I'm storing spices and dried herbs, as I always did, in glass containers into a spice only-drawer. It always worked fine. But looks that this house is particularly humid, because ground spices, and even seeds like fennel, started to become moldy, and they're not THAT old... I don't always have access to new spices, so I must buy them all around once in a year. Is there amy storage tip for avoiding mold?
Hello from the humid Philippines! I'll answer regarding the amount you keep in the jar out of storage.
I completely agree with the idea of vacuum-sealing any excess spice. That's what I do, because I can only buy certain spices (brown cardamom, sumac, etc) in bulk. I use a desiccant in my spice jars, which have rubber gaskets for an air-tight seal.
If you go to your local hardware store, you should be able to find screen material made out of plastic. Cut some to fit the bottom of your spice jar exactly, put some sushi rice (basmati also works if you can't find the really fat variety) and then put the screen on top of it. Then, put in your spice of choice.
The rice is enough to draw out any humidity you introduce into the jar each time you open it, so your spices stay fresh and are much less likely to clump together.
If you can't find non-reactive screen material, some cheesecloth will work. You can also just mix the rice into the spice (not recommended as rice is starchy, and the spice will clump around the grains as they absorb moisture).
Have you considered vacuum sealing? As you are buying larger amounts of spices to use for a whole year, here's what I would recommend.
Obtain some vacuum seal containers sized for the amount of each spice you are going to purchase. Get some smaller glass bottles for your daily use.
When you purchase your spices and get them home, fill the small bottles with enough of each spice to last 2 or 3 weeks.
Then vacuum seal the remainder of each spice in the larger containers.
As you use up the spices in the small bottles, you can refill them from the sealed containers and then reseal them.
Firstly, if you have any mold on your spices, then throw them out, do not risk it. When you say that you buy them at the same time of the year, I am assuming that you are buying quite quantities. Divide your spices into manageable quantities, then place into sealed containers, it might help to also go around the lid of the container with some tape to really keep them airtight. This works for me living in a humid environment. My curry powder is still great after more than a year using this method.
I worry about the rice method, as the rice will absorb the moisture and then become prone to mold in itself. It is possible to buy cheap humidity absorbers from most supermarkets, place one in the area where your spices are stored. Good luck...
I think the vacuum advice is excellent. If you can't do that or if you want to protect spices that you currently use, get some silica gel packets and add them to the spices.
First, I would suggest moving any spices that still seem good (that don't look like there's mold, and don't smell off, or anything like that), and any new spices you buy, to the fridge or even to the freezer for a bit - that should let them store safely for a while, so you don't get new spices growing mold.
Next, you should probably clean out the drawer pretty thoroughly - humidity is a problem, but the presence of mold spores are also a prerequisite for your molding spices. Having something moldy around makes it more likely other spices will contact the mold, from the spores getting on hands, or on jars, and getting inside as they are opened and handled. You might even swap your spice drawer out for a different drawer's contents, store something the mold spores won't effect in that drawer, if you're not sure of your cleaning.
For your new, or newly cleaned, spice drawer, you want to make sure it is as dry as possible, especially after cleaning, perhaps line it with paper or something if you think the mold might have settled into corners or edges. And make sure any jars and/or spices you add back are freshly clean (or new). You might, as MirekE suggests, add a handful of desiccant packets to the drawer to keep it from getting too humid in the future (maybe keep an eye out for the desiccants packets that sometimes show up in other things to keep in the drawer, if you don't want to keep buying fresh). Or occasionally aim a fan at the open drawer, so that air circulation can maybe dry things out.
If you keep checking and keep the drawer clean and dry, you might prevent the mold from re-infecting the drawer (and spices), or at least catch it early enough not to lose all your spices.