Since the collars are not deeply embedded in the soil (only a couple of inches/centimeters), they cannot protect the plants against slugs that live and move underground; for example, gray field slugs . Traditional slug pellets, which contained metaldehyde, have now been withdrawn by the Health and Safety Executive (HSE) for their damaging effects on birds and mammals. These edges are difficult to overcome for slugs, as they are too sharp for the animals to climb around the corner.

The bigger the bottle, the longer it can offer protection before becoming too little for the plants to grow inside.

I have found a DIY-guide on how to construct a slug-safe mini greenhouse with a simple means of production.

Regarding garden bells, on the other hand, the edge is not relevant because they normally stay closed. Slugs in gardens can cause heavy damage to leaves, resulting in large holes. These are irregular in shape, because slugs mouths are tongue-like with thousands of microscopic teeth. They do not eat in a typical bite-like way leaving circular holes like some caterpillars do. For this task, you need, for example, big old jars or large plastic/glass bottles. The bigger, the better.

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The product: Reusable physical barrier to prevent slug and snail damage. An internal diameter of 13cm makes them suitable for individual plants (e.g. Hostas), salads, vegetables and even any supporting canes. But the metal fences are relatively expensive and not affordable for some gardeners. Plastic fences, on the other hand, are often not effective enough.