What is the best way to harvest tomato seeds? There are quite a few methods described on the internet and everyone has an opinion as to what works best. Unfortunately most of these ‘opinions’ are not based on any reliable testing. To really answer the question we need to find out what science says.
In this post I will review the science behind tomato seed harvesting with a special emphasis on cleaning the seed.
Different Methods for Harvesting Tomato Seeds
I’ll review the various methods for harvesting tomato seeds in detail, in a separate post, but in summary they center around three options; fermentation, hand cleaning or rubbing, and treating with a chemical. Each of these methods is designed to remove the pulp and gel from the tomato seed.
The main problem is a gel capsule around each seed. It is believed that this gel contains inhibitors which will prevent germination, so it is important to remove it. It is not rare to find tomatoes with sprouting seed inside, so this inhibitor can’t be too effective.
Removing the pulp may also reduce the transfer of disease from one year to the next.
On social media people commonly report that, ‘ It Works’! Nobody ever defines what this means and it is one reason opinions on social media are of such little value.
For this discussion there are two criteria that are important:
- What is the % germination rate? Without germination, there is no point in saving seeds.
- How well does the seedling grow? High germination is not of much help if the seedling does not grow after it germinates.
How Long Should You Ferment Tomato Seeds?
Common advice on the internet suggests fermentation should be carried out for 2 to 7 days – longer is better since it produces cleaner seed.
The science indicates that fermentation periods longer than 48 hours results in reduced germination rates but this does depend on the temperature used. At 25ºC (77ºF) the decrease in germination is minor, but at 35ºC (95ºF) it was so extreme that a week of fermentation resulted in almost no germination. Seedlings also grow better with shorter fermentation times.
Another study found “Lengthening fermentation period (more than 48 hrs) resulted in a decrease of seedling vigor and percent germination.”
The optimum fermentation was 48 hours at 25ºC.
Should Water be Added?
Some people suggest that water should be added to the extracted pulp before fermentation. This is not a good idea since a dilution of the mixture also dilutes the effective killing of soil-borne diseases. At most add 10% water.
It is also a good idea to stir the mixture several times a day to reduce the formation of mold growth on top of the mixture.
Chemical Treatment of Tomato Seeds
A number of chemical treatments of tomato seed have been tested to see if they perform better than fermentation.
This study compared fermentation to hydrochloric acid, sulphuric acid, and sodium carbonate. None of these treatments outperformed fermentation. Acid treated seed did look whiter and cleaner, which may be of benefit to commercial outlets.
There are also several on line reports of treating the seeds with various home products such as backing soda (sodium bicarbonate), washing soda (sodium carbonate), and trisodium phosphate (TSP).
There seems to be no evidence that chemical treatment is any better than fermentation.
Is Fermentation Required?
Other studies also found that 48 hours is a good fermentation time and one reported that “Unfermented tomato seeds (rubbed seed) and seeds fermented for two days had similar germination rates“, showing that fermentation is not required.
My own limited experience is that simply rubbing the gel off the seed, by rubbing them on paper towels produces clean seed with a high rate of germination – close to 100%. Many people on social media use this method.
If we only consider germination, the fermentation process is not required. However, if you are producing a lot of seed it is much faster than the rubbing technique.
What about diseases? Does fermentation reduce diseases better than the rubbing method?
Fermentation “can destroy bacterial canker and other seed-borne diseases”. The rubbing method will not eliminate diseases that are on the surface of the seed.
If tomato canker is an issue, fermentation should last 96 hours (4 days) at a temperature about 18ºC (65ºF). This long fermentation can cause injury to the seed but is off set by a reduction of disease.
Late blight is a major problem with tomatoes and “as far as we know, the pathogen does not survive asexually from one season to the next on dried tomato seed.”
In most cases treatment for diseases is not required after a fermentation process.
Post Extraction Treatment for Disease
Diseases can also be reduced or eliminated by treatment after extraction. The most common treatment is with bleach. Soak the clean seed in a 10% bleach solution for 30 minutes, and then wash with plenty of cold water.
The bleach treatment removes bacterial pathogens and some viruses, like tobacco mosaic virus, from the surface of seeds.
Some people suggest using a hot water treatment instead of bleach, but this will not “kill pathogens associated with the embryo and will not remove seed-borne plant viruses from seed surfaces”, nor does it reduce salmonella.
Which Method Should You Use to Extract Tomato Seed?
The fermentation method produces clean seed, and reduces the potential of diseases. It takes several days and is a bit messy. It is the best option if you want to collect a larger number of seeds.
If you are like most home gardeners who need a dozen seeds for next year and you only collect from a couple of varieties, the rubbing method works fine. If disease is a concern, give them a bleach treatment.
Bottom line- fermentation is not required.
This section applies to almost all seed that is not damaged by drying.
Once the tomato seed is dry, it needs to be further cured for another 4 weeks, before being packed in an air tight container. Just because it feels dry, does not mean it is dry.
I actually prefer storing seed in paper and in an open container. This ensures that it does not develop mold from trapped moisture.
If you are storing them for a year or less, just keep them at room temperature. For longer storage, keep them in the fridge – never in the freezer.