← Species guide
New Zealand pygmyweed
High riskCrassula helmsii
A tiny aquarium plant that is one of Britain's most destructive pond invaders. A single fragment regrows into a new mat, and once established in a pond or ditch it is close to impossible to remove without draining.
How to identify it
- Tiny succulent carpet of pale green stems over mud and shallow water
- Needle-thin leaves in opposite pairs, 4–10mm
- Small white flowers on short stalks in summer
- Grows both submerged and as a dense mat over damp mud
Why it matters
- Forms solid mats that kill all submerged and marginal plants
- Chokes ponds and ditches, degrading habitat for amphibians and invertebrates
- Practically impossible to eradicate once established without major intervention
How to control it
- Check, clean, dry equipment and boots between water bodies
- Never release or tip pond or aquarium plants anywhere outdoors
- Small early patches: hand-pick every fragment, bag and dispose as controlled waste
- Established sites need specialist treatment — liner removal, draining or approved herbicide by professionals
