Forum home Problem solving

advice desperately needed on the evil that is alkanet/pentaglottis

I understand this creature of evil can be called "green alkanet", or "pentaglottis."  I have a few other names for it.  Here is my story.  If tdlr; please give me your advice on controlling alkanet.  I know it might take years, if ever, for it to be removed, but I'm worried I'm making it worse, and that it's getting uncontrollable. 

I recently moved into a flat where "maintaining" the medium-sized garden is part of the tenancy.  I bring to this role a complete absence of gardening experience, a lack of natural talent (one might say even anti-talent), and a tendency to shriek at the sight of worms. 

The previous tenant must have let it go for years, since alkanet owned that garden.  It fully covered the beds.  The beds are almost entirely empty apart from a couple of roses and some bamboo at the back. It ate the back third of the lawn, and was munching on the front third.  I miss my naivity when I visited, saw a meadow of three foot high foot plants and thought "what beautiful blue flowers".  And when the agent said it would be taken care of before I moved in...  

Well, certainly the three foot plants were gone a month later when I moved in, but already on that day there were new 3-4 inch plants in all the beds again, and in all of the back third of the lawn, and growing in the front third.  I ended up hiring someone to dig them out for 4 hours, and he said it had been allowed to be invasive for so long that the roots were over four foot, and it would keep coming back.

It stayed quiet for the best part of a month.  Perhaps it was only licking its wounds before planning its next move.  Then I noticed a few.  Dug out/hoed out.  Then a few more.  Last week I noticed ones that had grown big hiding behind the bamboo.  I couldn't pull them out.  Dug in and the top root snapped.  Last weekend, I pulled/hoed a lot of little ones from the beds. 

Today (Tuesday) I pulled another 47, including distressingly a lot in the bed I had already hoed only two days earlier.  I definitely had not missed them.  There are also some growing in the fragile new grass that I have (incompetently) sown.  I almost cried at the sight of that.  And then while attempting to dig in the bed, I found a lot more of what I think are tap roots. 

In case it's not clear, I am desperately in over my head here.   I've bought a lot of plants that I want to put in the beds to protect the soil and - if possible - to reduce the alkanet's spread.  But I don't know if it's safe to do this yet.  I've googled a lot but am finding conflicting advice about how alkanets work and whether I'm following the right methods.  In particular:

1) Should I keep attempting to dig out tap roots?  Bearing in mind the soil is hard, and my incompetence (today I proudly dug a foot deep before the root snapped), am I actually making it worse by inadvertently leaving multiple pieces of root as has been said in one post ?  Is that why they seem to be multiplying?  Or am I slowly "making it better" by weakening the root, and the sudden increase is due to dormant seed?

2) Will the alkanet still come up "through" plants I put in the beds?  I want to get ground cover, since it's 90% bare.  But to keep on top of the alkanet, I need to hoe before they develop a tap root too hard for me to dig up.  Putting plants in the beds makes that harder.

3) Is it a bad idea to plan to put bark mulch on top of the beds once the plants are in?  Can that suppress alkanet?  Or does it mean that by the time they push up through the mulch and I spot them, they will have generated that awful tap root? 

4)  Does Roundup kill the taproot itself if applied to the leafs, or will the root keep regenerating until dug up?

5)  Does alkanet only clone itself by seeds, or does it spread its roots elsewhere?  In other words, if I keep on top of hoeing the babies, does it matter that the tap roots remain?  I keep finding a cluster of very thin brown roots when I dig but don't know whether to rip that out as well, or whether it belongs to one of the few surviving plants or previous occupants. 

6)  Is it okay to leave the tiniest of baby alkanets in the bed after hoeing?  If it's big enough for me to identify as an alkanet, I dig it out and put it in a bag and take it away.  But the beds keep growing lots of "cress like" weeds that in their first few days are hard to tell apart from alkanets.  For those, I just hoe and leave them there.  Could this be causing the sudden regrowth this week?

7)  Should I stop spending money on plants and instead pay to have someone competent sort this out (again)?  I am okay with hoeing and digging out babies.  But ones that have the taproot are laughing at me.


  • Busy-LizzieBusy-Lizzie Posts: 19,166
    Glyphosate should kill green alkanet but it will also kill any other plants that get sprayed and the lawn. As yours is so rampant it will probably need more than one spraying. Avoid spraying the lawn and repeated mowing should keep down the alkanet.
    Dordogne and Norfolk
  • DovefromaboveDovefromabove Central Norfolk UKPosts: 79,298
    Hello @mktournament2000 and welcome to the forum 😊 

    That is a situation just crying out to be tackled with a glyphosate weedkiller. Not now … the plants are shutting down for the winter and it needs to be applied to actively growing leaves. 

    Apply it in the spring, exactly as directed on the pack. Leave it until all the leaves have turned brown. This indicates that the chemical has travelled back to the roots. 
    I would then dig out all the roots that I can. 

    Then I would wait a couple of months for the appearance of more alkanet leaves
    … if any appear treat them as before.  That should sort it and you can dig over and create a garden. 

    You will always have to keep an eye open for an occasional invader from around your boundaries, but a carefully targeted spray with glyphosate will deal with them. 

    Many folk (and I’m one of them) prefer not to use chemicals if at all possible.  Trust me, you won’t beat alkanet without. 

    Good luck 😊 
    “I am not lost, for I know where I am. But however, where I am may be lost.” Winnie the Pooh

  • LG_LG_ SE LondonPosts: 3,921
    My garden used to be full of the stuff and now it isn't. Occasionally one comes up, but it's entirely manageable now. It's a bit late for me to answer your questions one by one, but:

    1. In my experience, the only thing that works is digging every bit out. And then again and again. Sorry. But it does work.

    2. I have / had a few in paving or other spots where I cannot dig. On these I have used glyphosate. Repeatedly. Eventually it weakens them but is not as effective as manual removal, in my experience. However, occasionally it's the only option.

    3. Don't hoe them. However small, remove them root and all. Otherwise you're making it much worse.

    4. Yes, they will come up through other plants. Whether they are reduced or not I don't know, but I have had them enmeshed in the rootball of shrubs etc. Again, sorry.

    Lots of sorrys. But they can be beaten, and if you get the big ones, any regrowth from the bits of root tend to be easier to remove and quite satisfying.
    'If you have a garden and a library, you have everything you need.'
    - Cicero
  • SkandiSkandi Northern DenmarkPosts: 1,508
    Digging them up won't make it worse, yes you may have 5 plants come up rather than the 1 you started with, but they will be 5 much weaker plants, hoeing will also work, but you will have to keep up with it at least once a week for at least a year possibly 2.
    I would try roundup once if that doesn't seem to have the effect I would go with SBK or other brushwood killer.
    Don't worry about the bits in the lawn, keep it mown and they will die in a year or so.

    Don't mulch until you have the taproots gone, they won't care about the mulch at all and it will just be another layer for you to dig through. You could mulch with black plastic for a year /18 months if any areas are amenable to it.
  • Rob LockwoodRob Lockwood Midlands of EnglandPosts: 321
    edited October 2021
    Agree that you can have a good crack at digging them out - as you note the roots go quite deep but as Skandi says, if you don't get it all out, you've still removed 75% of the b****s food/energy reserves, so a second attack the next year to avoid it refuelling should finish it off.  Very few plants can survive without leaves to photosynthesize.  Alternatively, weedkiller I suppose.
    I leave 2 small patches as the bees absolutely love it, but of course take off the spent flowers because I'm not mad :) 
    For some of your other questions - pretty sure they don't spread underground as such, so seeds are the big enemy, but although I'm generally rabid about removing any new ones which get through the deadheading, it takes them a while to become a nuisance - there's one here with leaves around 6 inches but I'm not too bothered yet as it'll just pull out.  They're recognisable fairly early on from the little white dots or the bristles on the leaves.

  • Alkanet will spread via root IME - removing the flower heads once finished helps but not the end of the story unfortunately :)
  • When we first moved in here about six years ago we had a 4 x 2m border pretty much riddled with alkanet, and patches of it in the other borders. I used to try pulling/digging it up, and the roots of course would snap. Then I found the dandelion weeder tool, which is much better at getting deeper into the root. Long ones still snap, but I suppose it weakens them more as it goes deeper than a trowel. I now no longer blanch when I see it (which I probably always will because lazy), as it’s now manageable. Mulching in autumn/winter also helps a bit. Another demon tool is the Japanese Hoki Hoki knife, which again easily digs deep without disturbing the surrounding soil and encouraging seeds to germinate.
  • mktournament2000mktournament2000 Posts: 2
    edited October 2021
    Thank you, all!  Gosh it does seem like there's a diverse range of views for tackling this evil. 
    Well, I've bought a pick-axe off amazon to help get through the hard soil, so I'll give those tap roots another go this weekend, and perhaps they won't laugh at me this time.   I'll also look at those tools Kathryn mentioned.
    I've also sprayed some of the taller alkanets that seem to have tap roots going under the patio or into the neighbour's garden.  If they're not actively growing, I'd hate to see what that actually looks like.  I've read that alkanet is meant to be quietening down this time of year and can't survive in acid soils...well, apparently not this mutant strain, since new ones pop up daily, and my soil came up as mildly acidic in two tests.  Of course I could have messed those up too...
    I am really worried that I might be making the problem worse by hoeing seedlings.  I'm not hoeing anything I think/know is a baby alkanet.  I dig them up with a trowel and dispose of them.  But little cress like weeds pop up every day too, and for the first few days, I can't tell them apart from the alkanet.  There are far too many of them to pick up by hand and dispose of.  So I hoe them and leave them in the soil.
    Likely some of them would have been alkanet seedlings, but when they're only a day or so old and have no tap root, does it really do any harm to hoe and  leave them in the soil?  Can they still seed?   Could the hoeing really be responsible for the surge of tap root less babies I've had this week?  I plucked another 17 yesterday, after the 47 on Tuesday.  I was hoping that the surge was down to the seeds being dormant, and my growing ability to identify them earlier...but if I'm making it worse...I'll have to stop, which is worrying, because hoeing is the only thing I'm pretty okay at.
  • FairygirlFairygirl west central ScotlandPosts: 47,145
    We're fortunate here that it isn't a plant that's common at all [more common further east apparently] but, as others have said, you may have to bite the bullet and use weedkiller initially to get on top of it, then remove, or at least dead head, to keep the upper hand.
    Mowing will help to keep it in check in a lawn, but if it spreads by rooting, it won't stop it getting into borders/beds. 
    Anything small that you regularly hoe will eventually [in theory] give up, but without killing the tap root, it's still possible for it to grow, like dandelions do. If you hoe new seedlings, and leave them on the surface, that part will die off.
    The other little plant you describe is likely to be bittercress, and those are shallow rooted, so hoeing them will generally see them off very quickly. 
    It's a place where beautiful isn't enough of a word....

  • BobTheGardenerBobTheGardener Leicestershire, UKPosts: 11,337
    I successfully removed a small 1x1m patch of Green Alkanet a few years ago by digging out as many roots as I could, then waited until the regrowth was about 18" tall and sprayed it with glyphosate (was about May/June time, I think.)  I left it for about 6 weeks before removing the dead leaves.  It's now gone, and I only have to watch for new seedlings germinating in that area (which is still on-going.)
    Can you post photos of the weed seedlings in the areas you mention?  If we can identify those (which may well be something else), we'll likely be able to offer more specific advice.
    On neglected ground, seeds can build-up in the soil and lie dormant for many years, only germinating when they are brought to the surface by soil disturbance, such as any general cultivation.  There's an old gardening saying "One year's weeds, seven years seeds" which means it could take you several years of regular hoeing before the number of weed seedlings germinating significantly reduces.  If that's the case, then I'd advise that you dig-out any perennial weeds such as the alkanet and dandelions etc., then hoe the rest and cover with at least 2 inches (pref. 4") of mulch, such as composted bark (or bagged composted manure, if you also want to improve the soil.)  By excluding light with the mulch (which will need to be topped-up annually for a few years), the seeds in the soil below will slowly reduce in numbers, as they can only stay dormant for so long before rotting away (although some, like poppy seeds, can last in the soil for centuries!)
    A trowel in the hand is worth a thousand lost under a bush.
Sign In or Register to comment.