By Andrea Springmeier
Seed Savers Exchange
Every gardener and seed saver, from time to time, may discover a lost packet or old jar of seed which could hold the potential for an amazing crop of beautiful, delicious vegetables, or hold nothing more than a season of disappointment. In order to help you determine just how likely one possibility is over the next, you may want to test the germination rate of the questionable seed before you find yourself wasting hours of labor and precious garden space on something that won’t return the flavor. A germination test can be used to determine the potential maximum rate of germination for your seeds under optimal conditions. Home germination testing can be done with a minimal amount of effort, using common household items, and the results can help you to determine what crops to plant and how much seed to use in order to fill your garden (and stomach) with the bounty of the season.
Supplies you will need:
• Ziploc bags, plastic bags, or plastic wrap
• Rubber bands
• Pencil or marker
• Paper towels
• Cup or container
Additional helpful items:
• Household bleach
• Dish soap
• Nail clippers or razor blade
Before you begin
Determine the germination requirements for the seeds you would like to test based on the information on the packet or on your general knowledge of the seed. Some species may require special procedures such as soaking, pre-chilling, or cold stratification prior to sowing in the field. These procedures should also be followed prior to germination testing for optimal results. If you are testing a cool season crop, use a thermometer to find a suitable area in your home between 55-70 degrees F during the day. Warm season crops should be kept between 80-85 degrees F. It is helpful for testing purposes, if the temperature drops by about 10 to 15 degrees at night for all seeds. Some crops may require exposure to light to germinate and a few others may require complete darkness. This information is usually given on the packet or can be referenced from a variety of publications or web sites.
For the majority of species, the rolled towel test works well. To start, lay out 2 layers of paper towels and write the variety and start date on the edge of one, using a pencil or water resistant marker. Moisten them with clean tap water but don’t leave them too wet. Too much moisture can suffocate the seeds and promote bacterial and fungal growth. If water wells up around your finger when you press on the towel, then the towel is too wet. A spray bottle works best for evenly wetting the paper. Dipping and wringing out the towels may work, but you risk tearing the towels.
Set aside a designated number of seeds. You will want to use at least 5 seeds, but the more seeds used, the more accurately you will be able to estimate the germination of your crop. We typically use 50 seeds x 4 repetitions for each variety we test here at Seed Savers Exchange. Most commercial labs use 100 seeds x 4 repetitions as the standard. Using 20 to 25 seeds should be sufficient for home purposes.
If you prefer, prior to testing your seeds, you may soak your sample in a 20% bleach solution (1 part household bleach to 4 parts water with a tiny drop of dish soap) for a few minutes, then rinse them thoroughly with clean tap water. This will help to reduce the amount of surface spores and bacteria on the seeds. However, this is not essential to the test. The seeds you plant in the garden or in pots won’t be sterilized prior to planting, and they will be exposed to all the potential pathogens that are already in the soil. The main benefit to surface sanitizing your seed is that you may end up with a cleaner test that is easier to read.
Lay the seeds out on the damp paper towels in one or two rows about 1” from the top of the towels. Space them at least ½” apart for small seeds (tomato, pepper, carrot, etc.) and 1” to 1 ½” apart for large seeds (squash, beans, corn, etc.), leaving about 1”-2” of space at the sides. Fold the towels in half from the bottom up over the seeds and then roll the towels from one side to the other.
Place the roll in a plastic bag or cover with plastic wrap and set it in a cup or other container with the open end of the towel up. Place the container in an area that best suits its germination needs according to the packet and your thermometer. For crops that benefit from pre-chilling, the container can be placed in the refrigerator for a few days, and then moved to a warmer location to start the test. For warm season seeds, the top of the refrigerator, VCR, or computer can provide a good environment for germination, or you can try using a heating pad set to low. For cool season seeds, a basement, north facing windowsill, or cool corner can work. Try to provide some indirect light for most seeds, unless the variety requires dark to germinate. Do not place them in direct sunlight or you risk prematurely cooking your crop.
Check the moisture level of the towels each day to make sure they are still damp enough and add a few drops of water when necessary. You can start checking the seeds as early as 3 days after setup. If you find very moldy looking seeds, count them as dead and remove them, or the mold may spread to other seedlings and give you an inaccurate test. You can count and remove the healthy seedlings as they develop. Keep a record of how many days it takes a given variety to germinate so you can refer to it when testing similar seeds in the future. The test is over when all the seeds have germinated or the number of days to germination, as stated on the packet or from your records, is up. If you find swollen seeds that aren’t dead or rotted, or seedlings that have just started to emerge, you can continue the test for a few more days.
With some vegetables like beans and okra, you may find that some of the seeds look the same at the end of the test as they did when you started them—having no mold or bacteria on them, but not becoming soft, swelling, or sprouting either. These are called hard seeds and they are caused by the seed coats becoming impermeable to water as they dry out and age. These seeds may still be viable, but they may require that you scarify (scratch or nick) the seed coats before growing them in order to let water and air inside. A razor blade or nail clippers works well for this. Be careful not to damage the growth tip of the seed when nicking the seed coat. If this is an important variety for you and you don’t have many seeds left, it may be worth your time to scratch each seed before planting them in the garden.
After finishing the test, to calculate the germination percentage, you take the number of healthy seedlings divided by the total number of seeds used in the test and multiply it by 100. The resulting number is your percent germination. For example, if you had 20 healthy sprouts and had started with 25 seeds, your germination rate would be 80%. This can be used as a general guide to help you determine how many seeds to plant and how closely to space them. The lower the germination rate, the sooner the seed should be grown because the rate will continue to drop rather quickly once the seeds begin to deteriorate, even when kept under optimal storage conditions.
(Healthy seedlings) / (Total seeds tested) x 100 = Germination %
20 / 25 x 100 = 80%
One fact to note is that a germination test is not an estimate of the vigor of a crop. Many environmental stresses could reduce the field germination rate of your seed well below the germination rate predicted by the test. Also, as seeds age, their vigor steadily declines even when their germination rates remain relatively high. The best way to maintain a high rate of vigor as well as a high germination percentage is to maintain your seeds under optimal storage conditions. This means storing them at low temperatures under extremely low humidity and avoiding fluctuations in humidity and temperature as much as possible. Exposing seeds to large fluctuations can damage them more in a short period of time than storing them long-term under less-than-ideal conditions. If you choose to keep your seeds frozen, storing them in airtight containers in a deep-freeze is recommended. Frost-free freezers can damage seeds since they fluctuate between freeze/thaw cycles in order to reduce ice buildup. When removing seeds from cold storage, it is very important to keep them sealed until they reach room temperature. This will help limit exposure to moisture caused by condensation. If you will only be storing your seeds for a short time (2 years or less), storing them under dry refrigeration may be sufficient for maintaining quality. You may find that some species or even some varieties simply don’t have the same storage life as others. This is why regular germination testing is so important to a seed saver.
Now for the hard part. You have all these perfect little sprouts from your favorite garden varieties all ready to grow. So what do you do with them now that the test is over? Any seed testing lab wouldn’t bat an eye at tossing the seedlings directly into the nearest compost bin. But you are not a lab. You are a gardener and a seed saver. More than likely, the thought of wasting such lovely seedlings is enough to make you cringe. But if you are thinking about trying to save your test sprouts, you may have some work cut out for you. Many species don’t handle the stress of transplanting well at such an early stage of development. Their stems are exceptionally fragile at this point and they don’t like having their roots disturbed. Also, a sudden change in environment can trigger shock in the seedlings, and even if they do manage to survive transplanting, they may have been too damaged early on to really thrive or produce well throughout the season. If you’re determined to try and save the sprouts, you may want to leave them attached to the paper toweling and cut around their roots to minimize disruption. You can plant the seedlings, toweling and all, into pots containing a clean, loose seed starting mix, then place the seedlings under grow lights until, with any luck, they can grow large enough to harden off outdoors.
Overall, the best option is usually to dispose of the test seedlings and start fresh, either by planting in pots or directly in the garden. The information you obtain from the test can help you to determine how often to plant out a variety, how much seed to use, and how closely to space them. If your seed stores are getting so low that you feel you can’t afford to waste a few seedlings, then I would suggest skipping the test altogether and very carefully planting out what you have so you can save a lot more seed this season for the future.
Published with permission from Seed Savers Exchange.
Association of Official Seed Analysts. Rules for Testing Seeds. Revised 2008
Association of Official Seed Analysts. Seedling Evaluation Handbook. Revised 2006