of the amenity until the spread is removed. Denser colonies exist in damper places and riverbanks are most prone to rapid invasion. Typical with the invasive aliens it excludes indigenous herbaceous plants, and ultimately undermines riverbank stability. It is a major nuisance in disaffecting salmon spawning. Typical too with the most successful invasive it produces several thousand seeds per plant, which are light enough to travel good distances on a breeze and even further on water currents. The flower is typical of umbellifers, looking like the spokes of an upside down umbrella clustered with a frenzy of tiny white inflorescences. It flowers between June and September. The stem has distinct purple blotches and is both hollow and ridged. It smells a little like parsnip and in fact its rootstock is quite like a bloated parsnip. To get rid of it off your land, wear protective clothing especially goggles and gloves, it can be dug up and if not bearing seed, dried and composted. It is susceptible to repeated herbicide applications. It may take a few goes but persistence will win the day. Of all on the list this is the most dangerous one and needs tackling straight away. Protect children and pets for exposure to it.

Lastly but by no means least we come to Hyacinthoides hispanica, better known as the Spanish bluebell.  This perhaps is the toughest in the list to think of as an alien invader, worthy of extraction from the garden and wild. But don’t be fooled by its blue-eyed good looks. It’s a thug.   In many respects it is considered a more steroid version of a native bluebell, but beyond its vigor if you look closely, all of its bell flowers  are arranged around the stem. Natives hang their bells to one side. This a key clue as the Spanish hybridize readily with the natives and the natives take on that trait amongst others.

The Spanish entered Ireland via garden centers, sold as an ornamental and this is evident in their colour varieties, not just blue but pink and white. The problem is that the Spanish out compete the natives and lure away pollinators while simultaneous hybridizing native populations out of existence.  Seed dispersal of both original and hybrid is largely by wind and generally over short distances so its threat is currently not considered high, but we have an opportunity to not let it escape further into wild native colonies by not planting Spanish or hybrids in our gardens and insisting garden centers trade in native stocks only.  If you have non-native bluebells in your garden, dig them up, crush the bulbs, dry out the mashed bulb and compost or dispose sensibly. The problem is that they are a risk, they will alter ecology..  so no matter how ornamental remember ‘potential detrimental’.

So they may not have come from outer space, but all that means is that we can’t kill them off with the common cold. We need a strategy. Governments and conservation groups around the world are continually revising methods of control. The best control is self control, I know that means different things to different people but in this case it means don’t plant an invasive species in your garden and if you have one or two already there then it is up to your self to control it.  And if you really want to play a bigger part, your local conservation group are always on the lookout for volunteers.


Just in case.

About The Holistic Gardener

author of wellness books, columnist, keynote speaker.
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