Have you got any small plants that fall to pieces at ground level?
Whole stems get detached from the ground.
This is typically on soft stemmed plants.
This plant problem could be due to:
- An attack by a fungal pathogen, such as Rhizoctonia, Pythium, Phytophthora, etc. Most, or all, of the roots will be brown or black, and dead – not white or fresh looking. 
- An attack by one or more pests eating the roots away, such as Leatherjackets, Cutworms  or Vine Weevil.
When I saw the damage done to the Sedum in the picture above, I immediately thought it was Vine Weevil damage.
But after careful inspection of carefully pulling the compost to pieces I couldn’t find an offending Vine Weevil larvae anywhere.
It being winter time, the almost dormant Vine Weevil larvae would have been just lying, buried in the compost, but my conclusion is that the Vine Weevil biological predator (see at the bottom of the page for purchasing this) had found it before I did!
I did want to include a picture of one of the Vine Weevil larvae, but I can’t find any!
You can see on the picture that a couple of the stems have begun to root again into the compost.
The problem with Vine Weevil is that you can have a plant in your garden, or a neighbours, that is a host plant and from there the weevils wander off to lays eggs on your choice, smaller, softer plants.
The least obvious host plants for the adult weevils can be:
- evergreen Euonymus
They chew out notches in the leaf margins.
I have heard that some people go out when it is dark with a torch and some pliers to crush the offending adult weevils!
The larvae stage is the most destructive, and they look like little white bananas about 5mm long.
The most tempting plants for the Vine Weevil grubs are:
- Cacti and succulents
- other house plants
- Saxifraga (mossy types)
- young Yew (Taxus)
If you are interested in the Vine Weevil predator (which I highly recommend) it can be purchased from Amazon – the link* below will take you to the correct page:
Nemasys vine weevil killer standard pack
This is not a chemical but a live product that needs to be used right away.
It is mixed with water and then sprinkled on the ground around your plants with a watering can.
It is not a chemical and is harmless to children, pets and adults!