That is not drainage, but it should hopefully allow enough oxygen to the roots or they will rot and you will see the kind of stuff you are experiencing.
Earthbox is a fine way to grow but it has a few minor issues that can mess up your plants. Also, marijuana is what I call a 'dry' plant. If its too wet, they don't like it.
If you are worried about overwatering, then that is very easily fixed.
Basic Watering
Discuss different watering methods for growing your own weed from seeds. Our community members give helpful tips on how pot size and material, as well as environmental factors affect growing cannabis.
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Watering SOIL
To know if you need to water your cannabis plant get yourself a soil meter. Poke or drill 4 holes in your pot 1/4 way up from the bottom spaced around the pot...
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