Look for terracotta in-ground water jugs. You bury it beside the plant, fill it with water and the water leaches from the jug keeping the soil moist. You can also get a product called Soil Moist that expands and holds water up to 400x it's mass. They are a kind of crystalline product. If you go the Soil Moist route do a little research. There are two types - one leaches chemicals that will get into your plant. You want the other one...

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