I wrote about this experiment six years ago (Dremann, 1995), and this is an update containing the current results. In the autumn of 1988, I bought two samples of native grass seeds collected in May and June 1988 from California: Bromus carinatus and Elymus glaucus. I chose these species for longevity tests because their seed-storage ability is notoriously short lived when kept at room temperature, as evidenced by earlier tests I had conducted:
|Fresh seed||one year||two years||three years||four years|
|Fresh seed||one year||two years||three years||four years|
The seeds I used for these longevity tests had been stored at room temperature from their 1988 harvest, until tested in February 1990 for germination, and found to still be 100% germination.
On March 26, 1990, I placed 50 grams (2-3 ounces) of each of these species in a dozen eight-ounce (220 ml) mason jars having screw-down canning lids. In six of the jars, I only put seed in the jars. In each of the other six jars, I placed one paper manila #1, 28 weight coin envelope containing one tablespoon (approximately 10 grams or 1/2 oz.) of loose indicator silica gel with the seed.
The indicator silica gel used in this experiment was purchased from Silica Gel Desiccant Products Co., 734 E. Hyde Park Blvd., Inglewood, Cal. 90302, phone (800) 426-1529. The type used is part No. S5112, 6-12 mesh granular indicating silica gel and comes packed bulk in 1-1/2 pound and 5-pound cans (about 1 Kg. and 2.2 Kg.). The silica gel's color is cobalt blue when it is dry and turns pink when fully saturated with moisture, and reaches an in-between lilac shade when it reaches an equilibrium, when the seed's moisture is stable and fully dried. The lilac color indicates that the gel can still absorb moisture until it turns pink, but in the lilac stage the gel has dried the seed down as far as it can go.
The three jars of each of the two species of native grass seed were placed in the following storage conditions:
1.) Room temperature 12 to 30C. (55-85F),
relative humidity 35-45%
2.) Refrigerated 1 to 6C (33-42F)
3.) Frozen -6 to -1C. (20-30F)
Periodically the jars of seed were removed from storage, and samples taken to be tested for germination. The color of the silica gel was always checked and redried if it had become saturated. The silica gel was recharged three times (dried in the over at 65C. (150F) for an hour, until the color turns cobalt blue): the first time at one year into the testing period, and then every 4-5 years thereafter.
Before removing seed samples from the jars, the frozen and refrigerated jars stood for 24 hours to warm up to room temperature before opening the jars. Otherwise moisture from the air in the room could condense on the cold seeds and damage future germination.
The germination tests used aluminum trays ("Danish trays") about 28 x 38 cm (11" x 15") with an one inch tall rim around the edge, purchased at the household sections of a hardware store. Sterile rolled Johnson & Johnson® cotton in a one pound (0.5 Kg.) box was laid out and cut to size to fit the bottom of the tray. Distilled water was added to saturate the cotton and the excess poured off. Then seeds were placed on the cotton spacing them about 1 cm. (1/4") apart in each direction, and toothpicks placed between the different lots to keep them separated. Then a plate of single thickness window glass cut to size and with taped edges, was laid over the top of the tray to keep moisture in and let light into the tests.
Germination tests lasted 35 days, with periods of 3-4 days cold 1 to 6C (33-41F) moist stratification each week and then warmed to room temperature to 21-26C (70-80F) for 3-4 days each week. The first count of germinating seedlings occurred when seedlings were about 0.5-1 cm (1-2") tall that began as early as the fourth day with either the Bromus or Elymus. The Bromus and Elymus would all germinate generally between the sixth and seventeenth day of the test, but occasionally some would germinate as late as the 25th day of the test for Bromus and the 35th day for Elymus.
The tables below give the results of the germination tests conducted between 1990 and 2001. The following abbreviations found in the tables are as follows:
Rm No Sil = Room temperature, No silica
Rm +Silica = Room temperature, with silica gel.
Ref No Sil = Refrigerated, No silica gel.
Ref +Silica = Refrigerated, with silica gel
Froz No Sil = Frozen, No silica gel
Froz+Silica = Frozen, with silica gel.
|Rm No Sil||100||70||90||55||55||36||0||0||0||0||0||0||0||0||0||0|
|Ref No Sil||100||90||100||70||80||58||60||55||55||60||30||30||45||10||20||3|
|Froz No Sil||100||90||90||70||100||76||70||75||50||90||60||55||50||30||5||0|
|Rm No Sil||100||65||50||30||35||0||0||0||0||0||0||0||0||0||0||0|
|Ref No Sil||100||75||72||80||90||76||55||72||85||90||50||30||55||10||15||0|
|Froz No Sil||100||85||80||90||100||79||40||68||60||40||40||25||0||0||0||0|
You may notice the fluctuations in the germination rates from test-to-test. That is partially due to the official rules of germination test counting. You are only allowed to count 'normal' seedlings as a germinated seed, which means that the seedling has to have a well-developed root system and green leaves. Deformed seedlings cannot be counted, and you cannot count dormant seeds that may be present at the end of the test period. Dormant seeds that were present in one year's test could germinate in the next year's test and be counted as germinated seedlings; and then go dormant again when conducting future tests.
Conclusions: Refrigerated storage or frozen storage are better methods than room temperature for long-term storage of these short-lived native grass seeds. However, silica gel can extend the overall longevity of either of these species when stored at any temperature. You can gain at least two years when silica gel is placed with seed stored at room temperature, and silica gel helps maintain viability in refrigerated or frozen seed for at least an additional four years or more. Indicator silica gel appears to be relatively inexpensive insurance that guarantees a long life for short-lived native grass seeds put into long-term storage.
When you compare the energy costs of long-term frozen vs refrigerated storage, it appears that refrigerated storage is adequate, as long as silica gel is placed with the seeds. The use of lower-cost refrigeration rather than more energy intensive frozen storage could translate into significant cost-savings over time.
I will continue to test these Bromus and Elymus seed lots periodically until they all "zero-out," probably sometime later this decade, and will write another update in five years.
Dremann, Craig C. 1995. Bromus Carinatus & Elymus glaucus storage, longevity, genetic changes and ecotypical variations. GRASSLANDS (Journal of the California Native Grass Association, Davis, California) 5 (3) 2-5