Experiments in Sequoia Seed Germination IV
Indicator Germination Rates Based on Stratification Duration,
Comparison of Germination Rates for Frozen Seeds and Follow-Up
(Information Provided is for Recreational Purposes Only--No Liability Assumed)
There are three Redwood species:
Coast Redwood (of Redwood Forest fame and lumber): Sequoia sempervirens
Dawn Redwood (Chinese native, deciduous and similar to Bald Cypress in appearance): Metasequoia glyptostroboides
Giant Redwood/Giant Sequoia (Big Trees of the Sierra Nevada interior, Yosemite and Sequoia Natl Park): Sequoiadendron giganteum
In the NYC/Philly Area, Metasequoia grows easily, Sequoiadendron grows grudgingly and S. sempervirens generally
will not survive our winters. S.s. is seen growing in coastal Maryland, Virginia and DE (borderline zone 8 areas).
There are some tricks to growing Sequoiadendron in the area, so this site is dedicated to those interested.
For those interested in growing the tree from seed, some advice can be offered. In general, growing from seed will produce offspring best suited to local conditions. The following, however, represents only the early trials of determining the best methodology for seed germination. See the summary for more current advice:
Experiments were performed to determine the optimal duration of stratification based on resulting germination rates. Six stratification durations were tested. The optimal duration was roughly four weeks or 28 days. An experiment was also performed to test if frozen seeds had the same germination rates as unfrozen seeds. The germination differences were slightly in favor of unfrozen seeds. We also have an update on the seed appearance tests from a previous installment.
Based on the latest findings, the following is now considered best practices, and should supersede previous advice: [See the summary for more current advice.]
1) Segregate seeds by size, discarding
only the most damaged seeds. Small seeds will germinate much faster. The following
is for medium to large size seeds. There is some indication that seedlings from
larger seeds have better long-term survival rates.
2) Cold stratify them in a refrigerated (35 to 45 deg F), clean, sealed container of steam distilled water--allowing them to float on the water for roughly 28 days (4 weeks). One should give the seeds room so they don't overlap while floating. The less handling the seeds get the better they will float for that length of time. Seeds that sink may have a reduced germination rate, but seeds that are in the "Red/Brown" phase after stratification will be viable and seeds that are still "Green" will not be viable. Floating seeds remain 33% viable despite color indications, though more tests could be conducted on this.
3) Bring the seeds out of the cold and place them in a shallow dish where the seeds can be wetted with distilled water but not floating or sinking in it. Place the seed dish in a container to retain moisture and let stand warm until seeds begin to germinate in about one week to three weeks. Plant seeds as they begin to germinate. After several weeks the ungerminated seeds can be discarded.
4) Planting the seeds "pointy-side" down in wetted standard peat moss. Do not allow the soil to get saturated with water when planting the germinating seeds below the depth of the seeds. Soil below the seeds should be dry/moist but not wet. This all can be done in cups or containers in a dark room at room temperature. Do not expose the seeds to sunlight. You can also use potting soil or combinations of potting soil, peat moss and sand. Do not use sand alone. You can also use silty/shale mineral soils, though the seeds must be planted pointy-side up according to the planting diagram given below in the text. Mineral soils can be amended with 10% to 20% potting soil (not peat) to maintain looseness. Mineral soils should be dried before placed in the container, then wetted on top when the seeds are planted. This will avoid excess soil moisture problems. Make sure that the soil remains moist enough on the surface to allow the seedling to push through.
5) Watering seedlings only once per week is indicated. Soil should be clearly dry before watering or so well drained that it does not retain excess moisture. Seedlings that begin to flag are in need of water but overwatered seedlings will either wither or turn blue/purple and shrivel.
6) Seedlings should be planted in soil similar to the soil of its final destination if possible. The use of peat cups to aid in transplanting is under investigation. The use of tubes to hold the seedlings until transplanting will maintain a healthy root system. The early roots of seedlings can grow to over 2 ft in length, even for seedlings only 1" to 2" tall.
7) Dormant seedlings in the winter can remain in the dark, but should be kept above 25 degrees F.
8) Seedlings kept outside will need to be sprayed with Thiophanate-Methyl fungicides to prevent or halt botrytis damage. The soil may be drenched with such fungicides as well. Botrytis damage should be pruned with cleaned pruning tools once the infection is contained. Only dead branches should be removed.
9) Seedlings can be grown by the hundreds to select ones that are resistant to cercospora and phomopsis blights. One can also raise seedlings in areas free of hosts of such diseases, mostly junipers. One can also try using fungicides to raise a handful of seedlings to age 10 to 20, when the tree will be old enough to survive infection. Cercospora blighted branches should be pruned with cleaned tools early in the spring. The tree will need preventive spraying in May and June to prevent infection in June and July. Perhaps 5% of seedlings will have a strong resistance to these diseases. Clean tools with ammonia or bleach.
10) Small seedlings will need to be protected from rodents and birds if kept outside. They will also need protection from heavy rains. Established seedlings will not need much watering.
11) Seedlings grown in cups should only be transplanted when dormant. Care should be taken not to damage the roots too much.
This advice should only be considered
valid for Sequoiadendron giganteum seeds and no other species. For example,
the author would not recommend stratifying P. aristata seeds at all, but simply
sow them in moist mineral soil. Some species, such as P. jeffreyi have variable
germination requirements based on the climate of the location of the parent
Intial Germination Tests.
Tests of Seed Appearance.
Test of Soil Composition (with followup on seedlings from round 2).
The literature on Sequoiadendron germinating is not consistent. Advice ranges from warm stratification to cold, stratification periods lasting from weeks to months and other inconsistencies. It is hoped that some definitive results can be shown here to aid gardeners interested in this issue. During the winter and spring of 2002, casual experiments were performed to attempt to find some indications as to how to best germinate and grow Sequoiadendron giganteum from seeds. A difficult tree to grow in places outside of the west coast of the United States, growing a tree successfully requires a winnowing process whereby a large number of seedlings are started in the hopes that a few will survive the local environmental conditions. The low germination rates of these seeds makes the task more difficult, so increasing germination rates would increase the overall trial effectiveness.
Follow-up experiments are underway to confirm the results from 2002 and refine the methods outlined then. This experiment was designed to answer the question how long should one stratify seeds before planting. A second experiment was designed to show if there are benefits or liabilities to freezing seeds as a form of stratification.
The experiment followed the germination of seedlings for several weeks. There is no indication that germination methods have much effect on seedling survival, so other experiments will need to be conducted to determine how to improve seedling survival. Outside of California, seedlings that survive into maturity are likely to be mutant varieties (technically "cultivars" if not wild) of the species, either more cold or disease resistant. The discovery of random mutations in a seedling bed would therefore be of some interest to nurseries and gardeners. Indications of unusual needle color, size or shape, atypical growth rates, habit or form suggest a mutation is present. The results of the experiments of 2002 and early 2003 have aided in the survival of the seedlings here.
The experimental variables considered were:
1) Stratification duration and planting times in the first experiment.
2) Stratification temperature in the second experiment.
The following were fixed or made invariant:
1) Seed size.
2) Stratification conditions other than duration in the first experiment and other than temperature in the second.
3) Germination conditions.
Seeds were segregated based on size. Only the medium large to the largest seeds were used.
Stratification (duration experiment):
Stratification was done, as indicated, by floating 120 seeds in steam distilled water in a container in a refrigerator. This was done simply to match previous experiments to compare results. It is also very easy to do. The first day of stratification was considered day "0" for this experiment. Seeds were planted 20 at a time after 1, 2, 3, 4, 5 and 6 weeks of stratification. Previous experiments had been conducted with stratification lasting as long as 50 days, with 42 days the most common. Recently, experiments were conducted using a duration of 35 days and these tests were designed to determine the optimal number of weeks.
Germination (duration experiment):
Germination conditions for all seeds were nearly identical. Seeds were planted 5 at a time each week in 4 identical cups filled with a mix of peat moss and potting soil. Germination was allowed to take place at room temperature. The cups were carefully labeled. Seedlings were marked with pebbles when they germinated and their date of germination was recorded. Watering was done as needed.
Stratification (temperature experiment):
Stratification was done, as indicated, by floating 200 seeds in steam distilled water in containers, 100 seeds in a refrigerator and another 100 seeds in a freezer set below 30 degrees. The first day of stratification was considered day "0" for this experiment. The seeds in the refrigerator were stratified for 35 days. Seeds in the freezer were frozen immediately. The seeds remained frozen in the water for 4 weeks and then allowed to thaw in the refrigerator. They stratified in the refrigerator for one additional week. Thus, the total time of stratification for both containers was the same.
Germination (temperature experiment):
Germination conditions for all seeds were nearly identical. Seeds were planted on day "35" and "36". Seeds were planted 25 at a time in 8 identical pots filled with a mix of peat moss and a small amount of sand and potting soil. Germination was allowed to take place at room temperature. The pots were carefully labeled. Seedlings were marked with pebbles when they first emerged from the soil and the date recorded.
The materials and equipment used in these experiments were chosen for their ready availability to the plant enthusiast.
Cold stratification was performed in a standard refrigerator and freezer. Water used was steam distilled.
Soils and fillers selected were all commonly available at garden shops. Shredded peat moss was a very available brand. Sand added to soil was "desert" sand. The potting soil for the experiments were selected as labeled ("for seed starting"). It is mostly shredded peat moss.
Germination was done in disposable plastic cups of one size (12 oz.) or standard 6" clay pots. All containers were carefully labeled to indicate their contents.
Light source used on seedlings was 60 watts of warm fluorescent lighting held one foot above the containers and left on for 12 hours per day.
Here are tables and charts of the germination rates based on number of weeks of stratification:
Germination Rates based on length of stratification:
As can be seen in both table format and in chart format, the germination percentage increases to 60% for 4 weeks of stratification and then decreases. Because only 20 seeds per week were planted, these percentages are only crude estimates. It would have been better to plant 100 to 200 seeds per week. However, in the table one also sees that only week 4 has a well spread-out series of germinations. The other weeks tend to bunch their germinations together within a period of 10 days (the timeline on top is not linear). The germinations for one week of stratification were spread-out, but not evenly. This evenly spaced series of germinations indicates that seed viability was better maintained with four weeks of wet stratification than with more or fewer weeks. One should not expect 60% germination rates with Sequoia seeds in general, but rates as high as 50% have been reported. Not shown was a germination in one of the five week cups on day 107. This brought the total number of germinations to 40, for an overall rate of 33%, exactly in-line with previous experiments. The average overall rate of germinations in the table assumes parity among the weeks, which is not the same statistically as the overall germination rate. Likewise, the average overall days to germination assumes parity among the weeks as well, which is not the same as the overall average days to germination. One can say that the number of days until germination is reduced by reducing the time spent stratifying, but this is offset by reduced germination rates. It has been determined that continued soaking of seeds in distilled water after stratification will accelerate germination by about one week, so that germinations begin on day 35 instead of around day 42. Likewise, soaking seeds will reveal which seeds are germinating before they are sowed, which increases planting efficiency. Thus, based on this experiment it is recommended that Sequoiadendron seeds be stratified for 4 weeks (28 days) and then soaked in shallow distilled water until they germinate or not. Upon germination they can be sowed in loose and fairly dry mineral soil with only the top of soil wetted to maintain seedling moisture levels as the radicle emerges. Seeds should be planted according to the following diagram:
Here are a table and chart comparing germination rates based on refrigeration based stratification and freezer based stratification:
Comparison of germination rates for seeds frozen and not frozen:
Seeds that were frozen for 4 weeks, then stratified for one week in the refrigerator before planting compared to seeds that were stratified for 5 weeks had slightly lower germination rates, but were primarily different in that it took much longer for them to begin germinating. This indicates that Sequoiadendron seeds can survive freezing (if frozen almost immediately after submerging in water), but cellular activity is slowed down significantly. There are no advantages to freezing seeds, but this experiment demonstrates that seeds in the wild might survive being frozen. The final germination percentages would be higher than the 29% indicated for non-frozen seeds if a handful of germinations that occurred long after the results were compiled were included. Likewise, seedlings were attacked indoors by an infestation of crickets (in a cellar where everything is stored). Several seedlings were eaten or damaged at the end of data compilation period. This made verifying the final number of germinations impossible, though data was maintained throughout the process.
Follow-up on seedlings placed outdoors:
Seedlings from the seed appearance tests were placed outdoors in mid-April. Since that time the weather has been unusually cloudy and cool, with April and June being unusually wet. June in particular was wet, with a accumulated rainfall from 5/24/03 to 6/22/03 of over 12 inches. Seedlings growing in cloudy, cool and damp conditions develop soft foliage with light green coloration and long needles. Such succulent needles are high susceptible to botrytis attack, and by the end of June several of the seedlings were severely damaged or dead. To protect the seedlings from excess rain, tarps were placed over the seedling rack and the seedlings were sprayed with thiophanate-methyl based fungicide to keep the botrytis at bay. The weather since the last week of June has been hot and dry and the new growth is darker and the needles are shorter. This hardening-off in dry, sunny weather appears to protect the foliage from fungal attack. However, seedlings are stressed by temperatures above 90 degrees, particularly when their roots are not fully established and when their previous growth is light green and longish (soft-growth). They would have preferred temperatures in the mid 70's during sunny dry days and night temperatures in the 50's.
The seedlings growing in seedling or tree tubes are much taller than seedlings growing in pots and have much more established roots systems. In fact, roots longer than 18" are now common in the tubes, even for seedlings only 1" high. This indicates that much more growth energy went into establishing roots during the cold and wet than into foliage. Such long roots are beyond the need of ordinary watering since the soil in the middle to bottom of the tubes does not dry much at all. It is unclear if roots that have grown out the bottom of the 18" tubes should be pruned or not.
Here is a link to the current diagram:
Followup on seedlings from round 2.
Exotic Tree Home Page
Giant Sequoia Growing in NJ, NY and PA
More GS Photos in NYC/Philly Area
Photos of Area GS from Middle 2004
Photos of Area GS from Late 2004
Photos of GS Diseases
More Photos of GS Diseases
Conifer Winter Bronzing Photos
GS Photos from Other Photographers
Some Other Exotic Tree Species
Bald Cypress and Dawn Redwood Bark Photos
Germination Trials for growing by seed:
Summary of Tips for Growing GS from Seeds
Germination Trials I
Germination Trials II
Germination Trials III
Germination Trials IV