By Goringfolly
West Sussex,
United Kingdom 
Problem weed. I posted a question yesterday about this weed but nobody seems to have made a positive ID as yet. There are big swathes of it all over waste ground near the seafront here in Goring, so it must be reasonably common. I took a photo this morning of one in full flower; maybe it will help?
Below is my first posting.
I have a problem weed spreading from a neighbouring garden but cannot identify it. Can anyone help? The green shoots come up on spindly thin white stems that break away easily from a bigger underground root system with tougher brown roots. These stems run horizontally about 12 inches down and will travel quickly under a metre or more of concrete. The shoots will come up anywhere there is the tiniest crack. I have been spraying on a weekly basis for some years but only managed to slow it down a little. Digging it up is not possible when it disappears under a path or building. Spaying only kills the shoots, not the root system. Nobody seem to know what it is.
26 May, 2010
Answers
I think what's throwing us is the root system, I am wondering could it be that it just looks like the plants a growing from this large wooded root and in actual fact they are seedlings, bacause valarian seeds are really prolific..
26 May, 2010
its a valarian. old ones do become woody. well they did in a neighbour's garden.
26 May, 2010
OMGoodness just googled "white weed" as I do not agree about it being Valerian (see other thread- the leaves of Valerian are in pairs up the stem) and came up with the following:
http://stca.tas.gov.au/?p=69
Source: 'Southern Tasmanian Councils Authority'!
I thought yesterday that it was strange that when I google image searched for 'white weed' that a very similar picture on Flickr was located in the US!
So, I think it is 'white weed' / 'white top' / 'hoary cress' (Lepidium draba).
Edit: apparently not unknown in the UK so it's not as exciting as I thought! But it's still funny how I found it. More info here:
http://www.gardenorganic.org.uk/organicweeds/weed_information/weed.php?id=9
26 May, 2010
Agree with Bryony not a Valerian, as she said the leaves are in pairs. I have them in my garden. white, also red ones
26 May, 2010
Many thanks. I do believe that Bryony is correct in that it is Lepidium Draba or Hoary Cress. I have looked it up and everything fits the ID, habit, appearance etc. So thank you very much indeed. Now I know what I am up against I can try to get rid of it.
26 May, 2010
I compared the photo with the valerian growing all over walls in Cornwall & it's not the same plant.
The Gold Star goes to Bryony - I'm sure she's right too. Now all you need to find is a weedkiller containing glyphosate and dicamba ( as recommended by the Tasmanian government).
26 May, 2010
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Really looks like Valarian to me!
26 May, 2010