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. 

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.  

When setting up a moisture dependent tarantula, always consider substrate depth.

When keeping moisture dependent species, the depth of substrate is also important. The deeper the substrate, the easier it is to keep the enclosure from drying out too quickly. For fossorial, or burrowing, species, this is a no brainer as the amount of dirt needed to permit burrowing is usually substantial enough to trap moisture for quite a while. However, substrate depth should be a consideration when setting up moisture dependent terrestrial and arboreal tarantulas as well. Even if your spider isn’t going to burrow, offering several inches of substrate will make it much easier for the keeper to keep the lower layers damp without having to constantly add water.

What type of substrate should you use for a moisture dependent species?

The substrate you choose to fill your enclosure can determine how often you have to add water as well as how difficult it will be to rehydrate the soil once it dries out a bit. Although there are several types of substrate that work well for tarantulas, they all have unique properties that impact their ability to absorb and retain water. Here are some of the more popular ones as well as some notes as to what to expect with each. 

Coco fiber/coir is probably the most used substrate in the hobby, but some of its characteristics can make it problematic for use with moisture dependent species. Coco fiber generally absorbs water like a sponge, which makes it VERY easy to moisten down after it has dried out. Unfortunately, it also seems to give up moisture much more easily than some of the other alternatives. It’s light, fluffy nature can also make it less than ideal for burrowers. I used coco fiber for years, but transitioned away from it when I started keeping moisture dependent species. Although it has its negative aspects, it can still be used by keepers who are willing to be a bit more attentive to moisture levels. 

Topsoil is inexpensive and works great as a substrate, and I’ve found that it will hold onto water quite well. Unfortunately, after it dries out, it can be very difficult to rehydrate. Some complain about water pooling on the surface, creating mud puddles and not percolating down to the lower levels. Those who use topsoil for substrate will often add in sand, vermiculite, or chopped up sphagnum moss to improve soil irrigation. 

Peat is also a popular substrate choice, and it can work very well for moisture dependent species. Once hydrated, peat holds onto water very well, which means that it takes much longer for it to dry out. Peat is also acidic, which can supposedly help in the reduction of mold. Unfortunately, much like topsoil, it can be difficult to rehydrate once it has dried out. Again, keepers will often add sand, vermiculite, or chopped up sphagnum to increase water percolation. Peat is also very dusty when dry, which can provide for extra mess.

Bioactive Substrates are another, albeit more pricey, alternative. Bioactive substrates are usually specially formulated for moist environments and contain carefully selected additives like charcoal, sphagnum moss, mulch, and sand to help with water acquisition and retention. Many of these substrates are bagged slightly damp, which can also be convenient. On the down side, bioactive substrate can be quite expensive, with 4-6 quart bags usually selling for around $10. I currently use the Bio Dude substrate for many of my fossorial species, and it works great. 

Now, it should be mentioned that you can use any of these without adding vermiculite or sphagnum moss, and many folks do just that. Personally speaking, I’ve used vermiculite as a soil additive for years, and I find that it has many benefits when working with moisture dependent species.  I’ve found that water will seep into topsoil better when it has been mixed with vermiculite (preventing the “mudding” effect). Also, strategic use of vermiculite in different concentrations can be useful for determining where the moisture you add ends up. For example, I will often put a ¾ to 1” (1.9-2.5 cm) layer of moist vermiculite down first when setting up an enclosure before filling the rest with my substrate. When I pour water down the sides of the enclosure, it soaks down into this layer of vermiculite, keeping that lowest layer nice and moist. 

Water dishes are important as well.

If space permits (it can be difficult fitting them into dram bottles), always include water dishes in your tarantula enclosures. Not only will many tarantulas drink right out of them, but they assist in keeping the inside of the enclosures from drying out too quickly. In some instances, I’ve even used more than one water dish in an enclosure. This can be particularly useful in larger enclosures with more shallow substrate. Many folks also choose to surround their dishes with moist moss to create a little bastion of moisture on the surface.

What about slings?

As a general rule of thumb, all slings should be kept on moist substrate. Like with the adults, the substrate can dry out on the top as long as the lower layers remain moist. Most species of slings will do some burrowing, and that allows them to burrow to the moisture level that they need. As slings lack the waxy coating of their more mature counterparts that helps to trap moisture in their bodies, they are much more prone to dehydration. 

Now, with slings being kept in much smaller enclosures, it can be a bit tricky trying to add moisture without accidentally overdoing it or even flooding the poor creature’s den. I find the easiest way to add moisture to the small enclosures is by using a pipette or syringe. Both are wonderful tools, as they will allow you to carefully direct water into the substrate while controlling how much you add. Just locate the spider’s den, then use the tapered end of the pipette or syringe to carefully direct the water into the lower layer of substrate on the other side. 

Sphagnum moss can also be used to help keep your slings hydrated. Adding a pinch of sphagnum on the surface (I prefer New Zealand sphagnum) can add a little oasis of moisture on the surface. Because the moss acts like a sponge, it’s easily moistened and can provide a damp refuge and a place to drink for your tiny spider.

Do NOT restrict ventilation

In a lot of older tarantula husbandry literature, authors will recommend restricting or covering up ventilation in order to maintain higher “humidity” in the enclosures. Years of tarantula keeping experience has taught us that this is actually a terrible idea. Moist enclosures need good ventilation, preferably cross-ventilation, to ensure good airflow and to prevent stuffy, potentially deadly enclosures. Stagnant, fetid enclosures can kill spiders just as quickly as dry ones. Always make sure to properly ventilate your enclosures. Many will argue that you can never have too much ventilation, as the only downside is that you’ll have to be a bit more diligent in making sure things don’t dry out too quickly. 

What does it mean when husbandry notes tell you to maintain a “moist corner” for your spider?

This one tends to cause a lot of stress, as a “moist corner” could be interpreted many different ways. I tend to recommend this for spiders that may still crave a bit of moisture in their habitat even after they hit the adult stage. The fact is, there are some species where some of the specimens outgrow the need for moisture and show no preference for it while other specimens still seem to appreciate some dampness. In these instances, I’ll use the rain shower technique to moisten down the substrate in one corner of the habitat to keep approximately one quarter of the dirt moist. Once that corner begins to dry out, I will moisten a different corner and allow the previous one to dry out. This helps minimize any issues with mold.

Now, if you see your tarantula hanging over this moist area or even the water dish, conditions are too dry. You will need to moisten down a good portion of the substrate to make sure that your pet has the moisture it needs.

So, what if I house my moisture dependent terrestrial on a couple inches of coco fiber and use only misting for soil hydration? Is this wrong?

Short answer? No. The above scenario would represent a hypothetical “worst case scenario” for a moisture dependent species, as this individual would be using a thin layer of fast-drying substrate with only misting as a means of rehydration. If the cage were properly ventilated, this enclosure would dry out very quickly. The keeper in this scenario would have to be extra diligent to ensure that the enclosure doesn’t dry out too quickly. This means more work for the keeper and, more importantly, more disruption and potential for sub-par conditions for the spider. 

Misting isn’t wrong; it just isn’t the best or most practical way to hydrate your substrate. I use misting in my collection, but it’s meant to supplement adding water through a simulated rain shower. I’ve found that some of my arboreal species will appreciate a good soft misting here and there, and many will come out after the lights go out and drink off the glass.

That said, would it be advisable to use misting to keep a C. lividum or T. blondi adequately hydrated? Absolutely not. 

I live in an area that has high relative humidity. Could adding too much water to the substrate be overkill?

Absolutely. It’s important for a keeper to take into account his or her local weather when deciding how much water to add and how often. For example, in my state it tends to get very humid in the summer. As my tarantula room is not air conditioned, there is already quite a bit of moisture in the air during the warmer months. During this time, I reduce how moist I keep my substrate and how often I rehydrate it. 

Folks from humid countries, like the Philippines, often experience this scenario year round. In these locales, keepers often have the perfect natural climate for some of the tropical and moisture-dependent species. In these instances, it’s important to take this into consideration when moistening your substrate. Most keepers will find that a damp corner and a water dish will do wonders, and the need to remoisten should be quite minimal. In these situations, good ventilation is paramount to ensure good airflow and to prevent stagnation inside the enclosure. 

As with all things, practice makes perfect! 

Making the move to keeping moisture dependent species, like any new experience, will likely cause some trepidation and worry. However, it really isn’t that difficult to keep these amazing species, and most keepers will quickly discover that checking levels and “making it rain” will become second nature and even a fun part of the hobby.  

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