small seedlings in a seed tray with algae growing on the surface

Restoring the fertility of Japanese Indigo seed via cold treatment.

By Ashley Walker
Copyright (text and images) Nature’s Rainbow unless otherwise credited. 26th May 2024.


It’s generally acknowledged that Japanese Indigo seed only retains viability for one year and as time goes on the seed slips into deeper and deeper dormancy. Early in 2024 there was a request on Facebook to do some experiments with old Japanese Indigo seed to see if it was possible to shock them out of dormancy by freezing. A number of sources said it was possible so I decided to give it a go.

The following experiment was very rudimentary and basic and much more could be learned from a more scientific approach. However, the results show that more experiments would be worth trying.


Old seed from 2022 was packaged up in small zip lock plastic bags to keep the seed dry and placed in a freezer at -18ºC. This is the temperature that our freezer operates at and no attempts were made to change that temperature. Seed was left in the freezer for various lengths of time.

24 hours
48 hours
1 week
2 weeks

Once the time was up the seed was placed in a heated propagator and observed each morning.

In addition a stratification experiment was set up. Stratification is a cold treatment designed to mimic winter conditions and break the dormancy of the seeds of some temperate climate plants. Seed is sowed into compost and watered (to mimic natural conditions). The whole seed tray is then wrapped in a plastic bag and placed in a refrigerator at less than 5ºC for a number of days.

Seed tray wrapped in a plastic bag and placed in domestic refridgerator

Seed tray planted up with 2 year old Japanese Indigo seed wrapped in an old plastic bag and placed in our refrigerator for 2 weeks

14 days in the refrigerator was chosen for this experiment as this seemed a reasonable period which often works for dormant seeds. For successful stratification the seed must remain wet during the period of cold.


There were very few germinations of the frozen seed and these occurred several weeks after the seed was sowed into warm compost. Of the few seeds that did germinate, the seedlings are slow growing and don’t look too well. There was no apparent effect of the period of time spent in the freezer on the germination success.

black plastic seed tray module filled with compost showing single tiny green seedling

Minimal and delayed germination from freezer experiment.

Green seed tray with two small seedlings with thin elongated stems

These two germinations occured while the seed tray was still in the refrigerator at less than 5ºC

By contrast stratification was very successful with two seeds germinating while in the refrigerator below 5ºC. When the tray was removed and placed in a warm propagator, a significant percentage of the remaining seed (20-30%) germinated in about a week. This is a germination time very comparable with fresh seed.

The two early germinated plants subsequently died when exposed to warmer conditions and light but the rest of the seedlings are looking reasonably healthy despite a thick growth of algae on the compost surface.

Seed tray with Lable filled with compost and a thin layer of green algae on the surface and several healthy looking seedlings of Japanese Indigo

About 20-30% of the old seed has germinated in this stratification experiment one week after removing from the refrigerator.

Discussion and conclusions

Freezing the seed is not an efficient way of rescuing Japanese Indigo seed. Less than 1% of the seed germinated and the seedlings do not look healthy. Also germination is much delayed even if you only freeze the seed for a short time.

Stratification on the other hand is well worth doing, with a germination rate of around 20-30% and only a 3 week wait from start to germination. The only drawback being the algae growth which appears to be associated with the time in the fridge as my seed trays do not normally show this much growth.

I have had germinations of old seed left for a very long time in seed trays but this takes so long it’s not practical. See here

Future steps

I only tried 2 varieties of Japanese indigo in these experiments (Maruba for the freezing experiment and Senbon for the stratification experiment) so it would be wise to try others out as well. In addition, I did not do a control with the same seeds sowed into trays without any cold treatment. Doing this would have determined how viable the seed was to begin with. On past experience I would have expected zero germinations.

Stratification appears to be a simple and efficient way of breaking the dormancy of two year old seed so I can rest easy and not have to worry about losing any particular variety if bad weather prevents seed production.

References and links

Japanese Indigo and seed dormancy

Growing Japanese Indigo

Premature Flowering in Japanese Indigo

The naming of Japanese Indigo varieties