Make this horse a non-runner

Many thanks for your recent enquiry regarding how best to remove horsetail which is growing in between established plants. Let’s take a quick look at the culprit first...
  Field horsetail or mare’s tail is one of the most feared weeds. This intrusive plant has deep creeping roots which can reach as deep as 2m below the soil surface, and can reproduce from small pieces. Two types of shoot appear above the soil; light brown stems with a cone-shaped top appear in late spring and are followed later in the year by green 60cm tall stems, which are similar in appearance to tiny fir trees.

Treatment of this persistent weed can be carried out culturally and chemically. If you don’t want to resort to synthetic herbicides, then you’ll need to be diligent. Repeated hoeing and destruction of the foliage will weaken the plant, and generally ensure you remove any new growth as and when you see it. Forking out the roots to as great a depth as possible will also help but this is no quick-fix solution (as I mentioned above, even a tiny sliver of root is enough for the plant to re-grow) and can take several years. It may also be unpractical given you have established plants in the near vicinity.

If chemical control is an option you wish to consider the good news is it will work but again, is a long term affair. Applying glyphosate in late summer is effective when the weed is actively growing and bruising or damaging the plant (by trampling growth for example) before spraying often helps herbicide penetration, and ensure that any re-growth is treated promptly.   In your situation with the weed popping up between established plants, the suggestion of using a mulch cover is probably worth considering. I’d advise you use the Lightweight Porous Ground Cover as this material is designed for planting through and will allow water to pass through easily. The other cover up for consideration is the Woven Heavy-Duty Ground Cover which is used extensively in garden centres and nurseries to create plant standing areas; this material is not designed to be cut or planted through however, and its random permeability can lead to completely dry or waterlogged soil beneath!

Finally, I would suggest you cover the material you decide upon with between 5 and 10cm of soil or mulch after planting; this will help keep the cover in place, prevent drying out, retain moisture and suppress the weeds below.   

I do hope I’ve managed to be of help; obviously, trying to eradicate this pernicious weed is a likely to be a protracted task but worth it in the long term - many thanks once again for your enquiry and please do not hesitate to contact me with any other questions or queries you may have.