Surface Mounting Blumat
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At first glance, these may seem expensive. If you're using them as intended though - in trays with capillary mats, they're totally worth it. This is a growing system popular in Europe that's only slowly making its way into the USA. These fit on top of a capillary mat with perforated black foil on top of that to prevent algae and other things from growing. The surface blumat turns water on and off that spreads moisture through the felt and soaks up from the bottom. It works great with fabric pots and plastic ones with a flat bottom and extra holes drilled in, but not plastic pots with a big lip - the mat needs to be touching the bottom of the pot. Good for pots up to 2 gallon and larger for plants with deep but not surface roots. In a 5 gallon pot, the moisture will only go up about half way, so with that size it's better to use a regular Blumat sensor. One of these plus distribution drippers will supply enough water for 5 square meters of tray. The electronic version can supply enough for thousands of plants as long as the surface is flat (use a level to make sure).