Keeping Moisture Dependent Tarantula Species (And How to Maintain Moist Substrate)

Author’s Note: Although tarantula husbandry “care sheets” often make a point of stating “ideal humidity” requirements, these numbers are arbitrary and should be ignored. There is no such thing as an ideal humidity level, and serious keepers abhor the “h” word. I’ve been keeping tarantulas for years, and I never monitor humidity inside the cages. Trying to maintain ridiculously high (and bogus) humidity levels in tarantula cages is a great way to end up with dank, stuffy enclosures and dead tarantulas. In the hobby, we don’t say that tarantulas need high humidity; instead, we say that they are “moisture dependent.” And, the key to keeping moisture dependent species healthy is moist substrate. For more on this topic, please read my article “Humidity, Temperatures, and Tarantulas.” 

Keeping moisture dependent species for the first time can be particularly stressful, as there really isn’t much out there explaining how exactly moist substrate works. Many care sheets will mention “misting” or “spraying” as a means to keep the moisture levels in enclosures up. Some will even go so far as to prescribe a certain schedule for misting the enclosure, like every other day or twice a week. Unfortunately, misting is often not the best and most practical way to get your spiders the moisture they will need.

Although misting can provide a burst of moisture in an otherwise dry enclosure, it does little to maintain higher moisture levels for the long term. Spraying or misting only soaks down the surface of the substrate and the decorations in most instances. This is a great way to give your spiders an alternative to the water dish for getting a drink, and many species, like Avicularia, will siphon moisture right off the glass and plants. However, for species that require constant dampness, misting and spraying falls short. The problem is, if an enclosure is properly ventilated, the surface moisture will quickly evaporate, leaving the substrate to dry out within hours. 

This can be even more of an issue during the cold winter months when your home’s heat is drying out the air and leeching moisture from the enclosures. A keeper using misting or spraying to keep the substrate damp will be spraying much more often. Also, keep in mind that spraying and misting can be stressful for the tarantulas, as the sudden burst of air and water vapor can startle your spider.

Forget Misting; Make it Rain!

For moisture dependent species, in most instances, spraying and misting alone is not going to be enough. With tarantulas that require dampness, you want to ensure that the lower layers of substrate, not the surface layer, remain moist. In fact, the surface can dry out a bit which, along with good ventilation, helps prevent mold from forming. 

An old juice bottle modified with some holes to be a watering bottle.

When adding moisture, I like to simulate a ground-soaking rain shower. Instead of spraying, I have a bottle with several small holes in the cap that I use like a garden watering can. I will aim the water at the edges of the substrate, where the dirt meets the side of the enclosure so that the water can seep down between the two and reach the bottom layers. If you’re having trouble getting the water to seep down through, you can always use a pencil to push some channels down between the substrate and side of the enclosure to allow the water to travel down through. Although some of the surface will obviously be damp when I’m done, the lower levels of substrate will also be rehydrated. This means even after the top levels dry up, the lower layers will retain that vital moisture.

How do I tell if my substrate needs more moisture?

This one is actually quite easy. Moist substrate will always have a darker hue to it than dry substrate. Once the surface of moist substrate starts to dry out a bit, it will be paler in comparison to the lower levels of moist substrate. When that darker band shrinks below the halfway point of the total level of substrate, it’s time to remoisten. 

Many folks new to keeping moisture dependent species will panic when they see the surface of the substrate drying out, prompting them to add more moisture too soon. This can lead to overly dank, stuffy conditions. Again, we do not need the surface to remain moist, and a drier top layer is no cause for alarm as long as the lower layers remain damp. One must always keep in mind that a tarantula looking for moisture can burrow down to find the level it needs. In the wild, tarantulas will use their burrows to escape extreme temperatures and conditions, including drought.   Continue reading →