Online information about growing clematis from seed is limited. As a result, I decided to investigate clematis seed germination in more detail by running 120 test cases on a number of different types of clematis. This report summarizes known information and presents new information found as a result of this work.
Growing Clematis From Seed – Current Information
My go-to sources for germination information on all types of seed are Dr. Deno’s Seed Starting Books (Deno), and Ontario Rock Garden And Hardy Plant Society web site’s Germination Guide (ORGS). Dr. Deno’s books are available as a free download – see the above menu called Free Books for details. These are the two best sources of information on the internet. Both of these have some general guidelines for clematis, but the information is not very detailed and for some species the two sources disagree with each other.
These sources provide information on a per species or cultivar basis. In some cases cold stratification is required, and in others it is not required. In some cases a warm period is followed by a cold period and then a warm period again. What is clear is that different species require different methods.
There is also a website, developed by Brian R. Collingwood with the title Clematis From Seed (CFS), but the information there is extremely difficult to extract. The method used most by this author is to pot up the seed, place it in a greenhouse and wait. Germination conditions are only provided in a very general way. The site reports that cold stratification is not required but fall collected seeds seem to spend the winter in the cold greenhouse.
The International Clematis Society (ICS) says “Clematis seeds may take up to three years to germinate, but you should get some germination in about six months to a year. Collect ripe seed in the fall and plant in sterile seed starting mix, covering seeds with a thin layer of sand. Place the container into a zip lock polyethylene bag and place it outside in a shady spot (or a refrigerator) for several months during the winter so that they go through several freeze/thaw cycles. Then place the covered container in a warm location out of direct sunlight and wait for your first seedling.”
The stated requirement for stratification by ICS contradicts the recommendations by Deno, and ORGS for some species. No other references suggest that a ‘freeze/thaw cycle’ is required.
The British Clematis Society (BCS) suggests essentially the same procure as ICS. They put the pots in a ‘cold greenhouse’ without defining what that means. It is not clear if they feel stratification is required.
I have had fairly extensive experience germinating perennials, shrubs and trees from seed but have had limited experience with clematis; germinating a half dozen species/cultivars.
In summary, after looking at all of the available information I came to the following conclusions.
- Good detailed and reliable information for germination is not available
- It is not clear if and when stratification is required
- Documented methods use pots, mostly in greenhouses. Newer germination methods, more suitable to homeowners are not discussed, except in Deno.
- Germination periods are long, and yet the effects of GA3 have had limited testing.
Clematis Germination Procedures
Seed was obtained from ORGS, who were kind enough to provide left over seed from their very popular annual seed exchange. This seed originates from many amateur growers and so there is a possibility that (a) seed may not be viable, and (b) they may not be named correctly. There is no way for me to verify seed names.
A literature review was done for each type of seed to determine the best germination procedure. Using this information, a set of germination conditions was defined for each seed type.
The standard reference method was the baggy method at room temperature and this method was included for almost all seed types. Other germination methods could then be compared to this standard method to see if they provided an improvement in germination.
Each method was given a short hand code of the form: a number followed by a B or W to indicate the Baggy or Water method (described below). This was then followed by one or more of the following codes to further define the method:
W – warm (room temperatures throughout the process)
D – dark throughout the process
WCW – warm, cold (5C), warm cycles, with a temperature change every 30 days
CWC – cold, warm, cold cycle, with a temperature change every 30 days
N – nude (outer seed coat removed after several days of soaking)
GA3 – treated with GA3 hormone.
T – tail left on the seed
The initial number is a sequential number indicating the seed type. Each unique species/cultivar, from a unique source, was given a unique number.
For example “5BWTGA3” would be seed type #5, treated using the baggy method, in warm conditions, with tails left on, and treated with GA3.
The seeds were checked on a weekly basis, and germinated seed was counted and removed. Moisture levels were maintained throughout the process. Any molded seed was not removed but mold was not a significant problem except as noted.
Ten seeds were used for each test case. Seed was selected so that each test case, for a particular type of seed, had similar sized seed.
GA3 treatment was done at the start of the process. For the baggy method, I used the method outlined by Deno. For the water method, the GA3 was added right to the water in which the seed was floating. The GA3 water was not replaced with fresh water since GA3 will break down over time.
Seed coat removal was done after several days of soaking in water. The process is tedious and very difficult for tiny seed. On very small seed or on seed that held the seed coat very tightly, only part of the seed coat was removed.
One of the main reasons for conducting this study is to compare some common germination methods.
Most of the references reported above use the ‘potted’ method. Seeds are placed in a pot of soil, covered with grit, and left until they germinate. This method works, but it has one serious drawback. Since germination is very slow, you end up with a lot of pots, waiting for something to happen. For people who germinate many types of seed this is not a very practical method.
The method I have been using for several years is the baggy method. Seeds are placed inside a Ziploc plastic snack bag, along with a moistened paper towel. Over the test period the paper towel is kept moist. In my case, I am using well water that is on the hard side, but any water source should work. Using this method, a large variety of seeds can be stored in a small space which can be very important when you keep them in the fridge. I have used this method to germinate hundreds of species.
A second method, which I will call the ‘water’ method, is reported by amateur clematis growers in a few locations on the internet. Seeds are placed in water until they germinate. They are actually fully submersed until germination.
Some preliminary testing last year with old clematis seed showed that the method can work, and that viable seed does not rot in the water.
Another method that has been reported is the ‘nude’ method. The outer coat of the seed is removed at the start of the process. The inner part of the seed is left to germinate. Some preliminary testing showed that this can also work and some people report that it speeds up the germination process.
Test cases were selected to compare the baggy method to the water and nude methods to see if one of these produced higher germination rates, or faster germination.
Baggy Method vs Water Method
In all except one case, the baggy method worked as well as the water method. C. virginiana had better germination using the water method, but it also required GA3 to germinate. It is quite possible that the longer exposure to GA3 in the water method improved germination.
Results – Does Removing the Seed Coat Work?
Removing the seed coat did not improve germination for the water method. Visually, a lot of these seeds got coated with slime (bacteria?) and seemed to slowly decompose. Most water+nude tests were stopped in October because of decay.
Not enough test cases of nude seed in baggies was carried out to come to any conclusions.
Removing seed coats does have one advantage. In a couple of cases removing the seed coat make it clear that the seed was empty and not viable. This would save time trying to germinate such seed. However, removing the seed coat is tedious and time consuming. It is also almost impossible on very small seed – at least for my big fingers!
Clematis Germination – Cold Stratification
It is not clear if stratification is required. Both Deno and ORGS recommend it for certain species, but not others. ICS and BCS suggest it is required for all species, and CFS says it is not required for any species. I have successfully germinated clematis in the past without cold stratification, so it is certainly not a requirement for all species. It may however speed up the germination process.
Since the two most trusted resources, namely Deno and ORGS, suggest that stratification is not always required, this study investigated the need for stratification in only those cases were at least one of these reference sources indicates it is required or beneficial.
Results – Cold Stratification
As a general rule cold stratification does not seem to be a requirement. For some species it is either required, or helpful.
C. orientalis had poor germination with WCW cycling and no germination without a cold treatment, but the differences may not be statistically significant. Deno and ORGS both report that C. orientalis germinates warm.
There were two sources of C. stans. One source showed good germination with CWC treatment, and no germination warm. The second source showed good germination with CWC treatment, and reasonable germination warm.
Some non-clematis seed germinates in the cold but this was not observed with clematis, except for 1 or 2 seeds. Any cold treated seed did not germinate until it was returned to warm conditions. This observation contradicts Deno’s report that some species germinated better cold, but the tested species list in the two studies has some overlap, but are not the same.
Clematis Germination – Tails
Most of the references do not mention the need to remove the tail on seeds. ORGS recommends removing the tail because it might interfere with germination. It is known that some other non-clematis seed can be prevented from germination when tails are left off (or is that a myth?).
Most of the seed obtained for this study did not have tails in tact and could therefore not be used to investigate this phenomena. Where tails were present, trials were conducted with and without tails to see if there is an effect.
Results – Should Tails Be Left on Clematis Seed?
In the few cases that have results, the presence of tails did not stop germination, but may have lowered germination rates slightly for some species.
Based on limited data, it seems that the tails do not significantly affect germination.
Clematis Germination – GA3
GA3 is a plant hormone that has been used to speed up the germination process. http://botanicallyinclined.org/the-magic-of-germination/
Very little testing seems to have been done using GA3 on clematis. ORGS does mention it for some clematis species, and Deno suggests “GA3 or light is a requirement for some species”.
GA3 has reduced the time of germination for some non-clematis cases for the author.
Results – Does GA3 Treatment Help?
GA3 hormone treatment was only applied to a few species where the literature suggests it would be helpful or required. Test results show that it is required or at least helpful for C. pitcheri and C. virginiana.
For non-clematis seed, GA3 tends to speed up germination, and so both of these seeds may still germinate without treatment.
Most of the GA3 seedlings were grown on to see if the hormone caused any abnormal elongation. No problem was identified.
More testing of GA3 on clematis seed is warranted.
Clematis Germination – Darkness
It is not clear from the references if darkness is a requirement. It is suggested for some species by Deno and ORGS, but the other references do not mention it as a requirement.
Results – Do Seeds Need To Be Dark?
The only two species that were tested in the dark were C. viorna and C. viticella and in both cases there was some improvement in germination when done in the dark. As I write this, C. viticella ssp campaniflora (23BDWCW) is germinating well (50%, mostly in November), and the same seeds in light are not germinating. The number of test cases was limited and germination rates were low.
Any time spent in the cold would be time spent in the dark since the seeds were stored in a small bar fridge.
Further testing in the dark is warranted.
Clematis Germination Results – General
The experiment was started at the end of February 2015. To see the detailed results click on this link: Growing Clematis from Seed 2015.
This file will be updated from time to time until the end of the test period. Last updated on Nov 16, 2015.
In general, clematis seed is very slow to germinate, which is in agreement with other references. After 8 months 30% have not germinated or have had very few seeds germinate. In most cases germination is spread over many weeks or even months. Only a couple of species germinated quickly.
The baggy method works as well as or better than the water method. I see no reason to use the water method.
Germination by the baggy method was not compared to the pot method. However, since most clematis do not seem to need cold stratification, it is possible that the baggy method will result in quicker germination in northern regions because seed is not sitting outside in the cold all winter waiting for warm weather.
Based on all of the references and the current tests, I would recommend the following as a general germination procedure for most clematis. Use the baggy method, warm, in the dark, with tails removed. If you have GA3 use it, but it is probably not required.
Conclusions – Species Specific
Based on this study I would make the following recommendations. Where my conclusions differ from Deno or ORGS, I have added a note.
- fusca – easy to germinate warm
- ispahanica – easy to germinate warm
- crispa – easy to germinate warm (both Deno and ORGS suggest C-W-C-W cycles and say germination is prolonged. Could my seed be something other than crispa?)
- virginiana – requires GA3 and warm (Deno suggests warm and light or GA3, ORGS suggests just warm. Personal communication confirms that fresh seed germinates warm without GA3)
- pitcheri – may benefit from GA3 and warm (Deno and ORGS suggest a warm-cold cycle)
- orientalis – easy to germinate warm
- viticella – warm and dark (Deno – warm and light with no germination in the dark, ORGS – warm-cold cycle followed by 10C for germination. It is quite possible that my ‘viticella‘ were not pure species since the term is used as a general term for un-named hybrids as well)
- integrifolia – easy to germinate warm
- stans – try warm for 2 months and if no germination give a cold treatment (ORGS suggests a cold-warm cycle)
- heracleifolia – easy to germinate warm (ORGS suggests a cold-warm cycle)
The following are some results from prior years. The baggy method was used for these as well.
Clematis hirsutissima – exposed to outdoors in late winter, followed by several warm –cold cycles. Took a year to get some germination.
Clematis integrifolia ‘Mongolian Bells’ – easy to germinate warm.
Clematis alpina ‘Willy’ – a cold-warm-cold cycle produced very low germination rates.
- Dr. Deno’s Seed Starting Books
- ORGS – Ontario Rock Garden & Hardy Plant Society, Germination Guide
- Clematis From Seed
- International Clematis Society – Germination from Seed
- British Clematis Society