They were designed for people for whom space is at a premium and instead of a lid they have a planter so you can grow herbs or whatever you fancy on top. Not everyone is green fingered so there is the option of a wooden lid instead of a planter.
The 'urn' allows you to collect the liquid produced and use it as a nutritious plant feed, having an urn means these wormeries can be used inside as well as out.
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