Wednesday 29 January 2020

Plants that become weeds

Who wants a twenty acre wood of cardiocrinum?

Garden plants that take over

Horse radish is very difficult to eliminate
The press reported when Jeremy Corbyn gave Tom Watson horseradish plants from his allotment that it was a show of good will.
It was a poisoned chalice.

One of my achievements when I took on the overgrown Bolton Percy churchyard almost fifty years ago was to eliminate this weed. It had roots two spits down and roots as thick as my arm. It took two years of glyphosate spraying to remove it.
If conventional perennial weeds are allowed to stay undisturbed in the ground for a very long time they too become so strong that they are much harder to eliminate than the text books might claim and need extra applications of weedkiller. (Not always - a strong stand of undisturbed couch grass will succumb to glyphosate in one go  and as I previously described I eliminated many year’s of established and crucially undisturbed, convolvulus very easily in Steven’s garden).

Some plants have the capacity to build up their strength for year upon year if left alone. Have you ever tried to dig out a ten year old hosta? 

Left for decades  some plants will become  monsters. Witness Japanese knotweed originally planted in Victorian gardens and how now ten foot high plants rampage through a landscape. As a new individual plant it is a pussy cat and easy to control.

I do not want my legacy to be Equisetum hyemale taking over Bolton Percy Cemetery (I have already eliminated it)

Indeed I would go so far as to suggest some potentially problem plants are difficult to get started. Gardener have often told me how their lily of the valley just won’t grow. Others struggle to eliminate strong stands or slow its spread.
Some plants year upon year build up huge resources. Beware.

Books warn not to plant aggressive houttuynia. This one took several years to achieve 
It is not my intention to frighten you today. Quite the opposite. For myself I love plants that are tough and can take care of themselves. If they spread horizontally and fill up large spaces that suits me fine. I relish the spread of such as alstromerias and Autumn anemones.
This Autumn anemone makes seven feet on Bolton  Percy heavy soil. It struggles to three foot on my own sandy soil
Some folk fear alstroemeria because they 'run'
There are perhaps two main characteristics of potential rogue plants They spread horizontally and/or make deep root systems or other resistant perennial structures. 
Bluebells such as these would be very difficult to eliminate should you wish to do so 
I will almost ignore today those plants that take over by self seeding. 
Perhaps just a word about bluebells. From seed they are slow to come up to flowering and even from strong bulbs they take a while to make up to a decent display but once established are really thugs. Beautiful thugs and in Worsbrough cemetery cover huge areas.

Perfect storms
There is more to it when certain plants take over than having planted the wrong subjects or long years of inaction. A major factor is whether the plant loves your conditions. Things such as soil type and fertility, drainage and light conditions. Just as a good gardener goes out of his way to plant in the right place for maximum success, for some of the thugs you can have too much of a good thing!

Monty says never plant skunk cabbage, it might take over. Here in a bog at Bodnant perhaps after a hundred years they are somewhat over grown but look quite superb
I moved from a heavier clay soil to my Seaton Ross sand. I was overjoyed to grow plants I could not before.
It was a great disappointment to find for others I could not. My beloved Aster ‘Violet Queen’ now needs all my skills to maintain it. The Japanese anemones although still lovely only make half the height than in Bolton Percy. Achilleas, hepaticas, bulbous iris and echinacea just fade away. Phlox do well for me but take years longer to establish strong clumps. (On the other hand Phlox subulata creeps freely and is a sheer delight).

Several thugs that the garden gurus tell us not to touch with a bargepole are for me shy maidens and difficult to grow. I delight that my Houtonya spreads at all. I bought expensive skunk cabbage and after twenty years I rejoice that it is big enough to be noticed.

Horror story follows about this shy plant that transforms into a monster
An extreme case of a ‘delicate’ plant ‘going native’ is the lovely poppy-like herbaceous perennial Romneya coulteri (shades of Mrs Coulter in Phillip Pullman’s wonderful ‘Dark materials’). I just cannot grow it at all. Not in any of my very different gardens and I have tried several times.

Peter Williams received an appeal from Glasgow. I think about a well drained sandy light soil - I never saw it but clearly a dead ringer for Romneya’s native habitat. The lady owned  an old sandstone property which was being ravaged as the roots ate up the foundations. Every story you have read about Japanese Knotweed was mirrored and exceeded here. I am not sure whether my own advice helped the dear lady. I wish now I had taken more interest, helped her more  and got some pictures
Peter Williams tells me he does manage to grow Romneya. In the past with great difficulty. It is now making very strong suckers. Some garden threats lie there as sleepers.

Woody wanderers
It goes without saying that self sown and inappropriately planted trees create dense shade and damage foundations. Not my subject today.
Many climbers are woody and some can be a menace. 
I have written before how global deforestation often gives aggressive climbers and sprawlers a foot hold. In the UK too some become monsters.
Most of us have witnessed how aged Russian vines are a nightmare to remove and it might take days to cut away from old buildings.
More innocuous genera do have their monsters. Clematis armandii and passiflora caerulea  are both tender plants that in the North can be difficult to grow. Given space and time and absence of pruning in a warmer climate they are a real menace.   The ever popular Clematis montana  has its moments too.
Give wisteria several decades and your parkland will become a jungle.

Always think twice about planting bamboos There are some none-creeping kinds but for the others beware.

Peter Williams hard prunes his Kiftsgate every year. If he did not do so he would have a monster on his hands
When I had my own clients my dear friend Jackie Barber grew that rampaging rose, ‘Kiftsgate’. It is super vigorous, lovely, very thorny and will swamp even quite vigorous trees. I dreaded the uncomfortable prickly day that her single plant would take me to prune each year.

I have just returned from Madeira where it strangled one hundred feet cliffs - sorry forgot camera!
There are herbaceous stranglers too. For most of us ipomea is a delicate annual. In Vico Equense (my son lives there) it is truly  perennial and over the cliffs and adjacent gardens creates huge tracts of blue. Think of our own bindweed. Naughty but nice.

Bits and bats and bog plants
Give it five minutes and hippurus will take over a huge lake. (Carefully directed glyphosate easily zaps it in your small pond at home)
This section has a watery theme. Bog areas and ponds are particularly vulnerable to rogue plants going native. Plants that have evolved to like wet conditions just keep on growing in sunshine and on really good sites never lack for water and have little competition from regular plants.

Leucanthemum, the shasta day
We always take our umbrella when we go to see Rowena and Harry in very wet Preston. I mentioned this post to Rowena and she confided in her problem with leucanthemum, the shasta daisy with which she has a love/hate relationship. Their garden is on 30 foot deep heavy clay, there is heavy rainfall and water springs appear in awkward places. Many of the fine plants she puts in die. Her shasta daisies are lovely. (She has a beautiful white border and guess what is the dominant plant). It spreads everywhere  and even in relatively dry weather her clay soil generously sustains it. It is starting to penetrate into the mortar of her brick wall. For the moment the house is ok.
Dammit it has died out on me. To be fair to this vigorous plant I have only tried the poncey double and fancy-petal varieties. 

Back to  bog plants some of which are are prominent in lists of alien plants illegal to grow and some have prohibitions on selling and planting. 
Over the top but some aquatics in water courses are a serious menace.

My gunnera loves its boggy patch. I wonder what it would do if left untouched for fifty years
The answer to the above question is that its spread would cease when it met drier soil - or strong plant competition (I might be able in a few months be able to tell you what complete submersion for six months does to it - my garden is still half under water - drains now in place)

One answer to over aggressive plants is to let them fight it out with each other
 Aristocratic weeds
Peter and I attended a lecture that pictured a neglected ten acre site completely taken over by cardiocrinum lily. It really hurts your wallet to buy one. They take perhaps eight years from seed to flower and set seed before each plant dies. Absolutely magnificent but a nightmare to tackle. Its what you get after fifty years of leaving alone. They retail at twelve quid a  throw.
I suppose when your lake is taken over by water lily you might describe that too as an aristocratic weed.

I wonder if you have any other candidates? 

More about the Romneya nightmare
Peter Williams has furnished me with some of the details of his anguished correspondence. I pick out some of the details from the emails he shared

….the builders are immediately moving in here, as the house has been invaded by a nasty, vigorous plant from the garden which is growing between the plasterboard and the masonry in the dining part of the kitchen. 8ft high and 3 ft wide. Has 
entered from garden under the bottom layer of our 6 layers of flooring insulation…… 
….Romneya coulteri, an American poppy, but not just any old poppy. A poppy on steroids. As well as having to have the kitchen dug up (and we don't know how far it has spread in the house), we then also  have had the patio slabs lifted to find out where it has entered the house, and then most of the large border excavated to eradicate it between the patio and the far end of the garden… 

….We hope the inside work will be finished by Christmas, but don't yet know if the outside work will start immediately or will wait until the spring. It has also spread into our neighbour's garden, who, like us, have been cultivating it assiduously as it is a very beautiful, large showy plant with lovely huge white flowers in late summer. No sign of it in their house, though…..

…… Not covered by insurance, as this has been gradual and not an "event". It was planted by the previous owner of our house, who was an avid gardener. It is very difficult to get it established in UK, as it likes desert conditions, but she managed it! …..

What a nightmare


  1. Lily of the valley is my birth flower and I can't get it to grow after several attempts.

    We gas a bamboo in the garden that was becoming a pest so we dug it up. and replanted in ay the allotment where it behaves itself perfectly!

    1. I too wold like to establish some lily of the valley - but have failed. I didn't dare take any from my father's garden before the house was sold because it was mixed in with horse tails!

    2. I understand your fear Neil!
      You could have carefully washed pieces out and planted them

  2. I'm actually fond of horseradish, especially on bland foods like fish balls. But a little bit goes a long way.


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