Thursday, 25 May 2017

Hairy Bittercress, Cardamine hirsuta

Hairy bittercress and thale cress, ubiquitous garden weeds

If you don’t know what bittercress looks like you will soon if you read further
And if you read to the end you will know about thale cress

Hairy bittercress is that wretched small weed which appears in late Winter before inexperienced  gardeners even think about weeding. It germinates in empty spaces and soon starts fizzing out seed all over your previously immaculate  garden. Described as ephemeral, continued short generations bedevil you for the rest of the year. Unless you are alert to its rapid reseeding after as little as a month following emergence you will never be rid of it.

Cardamine hirsuta

Rare as a weed when I was a child, it is a modern phenomenon. (Considering my age, not that modern). As a common weed it has been encouraged  by modernity and has been a product of the expanding garden centre industry over the last sixty years. How often has it lurked at the top of the pot of your new plant? Almost always!

Bittercress is the curse of the nurseryman. He has historically controlled weed in containers by using sterile or sterilised compost. He also uses residual herbicides to prevent germination of weeds that have blown in. Bittercress rapidly establishes on the otherwise clean standing ground for container grown plants. With its ‘exploding’ mechanical dispersal - known to the botanist as ballistic dispersal, bittercress seed is thrown in the air and is flung as far as a meter and straight into plant containers. It’s sticky seed coat helps it hold on.


My friend Peter Williams once studied the mechanical distribution of bittercress and how far it would jump!
If you want to see bittercress popping all over go to this link

Over the years bittercress has proved to be resistant to the nurseryman’s residual herbicides. The only one that actually worked was withdrawn because of licensing costs. 
(Residual herbicides have never caught on with amateur gardeners  - other than as an ingredient in path weedkillers - but are widely used by farmers and growers)
And of course bittercress has been distributed to garden centres far and wide.

Description and identification


Cardamine hirsuta
Classical kind
An extreme variation. This bittercress is very low growing and is rife in extremely wet conditions. 
Being such a nuisance hairy bittercress has been much researched - especially in America. Although easily identified there are several forms. Personally I have two and possibly four. It is difficult to be sure because plants can vary so much with time of year and growing conditions. A survey of US nurseries identified four major forms (arguably ‘species’). The RHS name two species in the UK. It is difficult to botanically classify the various ‘tribes’ and ‘strains’. I do find my varied forms maintain their distinct habit and do not appear to cross fertilise. 
You will have no difficulty in identifying all of them as hairy bittercress when they take over your garden. Superficially they do not seem very hairy to me. Leaves are actually quite smooth. The larger strains (or possibly just the more luxuriant specimens) make quite delicious salad and are not really bitter at all. Shame about the inevitable mouthful of soil.

Bittercress control
With difficulty! The actual killing is easy - its the timing and access that makes it difficult.
If you have got a clear run in more open spaces 1in 50 commercial strength glyphosate will surely kill it it. Unfortunately in the days it takes to die if it has started to seed it will continue to spit out more seeds to start all over again. Dare I say it, perhaps speedy diquat containing amateur products are better? (I do not personally use or advocate unsafe professional liquid-concentrate formulations of diquat for gardeners)


The only way to control this bittercress and keep the self sown nigella is to pull it out

Hand weeding is fiddly, takes a long time and is often too late to prevent seeding. It is actually quite difficult to pull out when very small. I frequently break the seedlings’ necks by sliding my foot over them.

Peter’s border is really quite weedy

But his former student Julie is a beautiful weeder
If you have dry windy days as we are suffering at the moment (written in April) hoeing is very effective. If you are sufficiently delicate to sever the weed at ground level it will quickly shrivel and die. Beware over enthusiasm and undercutting more deeply as they will reroot again. This is all very well but the soil surface is rarely dry when bittercress germinates all over on a mild February day


This shrivelled bittercress will not recover
Peter Williams agrees with me that in favourable conditions hairy bittercress will set viable seed within three weeks of germination. If you have a heated greenhouse this can be through 12 months of the year. Outside give it nine months when it might be  active. Although it can  be a problem in Summer and Autumn I find dry soil surfaces restrict it and other than in wet spells I am barely conscious it might be there.
Unfortunately the weather patterns this year for me have been particularly conducive to bittercress and I have never had more!

My own control policy is if I see a bittercress seeding - which is most of its life - I kill it on sight. I bend my back and pull or scrape it bootwise or grab my hoe or point my spray. It must not seed! But oh dear how this year I have failed.
Many gardeners ask me how to control this weed when it is propelling its seed all over their borders. Sorry it’s too late.
The only way is constance vigilance, stop it seeding and get it out of the system. You might be in full control in a couple of years. That is if you don’t dig buried seed to the surface - although I must  admit it likes the settled soil surfaces typical of non diggers like me!
I know of one gardener who advocates very deep mulching to control weed from seed. If you have a clean start with no bittercress seeds popping and are perhaps prepared to cover the contaminated soil three inches deep (phew) I think this will work. Beware any aerial bombardment however. Nothing likes an organic mulch as a seedbed more than newly scattered hairy bittercress.

Hairy bittercress in pictures


Can you spot it?
Dwarf form carefully weeded
How did I miss these two
You might find it in low quality lawns
Sprayed in seconds - it's all weed
The dwarf form is quite sneaky - and at first pretty
It's lost a petal - there is always four
Hairy bittercress
Littles and large
Not all cardamines are weeds, some are  very select garden plants
Dwarf form sets seed

Thale cress
Leaves are distinct, flowers are held higher, it looks more delicate and I don't think it goes pop
We have another horse in the stable. Sometimes actually growing alongside bittercress although this weed tends to be a nuisance in drier conditions. Superficially it looks very similar to hairy bittercress. It loves my own fine sandy soil.
It does not disperse its seed with the same vigour as bittercress but if does seem to set seed even quicker - I can scarcely believe this is possible. I was shocked in Peter’s garden today how tiny his flowering thale cress in his dry border could be. I have spent all Summer trying to see if thale-cress forcefully disperses its seed but it does not go pop for me.

Arabidopsis
Thale cress is really quite delicate and is very  quick to set seed
Three ' weeds' in a row - thale cress, honesty and Dicentra cucullaria.
(My next post is about variegated honesty)

More is known about the genetics of thale cress than perhaps any other plant. It is the darling of researchers. They love getting speedy results from its very short life cycle!  
                                                 ......I don’t.


Links
A fairly 'heavy' link to US research on bittercress

I have written about several weeds now. Look for your problem weed in my search box!

8 comments:

  1. I didn't realise there were different varieties. Have you ever been a bit tardy weeding it out and the act of weeding has made the seeds pop, spray into your face and sometimes find their way into your mouth? Yuk!

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    1. It's more the insult - we have won!

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  2. I have a lot of the thale thingy, sometimes they are really tiny but already seeding. They hang out at the base of roses, where I don't see them and sometimes can't reach them. Sneaky.
    A lot of the native brassica/mustard plants are food plants for moths, so there is probably a very tiny moth larvae that thinks these are yum!

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    1. Weeds that hide under a canopy and sneaklilyset seed or later pop up through it like epilobiums do are a real nuisance Sarah. I often push my hoe or insert my downward pointed spray lance under the plant cover 'blind' when I know they are there.

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  3. Must be my number one pet hate Roger,on top of a list which includes wood avens, herb-robert and cleavers! Like you I never saw it before containerised plants came along but it has stayed with me since, seeding itself into all sorts of rarely accessed places.

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    Replies
    1. I do agree Rick and like your list - and hate those weeds - and know them well!
      Many gardeners would not recognise wood avens and herb Robert as weeds but they certainly are. It is a mark of a good gardener such as yourself to make a list like yours - It's rather ironic that when you have mastered the well known weeds, others take their place

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    2. I am on the side of not recognising herb robert as a weed. It has lovely foliage and the bright pink flowers are really attractive. However, I will not let it grow anywhere. That is also true of many plants we would all agree are not weeds but can become a nuisance, think of London Pride (saxifraga x urbium) or in my current garden antirrhinum which appears between the paving slabs almost everywhere where the hairy bittercress isn't growing! And where would you put Valarian? We have it popping up in colours from deep pink to white both in our garden borders and in the grass verges and hedgerows by the roadside I don't know if it arrived on its own or someone previous gardener planted it.

      Now cleavers is a pest, but have you noticed that if you pull it up the ground underneath is quite clear of weeds; that is of course if you have let it grow enough and by then. like the bittercress, it will have dropped its seed, or it will have stuck to you so you can drop it in a new place.


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    3. Very philosophical Neil.
      Its very corny for me to repeat a weed is a plant in the wrong place! I love my Valerian in the Worsbrough cemetery I maintain and I treasure the white herb Robert there.
      You are right to point out that if you get rid of one weed another takes its place.
      I prefer to cover the ground wth such as your London Pride

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