how to dry and store seed

In our damp UK climate, drying seed is one of the greatest challenges we face as seed growers. The ideal conditions for storing seed are cold and dry. That’s all you really need to know about the best conditions for storing seeds! However, if you are interested in why this is the case we will have to delve into a little plant biology.

Although in many ways dormant, seeds are tiny living beings and are sensitive to fluctuations in ambient humidity and temperature. As levels of humidity and temperature increase so does the metabolic activity inside the seeds, which means that they are using up their precious energy stores at a faster rate. These stores of energy are what the seeds depend on once they have germinated – but before they have leaves to harness energy from sunshine – so if they are depleted, the vitality of the seeds will be compromised. If we can get the temperature and humidity down to low enough levels then the seeds very nearly stop metabolising and can therefore be stored for many decades without losing their vigour. The seeds stored at globally important seed banks such as the Millennium Seed Bank and the Svalbard Global Seed Vault are dried to the point of dormancy and then stored in sealed containers at temperatures as low as -18˚C.

In the domestic setting we have a few choices for storing our seeds depending on how long we want our seeds to last for. The three options we would suggest are:

  1. Indoors at ambient humidity and temperature in breathable bags e.g. paper or cloth. This method is totally fine if you just need to store your seeds for a year or two. However, we would not recommend this method if your house is particularly damp – if you have a problem with mould in the corners of your rooms then this method is not ideal.
  2. Dried fully and stored in airtight containers (e.g. jars or zip-lock plastic bags) at room temperature. Seeds stored in this way will keep for a long time. More than 5 years for many species. This method might be useful if you have a large batch of seed that will take you a few years to get through.
  3. Dried, sealed and stored in the freezer. This method should preserve your seeds for 20+ years.

For the last two methods outlined above the seeds must be very, very dry. This can be achieved through various means which we will come to now.

Drying with rice

As rice is an absorbent material (it’s a seed too!) it is able to pull moisture out of the air. On a small scale rice can be used to dry seeds as follows:

  • Measure out 5 times the weight of rice to seeds
  • Bake the rice in the oven to dry it out completely
  • Place rice and seeds together in an airtight jar with some means to separate them (e.g. put seed in a paper bag)
  • Leave for 2 weeks
  • Remove seeds and store in an airtight container

Drying with silica gel

On a small-medium scale seed can be dried using silica gel. This is the stuff that comes in little sachets inside new shoes and some other goods. Silica gel is a form of silicon dioxide which is highly moisture absorbent. It is available to buy loose or in sachets of various weights and comes in the form of beads (not really gel as the name suggests). You can dry seeds with silica gel as follows:

  • Measure out an equal weight of silica gel to seeds
  • Place silica gel and seeds together in an airtight jar with some means to separate them (e.g. put seed in a paper bag)
  • Leave for 1 week
  • Remove seeds and store in an airtight container
  • B. as the silica gel will absorb water from the seeds, it will need to be dried out again at some point. If your seeds are not that wet in the first place then you might be able to dry a few batches before it needs dehydrating. You can tell how much water the gel has absorbed by weighing it before and after use. It is also possible to buy silica gel which changes colour as it absorbs moisture, but make sure you do not buy the stuff which goes from blue to orange as the pigment is highly toxic. Orange-to-green, or orange-to-clear are safer.

Dehumidifier method

Seeds can also be effectively dried using a dehumidifier in a small air tight space. You can make a sealed space within a room by using plastic sheet, or you can use a small room. Seeds should be laid out in thin layers on mesh racking and placed in the space with the dehumidifier turned onto max power. How long you need to leave on the dehumidifier depends on many variables such as its power, the size of the room, the quantity of seeds to dry, and the moisture content of the seeds to begin with.