It is important that seed be dry before storage. By this we mean not just touch dry on the outside but that the internal moisture content is also low.
Why is this important?
There are three main reasons we care about seed being properly dry:
1. Viable storage period
2. Mould damage
3. Freezing for pest control
The time a seed can be kept in storage is affected by its internal moisture level. The more moisture it is holding the shorter the time it will remain viable in storage. Seeds with higher moisture respire at a higher rate and use up more of their stored energy and so age more rapidly in storage. The dryer the seed the longer you can store it.
Seeds can easily be damaged by mould if they are not dry enough before storage. This is more common if you are storing in airtight containers, which you may want to do as it also extends the storage time.
If the seed is being frozen for pest control or for storage, internal moisture can damage the seeds. Water expands when it is frozen and if that expansion occurs inside the seed then the damage can affect either the ability to germinate or the vigour of the plant.
How do we tell if the internal moisture level of the seeds is low enough?
With peas and beans it is relatively easy to check whether they are dry enough by using the hammer test. Put a sample seed on a hard surface and hit it firmly with a hammer. If it squashes at all then the internal moisture is still too high for storage. If it shatters into little bits then it is dry enough for storage.
Seeds of curcurbita, such as pumpkins and cucumbers, should snap cleanly when bent rather than fold or tear.
Another method is to put a sample of the seeds into a jar and place the jar in the sun for 30 minutes. The heating of the air inside the jar will cause condensation to form on the inside of the jar if the seeds contain moisture. Note that the temperature in the jar may be enough to damage the seeds so don’t put all your seeds in the sun – just a sample.
We don’t have a direct test that is easy to do at home and that doesn’t potentially destroy the seeds. But we can pay attention to the temperature and humidity that they are being dried at which can give us a good idea and we can also use hygrometers or humidity indicator cards as aids.
A note about Relative Humidity
It helps to understand that the moisture content of the air is determined by temperature as well as relative humidity. Air at 25ºC and 50% relative humidity is holding twice the moisture as air at 15ºC and 50% relative humidity.
In late summer and autumn, when we are typically processing and drying seed, our local daytime temperatures are usually at least 25ºC and relative humidity above 60% is quite common. This can make drying seed a challenge. For seed savers north of Sydney on the east coast their humidity levels can be a challenge for most of the year.
What can I do to improve drying?
Fans can be used to keep air moving over the seeds to help wick moisture away. Constantly moving air will aid in drying seed as long as the level of moisture in the air is lower than the moisture of the seed.
Some air-conditioners will lower the humidity of the room – but not evaporative coolers, which will make it worse. In very humid climates an air-conditioner might not lower the humidity in a room sufficiently for good seed drying as that isn’t its primary function, but it can certainly help.
Food dehydrators should only be used if they can be set to below 35ºC as temperatures above this can damage seeds. Dehydrators act by keeping the warmer air moving. Warm air can hold more moisture and will be better at wicking moisture away from the seeds as it moves past.
Dehumidifier units can be used to lower the humidity of the room the seeds are being dried in. How low dehumidifiers can get the relative humidity of a space is dependent on the specific unit. Many will not get lower than about 40%. But remember cold rooms at a particular relative humidity have less moisture in the air than warm rooms at the same relative humidity. Cool and dry is the ideal.
Some other techniques for reducing the moisture in your seeds are to use moisture absorbers like silica gel or bentonite clay or even oven dried rice.
You might be tempted to reuse the little moisture absorption packs that are packaged with other products you buy. But if they have been sitting around for a while they may have absorbed their full quota of moisture from the air already and do you no good unless they can be reactivated. Since it is hard to know if they are of a type that can be reactivated it is simpler to just either purchase reusable silica gel or use an appropriate kitty litter product from the supermarket. Just look for a kitty litter that is only silica gel or only bentonite clay. There are a number of products including fairly cheap generic brands.
Both the silica gel and the bentonite clay can be reactivated in the oven by spreading thinly on a tray and cooking at 120oC for 1 to 5 hours.
Rice is also often suggested as a moisture absorber. It isn’t as efficient as silica gel or bentonite clay, and it will also need to be oven dried before use each time. Rice can’t be reused indefinitely. After a few trips through the oven its moisture absorbing capacity will be degraded and it will need to be replaced with new rice.
It can be helpful to use an indicating silica gel so you can clearly see when it needs re-drying. It can also be useful to use humidity indicator cards which will give you an idea of the relative humidity inside your seed jars.
To dry the seeds put them in a sealed container along with moisture absorber (silica gel or bentonite or rice) and an indicator card if you are using them. Leave for at least a week to equalise the moisture between the seeds and the absorbent material. If you are using rice, have about twice the quantity of rice to seed. For silica gel or bentonite about equal quantities of seed and moisture absorber should work. Having proportionally more moisture absorber won’t hurt.
If the humidity indicator card in the jar shows the seed is dry enough, the seeds can be removed from the moisture absorber and put into storage. Otherwise the moisture absorber can be changed out for a fresh lot to continue the drying process.
A number of these methods might be used on any one batch of seed. For example a fan might be used for initial drying and then the seed placed in a jar with absorbent for final drying before storage.
What should I do if I can’t tell whether the seed is dry enough?
If you are in lower humidity areas (i.e. south of Sydney) you can store your seed in a way that does not hold moisture in. Store your seed in paper bags and envelopes. The downside of this is that you are not slowing down respiration of the seeds and they won’t last as long as if you had them in airtight storage. You may also have pest problems with insects or rodents eating your seeds. But for home gardeners in lower humidity climates just storing seed for the following season, a paper bag in a cool, dark location will probably be fine.
The more humid your climate is the more you will need to consider using the methods we’ve discussed to get your seed really dry, especially if you wish to be able to store your seed for a few season, keep pests out of your seed and have little loss of viability.
Contributors: Liz Worth, Julie Davies