Seed Saving Part 4: Storing And Testing

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June 1, 2020

This article was originally published in Garden Culture Magazine UK33 and US31.

Seeds, like all living things, have a lifespan. How long they last depends on factors including the seed variety and conditions during growth and processing. However, the way seeds are stored has an enormous impact on their vigor and longevity. Correctly preserving them will ensure your efforts to save seeds reaps long term rewards. 

Alive but Asleep! 

Inside every seed is a plant embryo surviving on a store of nutrients for the developing plant. Once the stored food is used up, the seed dies and won’t germinate. Jude Fanton, the author of The Seed Saver’s Handbook, explains: “In open-air conditions, the seeds will absorb moisture and the nutriment stored inside the seeds will start to react with the oxygen. With a little temperature rise, the seeds will release carbon dioxide and generate more heat. Soon, their respiration rate will rise to an unacceptable level for safe storage.” 

So, to extend the life of a seed, the rate at which the embryo uses its food needs to be slowed down, similar to pressing the pause button! When seeds are stored properly, they are effectively ‘sleeping’ or in a dormant state. They breathe or respirate very slowly but are still alive. It’s a balancing act between keeping the seeds inactive until you are ready to ‘wake’ them up and sow them. Do this by keeping the seeds dry in a consistently cool, dark place away from microbes, insects, and rodents. 

Storing seeds in clear glass jars on a well-lit shelf in a warm room will significantly reduce their life and vigor! After years of saving seeds and learning from my mistakes, I’ve realized careful planning is needed to optimize seed storage. 

How to Prepare Seeds for Storage 

  1. Remove Moisture 

Humidity or high moisture content is the enemy of seeds in storage. It’s vital to ensure the seeds themselves are very dry, and the container they are stored in has as much humidity removed as possible. Seeds can cope with temperature fluctuations much better with low moisture levels. For this reason, the majority of vegetable seeds should be stored at under 10% humidity and ideally at 5%. 

Moisture in the seed: Seeds that haven’t been dried before storage may attract fungi and bacteria. These microorganisms can cause mold or mildew to grow and eventual decomposition, destroying the seed and turning it into compost. Excess moisture in the seed can also start the germination process if the conditions are right. 

Vapour in the storage container: If there is excessive moisture inside a sealed container, the seeds may start to sweat and mold. Depending on the thickness of the seed coat and how quickly the seed absorbs moisture, they may begin to utilize their food store, risking germination. If using glass jars, one way to avoid surplus humidity is to minimize the air space between the seeds and the lid. 

  1. Keep Seeds Dry 

Silica sachets

One of the simplest ways to absorb any residual surplus moisture from dried seeds is by using silica gel. This is an absorbent granular compound that soaks up moisture from the air and gradually changes color to indicate when it is saturated. Silica gel comes in crystals or sachets that enclose the granules and can be used for seed saving purposes in two ways. 

i. Final Seed Drying:

Add equal quantities by weight of dry silica gel granules and seeds in a tightly sealed jar (e.g., 50gms of seeds to 50gms of silica crystals). The jar should be just big enough to fit the seeds and crystals with the minimum air space possible. Check the gel color daily for 2-3 days to see if it has changed color (e.g., from orange (dry) to green when saturated with water). If you have adequately dried your seeds already, you shouldn’t notice too much change. If the gel rapidly changes color, there is high moisture content. You may need to leave the seeds longer (up to a week). Avoid leaving them too long or the seeds can die. However, if the gel only changes slightly by the second or third day, the seeds are likely dry. Quickly remove them and store in their permanent container and location. 

ii. In the Storage Container:

I use the silica gel in dark or opaque storage containers when I pack my seeds. I add one or two silica gel sachets to each container. If you have the gel crystals, pour about 1cm/0.4” into the bottom of the jar or container and add a layer of cotton wool, then the seeds. This will keep moisture levels stable inside the container. 

Silica gel is also reusable. After it absorbs moisture surrounding the seeds, it changes color as it becomes saturated. The crystals can be dried out again and reactivated by heating the granules in a flat tray or baking dish in the oven at 115°C/240°F to 125°C/260°F. When the granules return to their original color, allow them to cool and store in a sealed container until you are ready to use again. 

The granules are available from chemists and online. The silica gel sachets can be repurposed from tablet bottles or new shoeboxes for small quantities of seeds. I reuse both the tablet bottles for storage and the sachets to keep my seeds dry. I’ve successfully stored my dried seeds with silica gel in the fridge, and they’ve retained good viability for years. 

If you don’t have access to the silica gel, try using very dry rice or grain. Bake some rice in the oven till it is bone dry and allow to cool in the oven. Then use in place of the crystals in the bottom of a jar or container. Alternately, powdered milk will do the job. 

  1. Minimize Light 

Because many seeds require light to germinate, they should be stored in a dark container or envelope inside a sealed container. Small cheap colored envelopes suitable for seed saving are readily available and can be used in conjunction with self-seal zip lock bags. 

  1. Keep Seeds Cool 

Warmth helps seeds germinate, so storing them in a cool place allows them to maintain dormancy. For most vegetable seeds, 5°C/40°F is an ideal temperature. If possible, for longer-term storage, a fridge is a perfect location. 

If you haven’t got space in a fridge or freezer, then don’t automatically choose your garden shed! Our homes and outdoor structures fluctuate from hot to cold throughout the seasons. So it’s vital to choose a spot where the temperature will likely be consistent all year. That may be a garage, basement, insulated polystyrene box, or other space on the coolest side of your house. 

  1. Prevent Insect Damage 

Seeds are a food source for insects, especially weevils! I’m used to seeing tiny insects in most seed heads when collecting and processing, so assume they are there. Even if you can’t see them, insect eggs may be present and will hatch when the temperature is favorable. 

There are three ways to prevent insects from destroying your seeds. 

  1. Freezing: By adding seeds to a sealed container or zip lock bag and freezing for two days, most insects and their eggs will be killed. Some weevils are pretty tough and may even survive the freeze, so you need a backup plan! 
  2. Desiccation: Food grade Diatomaceous Earth (D.E.), or natural amorphous silica, is a sedimentary rock made up of aquatic single-cell animal skeletons called diatoms. In a powdered form, D.E. can be used in small quantities to ensure no insects eat the seeds while in storage. It works by causing insects to dry out and die by absorbing the oils and fats from the insect’s exoskeleton. Its sharp edges are abrasive and accelerate the process. It’s economical, easy, and safe to use. Add a teaspoon of D.E. to a bowl with your seeds and stir until they are lightly coated in the fine powder. They are then ready to store. 
  3. Sealed Storage: Glass, metal, and plastic airtight containers are the final way to prevent insect damage. 

How to Label your Seeds 

Seed labeling

Know what you sow! Label the seeds you collect and re-label as you change containers. Basic information to include on the label is the variety name, original source, and the date collected. Record any information you noticed about the crop while it was growing (e.g., days to maturity; plant height and habit; fruit size, color and shape; flavor; yields (kgs); disease resistance or problems; and storage qualities). I note down any special observations such as ‘slow-to-bolt’, ‘drought-hardy’, or ‘sweet flavor’ because it helps me remember why I saved those particular seeds. 

Seed Containers and Storage Location 

seed saving

Make sure you pack your seeds on a dry day or at least in a cool indoor environment. A teaspoon and funnel make it easy to measure quantities and slide seeds easily into zip lock bags and narrow containers. 

I place small quantities and tiny seeds in little self-seal plastic bags with a silica gel sachet in each. As the bags are clear, they are then put into dark seed envelopes and labeled. These are then put in a box or larger zip lock bag. 

For larger quantities and bigger seeds like peas, corn, and beans, I use tablet containers and recycled screw-top jars with tight-fitting lids that are put inside a box to keep them in darkness. Metal boxes with sealed lids and airtight plastic containers are also suitable for seed storage. 

Use whatever you have easy access to and that will fit in your fridge, freezer, or cool location. Take into consideration if rodents are a likely threat and choose your container accordingly. You may need to protect your seeds in thick plastic, glass, or metal containers. 

If you keep your seed in the fridge or freezer, try to minimize how frequently you remove them to avoid temperature fluctuations reducing their longevity. When removing them, allow the containers to reach room temperature before opening to prevent condensation from forming inside them. Then get them back into storage as quickly as you can. 

How Long will Seeds Last in Storage? 

If stored correctly in cool, dry, dark conditions, most vegetable and flower seeds will last for three to five years. Some will stay viable for much longer. Generally, these include beans and peas because the seed coat is thick and the seeds themselves are larger. Smaller seeds tend to have a shorter life span. 

Not all seeds have a long life, even if stored in perfect conditions. Seeds from the Umbelliferae (carrot) and Allium (onion) families are relatively short-lived. These include parsnip, celery, parsley, dill, coriander, and fennel along with leeks, chives, onions, and shallots. These varieties may only have good germination and vigor for a year or two. It is especially important to keep short-lived seeds very cool and dry. 

Consistency is Key 

Author of Seed Sowing and Saving, Carole B Turner stresses the importance of maintaining consistent storage conditions. This was only something I discovered after years of storing seeds. I had failed to understand that even though I was storing my seeds in a cool, dark location in an insect and rodent safe container (in my fridge), I had not taken into consideration the impact that fluctuations in temperature and moisture had when I took seeds out to sow each season. Leaving the seeds out at room temperature, particularly in high humidity, while I sorted them or decided what to plant, was potentially causing the sleeping embryos inside the seeds to begin growing and then stop. 

Each disturbance initiated wake up signals which were suppressed again when I returned the seeds to the fridge. It was like flicking the switch on to grow and then turning it off again. So basically, the seeds began using up food that was meant to be saved for the germination process. I was accidentally weakening the seeds every time they came out of the fridge and sat around at room temperature for a few hours. 

Seeds may start to germinate at 20% moisture. In a humid climate like mine, that doesn’t take long to happen. Variations in hot or cold temperatures and light or darkness can also initiate germination. So to avoid this happening, I now keep a list of my seeds and organize them in the box in alphabetical order. When I am ready to sow, I can quickly grab what I need without potentially reducing the viability of the sleeping seeds. 

What if you decide not to store in a fridge? It doesn’t matter. The same principle applies. Decide on the best location for them and keep dormant seeds at that temperature consistently – for their entire lifetime. If you start storing them in a wooden box in a drawer in your garage, keep them there! Avoid getting seeds out when conditions are hotter, colder, or more humid and when you do, be quick about it! Remove as infrequently as possible. 

Germination Tests for Seed Viability 

No matter how well you store your seeds, the big question is: Are they still capable of germinating and growing into strong, healthy plants? There’s an easy way to test for vigor and seed quality and find out. Take a random sample of at least ten seeds or up to a hundred if you want greater accuracy. Lay a paper towel on a flat surface and spray with water until it is thoroughly damp but not dripping wet. Place seeds in rows (you may need several paper towels for larger numbers). Cover firmly with a second moistened paper towel, sandwiching the seeds in between and roll-up. Store in a sealed plastic bag or container in a warm spot. Mark with the date. 

Leave for a few days or as needed for the germination period for that seed variety. Remove the paper towel and unwrap the seeds gently, so the delicate shoots and roots are preserved. 

To work out the viability rate, count the seeds that successfully germinated (a healthy root and shoot). Multiply this number by a hundred to calculate the percentage. If nine out of ten seeds germinated, they have 90% viability. If the rate is less than 50%, it’s advisable to sow those seeds soon and regrow a new batch of healthy viable seeds to save. 

Rotating seeds in your seed bank, especially those with a short lifespan is an essential aspect of seed saving. It’s also good practice not to sow all your seed at once. Hold some back in reserve in case of a failed crop. 

What Seeds Should You Start Saving First? 

Eager to get started? Try easy ones like beans, peas, dill, basil, coriander, parsley, chives, leeks, lettuce, tatsoi, mustard, capsicum, chilies, tomatoes, and rocket. 

With rising seed prices and food security at stake, why not give seed saving a try? By collecting seeds from your best plants, you can preserve heirloom varieties, save money, and breed resilient plants that best meet your needs and climate conditions. 

Materials List for Seed Storage 

  • Dark, airtight containers; clean jars with lids; tablet containers; dark-colored plastic or metal tins; self-seal bags and opaque envelopes for seed storage. 
  • Labels and pens to record seed data. 
  • Rodent-free box for long-term storage. 
  • Silica gel crystals or sachets; dried rice or grain; powdered milk to absorb moisture. 
  • Funnels for adding seeds to seed packets. 
  • Teaspoons (for measuring into self-seal bags). 

Seed Saving Books and Resources 

  • Breed Your Own Vegetable Varieties. Carol Deppe. 2000. Chelsea Green. 
  • Seed To Seed: Seed Saving and Growing Techniques for Vegetable Gardeners. Suzanne Ashworth. 2002. Seed Savers Exchange. 
  • The Seed Savers’ Handbook. Michel & Jude Fanton. 2008. The Seed Savers Network.
  • The Complete Guide to Saving Seeds. Robert E. Gough & Cheryl Moore-Gough. 2011. Storey Publishing 
  • Seed Saver’s Exchange (SSE) – A network of people committed to collecting, conserving, and sharing heirloom seeds and plants. 
  • Seed Swaps – Gathering of gardeners and seed savers where seeds are freely exchanged. 

More from our seed saving series:

The Case for Rekindling The Lost Art of Seed Saving 

Seed Saving Part 2: Selecting Seeds and Controlling Pollination

Seed Saving Part 3: Harvesting and Processing Seeds

Anne Gibson
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Anne Gibson

Author, Speaker + Organic Gardening Coach at The Micro Gardener
Anne Gibson, The Micro Gardener, is a speaker, author and urban garden community educator on the Sunshine Coast, in Queensland, Australia. Anne is passionate about inspiring people to improve health and wellbeing, by growing nutrient-dense food gardens in creative containers and small spaces. Anne regularly presents workshops, speaks at sustainable living events, coaches private clients and teaches community education classes about organic gardening and ways to live sustainably. She has authored several eBooks and gardening guides. Anne shares organic gardening tips and tutorials to save time, money and energy on her popular website @ themicrogardener.com.
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