Drying racks are provided in Terraformation seed banks, and this is also something you could easily set up yourself. Any kind of rack system or shelving that allows for shallow trays with good airflow could be used for this purpose. To use the racks, simply spread the seeds out in a single layer in one or more trays, with only one seed accession per tray. It is critical that seeds are in a single layer, so that all the seeds dry evenly. Otherwise, seeds underneath other seeds will retain more moisture. For a Terraformation seed bank kept at 35%–40% relative humidity (RH), these racks can be used to dry seeds for frozen storage.
Electronic drying cabinets (which may also be called dry cabinets, electronic humidity control chambers, desiccation chambers, or similar names) allow a user to control the internal RH. Terraformation seed banks are equipped with four drying cabinets. These are relatively inexpensive and sufficient for restoration seed banking. We recommend setting three cabinets to 30% RH and the last one to 40% RH. You can use the cabinets at 30% to dry seeds for refrigerated storage at 5°C, and the cabinet at 40% to dry seeds for frozen storage at –20°C. Of course, you can also adjust the cabinets for your seed bank’s needs. As with the drying racks, spread the seeds out in a tray appropriate to the size of the seed accession.
One other useful option for seeds that will be refrigerated is to dry them on drying racks for a while, then move them to the drying cabinets to reach 30% RH.
Seeds should be allowed to dry for roughly a month (although this time varies by species), to make sure all the internal parts of the seeds will be dried homogeneously. Then, prior to packaging and storage, equilibrium RH (eRH) should be checked using a hygrometer.
Using desiccants
In addition to drying tools provided in Terraformation seed banks, there are also a variety of non-electronic dry boxes, often made of clear acrylic material for viewing internal contents, with hermetically sealing doors. This can work quite well as a low-tech option with no power required for the drying unit itself. However, these dry boxes will require some type of desiccant inside in order to dry the seeds, and the desiccant will also need to be recharged (redried to the original low humidity), which will require some use of power.
Some dry boxes have corresponding silica gel desiccant cartridges, in which case it is good to have several of these cartridges on hand, because it is likely that seed drying will require frequent cycling/recharging. You would also need to have an oven available to recharge the cartridges using low heat.
As a cheaper alternative, you may be able to purchase silica gel beads in bulk (not in a cartridge) and simply place them in a tray inside the dry box to do the desiccation work. They will still need to be regularly recharged by spreading them out on a tray in an oven on low heat.
It is important to use the right amount of silica gel to dry your seeds. If too much silica gel is used, seeds would dry too quickly and too much, causing damage to the seeds or eventually killing them. It is advisable to use a proportion in weight of seeds to silica gel of 1:1 to start with, and then replace these with new beads in a 1:4 ratio when the silica needs to be recharged.
Important note on silica gel: Color-indicating silica gel beads are important, so that you can visually tell when the silica gel has become saturated with moisture and is no longer able to dry the air and the seeds inside the box. However, we recommend not to use blue-to-pink color indicator beads, because these contain a chemical that is a carcinogen if the beads are broken into particles that can be inhaled. Blue-to-pink beads can be OK if contained inside a cartridge, since this would avoid breakage, but we don’t recommend using them in the open, especially if they are handled over and over again in recharging. Instead, orange-to-green color-indicating silica gel beads are a safe alternative. They are recharged in the same way but do not contain hazardous chemicals. Since color-indicating silica gel is more expensive than regular white silica gel, one option is to buy both types, mixing in some color-indicating with the regular white silica gel (they should all need to be recharged around the same time).
In some places in the world, you can purchase mini-dehumidifier units that contain silica gel inside and can be recharged simply by plugging the unit into a standard electrical outlet. These are convenient, and a seed bank could purchase enough of them to keep recharging and cycling through their non-electronic dry boxes. With a larger box full of fresh seeds, they may need to be recharged every week or few days. However, drying seeds on drying racks first, then moving partially dried seeds to cabinets would reduce the frequency of recharging any of the options above (essentially using dry boxes for fine-tuning and for collections of very small seeds).
Other cheaper desiccants are charcoal and rice. They can be easily found in most places, but be aware that they do not have the same drying capacity as silica gel. If you need to use one of these options, make sure to recharge the materials often, and to use an excess of desiccants compared to the weight of your seeds.
Using ambient conditions
Drying orthodox seeds under ambient conditions (the natural temperature and humidity in the local environment) is a technique used when suitable drying equipment is not available. However, this technique is much less effective at preserving seed viability than using a climate-controlled seed bank. For the best possible results, drying under ambient conditions can be carried out with the help of climatic information and permanent monitoring. Orthodox seeds may not reach the optimal moisture content for storage when there is not accurate equipment; therefore, much of the effort and resources used for collecting seeds can be wasted if seeds lose viability and die. To dry orthodox seeds under ambient conditions, you need to spread them in a thin layer in a well-ventilated position, held above the ground under partial shade.
Recall that the relative humidity (RH) describes the amount of water vapor held in the air as a percentage of the total amount of water that could be held at that temperature. Therefore, it is necessary to estimate the environmental conditions in your area by reviewing meteorological records and identifying the ranges of temperature and RH values. Changes of RH can be observed during the day and at night (nighttime rise in RH as temperature falls). It is important that you know when the RH increases in order to protect your seeds, for example by locating them in closed areas during that period of time or storing them in sealed containers to prevent the absorption of moisture as the RH rises.
As a best practice, seed banks measure the eRH (equilibrium Relative Humidity) of seeds, or the RH of seeds in equilibrium with the air around them, to determine when seeds are dry enough to be placed into storage containers. Measuring eRH requires instruments such as hygrometers or sensors that can be placed inside a sealed container with dry seeds. If it is not possible to measure eRH of your seeds, ambient RH can be used — but with caution, as seed eRH values are likely to be higher. Assuming ambient temperatures during seed collection between 20 and 25°C, 50% maximum RH would be a measure of acceptable air humidity if the seeds are to be dried under ambient conditions.
Keep in mind that most recalcitrant seeds would still lose viability even if dried only to ambient conditions, as they are intolerant of rapid drying and extreme temperatures. Recalcitrant seeds should be stored in very humid conditions for the shortest possible time.