Hello from Zimbabwe. I’m building several seed banks in very hot areas for storing indigenous tree seeds. I’m not a structural engineer, so have some (probably basic!) questions regarding humidity. I have added a cavity wall into the design, faced it away from the sun, wide eaves to limit sun also, etc, so hopefully the temperatures will be lower and not fluctuating too much. There is also damp-proof course in the design. My question is how many vents to add. If I add vents to add airflow to decrease RH, then it will draw in the hot air from outside, increasing the temperature… How to decrease humidity without increasing the temperature? There is budget for a small solar system enough for lights, maybe a small fan, but not an air-conditioning unit nor dehumidifier. The seeds are being dried, then stored in sealed buckets (paint buckets essentially), so hopefully the humidity isn’t too much of an issue, but I’d still like to get it clearer in my mind. Thanks in advance, Beth.
Hello Beth. So glad to hear of your efforts to establish seed banks for storing indigenous tree seeds. Interesting question about decreasing relative humidity, with passive ventilation, without increasing temperature. I am not sure about the number of vents but will share a few thoughts drawn mostly from our experience with earthbag structures for storing seeds (see ECHO Technical Note no. 96). A primary goal of an earthbag structure or something similar (e.g., cisterns, underground storage) is to stabilize temperature (see ECHO Asia Note no. 38 for some data). Humidity in these structures is more difficult to control and can be high at times. Hermetic/airtight sealing is probably the best way to address humidity in situations where there is no electricity for dehumidiers. So, I think you’re on the right track with using sealed buckets. Hermetic storage only works if the seeds being stored are dry to begin with, so you don’t end up trapping moisture in the container. Techniques to consider in combination with sealed containers are:
-
Filling the containers full. This minimizes air/oxygen in the container, reducing activity of any weevils present. See an ECHO Research Note called Fill it Full.
-
Placing desiccant material in the container with the seeds. One approach is to use oven-dried rice. Dry the rice at about 135 degrees C for about an hour or until the kernels slightly darken or stop losing weight, indicating all the moisture has been driven out. Place the dried rice in the bottom of your storage container, let it cool for a bit, wipe off any condensation from the sides of the container with a cloth, place seeds in the container (with perhaps a cloth or something on top of the rice to keep seeds from mixing with the rice), and close/seal the container. When you open the container to access the seeds, minimize the time that the container is left open so that the seeds don’t rehydrate. Replenish the desiccant as needed.
-
Vacuum-seal the seed storage container. There are some inexpensive, low-tech ways to do this, including modified bicycle pumps, brake bleeder pumps, and even syringes. See ECHO Technical Note no. 93 for more info.
With stabilized temps and airtight containers you would not necessarily need vents. On the other hand, lower humidity in the storage structure is certainly advantageous. Here are a few general thoughts that come to mind:
-
You’ll want to be aware of the humidity of the outdoor air being drawn in. Outdoor humidity fluctuates during a 24-hour period. Looking at weather station data here in Florida for instance, humidity often exceeds 90% during the night, peaking during early morning hours, and declining to 60% or less in the afternoon. A vent that can be opened and closed would allow for closing the vents at night to avoid incoming humid air. There might be seasons when outdoor humidity stays low, in which case this would be less of an issue. I can see where opening and closing vents is a hassle and adds complexity to the design. With passive ventilation, with vents that stay open, you might see lower overall humidity but with fluctuation that reflects outdoor conditions.
-
Consider placement of vents. Earthbag seed banks will typically have openings in the roof or a gap between the top of the wall and the roof for warm, humid air to escape, taking advantage of the fact that warm air rises. The temperature data recorded by ECHO in Asia (see earlier link for Asia Note no. 38) suggest that, even with some air movement/ventilation, temperatures inside were quite stable. Wide eves and shade trees around the storage structure would help ensure that air coming in from the outside is as cool as possible. One idea that comes to mind is to create and burry a U-shape vent with PVC, with the top of one side of the “U” outside the wall and under an eve (to keep out rainfall) and the top of the other side of the “U” inside the storage space. Air entering the pipe from the outside, would perhaps be cooled to an extent by passing below ground before entering the structure. Could be something to experiment with.
-
Consider protecting vents. Use of chicken wire or screen of some sort could be important for keeping out rodents.
The last thing I would say is that there are some pretty inexpensive hygrometers that will allow you to monitor humidity. A search for “button hygrometer” on Amazon led me to an Amazon page listing a 2-pack mini hygrometer for $7.97 that shows both humidity and temperature. Hopefully this gives you a few things to think about. Others may have added perspective.
Tim
Hi Tim, thank you SO much for your detailed and informative reply, I really appreciate the time taken, the wise advice, and the practical experience underpinning it! So thank you.
Good thoughts, and yes, I agree the main thing in our control re. humidity is storing the dried seeds in a well-sealed container, so I’ll concentrate more on keeping temperatures low and stable. We haven’t tried adding the dessicants, so will do. Our seed dryer is based on the Thailand solar dryer design, and is working well, although we need to upscale it. We also haven’t tried using a vacuum pump, but do keep the buckets full. We use ash to surround the seeds and limit air-gaps, as well as lighting a match prior to closing the lid. Not sure how effective that method is in comparison, but I can do some trials maybe.
If interested, photos attached of our current seed bank design (minus some new changes), our storage buckets in action, and our seed dryer. There’s also a small video of our seed storage here: https://www.youtube.com/watch?v=9ldDbx_3WMI That was our first prototype, but we’re trying to improve for the next ones.
I have a hygrometer donated by our technical partners at US Forestry Service, so I will monitor the RH once the seed banks have been built.
Thanks again, Beth.
For storing orthodox seeds in dry conditions you can see some more information on desiccants in ECHO technical note 63. We use silica gel in our seed science lab, but may be expensive on a large scale. Look for desiccants available to you - bentonite desiccant clay, cat litter, etc. The ash you are using will also have some desiccating effect. Lighting a match is an interesting idea as minimizing oxygen in the container slows seed aging and combustion could help reduce oxygen but I’m not sure how that would compare with a low-tech vacuum system.
Thanks for that. I’ll need to do some trials 
Rice useful as a dessicant also. Cheers.
I would like to second what everyone here is said. More on that below. But if you are able to purchase plaster of paris, wallboard, or another source of gypsum of the type used in Western construction if you go to the city, it is worth noting that this makes a fairly good desiccant that can be made (and later reused) by heating it to at least 400 degrees F for an hour, as described on page 40 or 41 here: https://info.ornl.gov/sites/publications/Files/Pub57069.pdf
I will add that airtight containers are very important for another reason - they are effective in limiting insect damage. Some insects can live and reproduce on even very dry seeds without water by metabolizing the sugars and starches and producing water as a by product, but they cannot do so without oxygen. So filling it full, eliminating as much oxygen as possible, and sealing it tightly are all great. Lighting a match may also provide a bit of sulfur dioxide to discourage rot, similar to its use in preserving dried fruit.