Japanese Indigo seedlings growing after dormancy is broken

Japanese Indigo and seed dormancy

By Ashley Walker
Copyright 14th February 2020

Persicaria tinctoria seed dormancy

Don’t throw out your old Japanese Indigo seed just yet.

Strange the things you think about when you are trying to get to sleep. For a week or two now I have noticed some germination activity going on in an old seed tray I moved outdoors at the end of last year. A few days ago (8th Feb) I took a closer look and realised that the seedlings were Japanese Indigo, so I’ve been thinking about this ever since.

The weather here has been very mild since before Christmas, with only a few night time frosts. During the day it’s been warm enough for Japanese Indigo to germinate but how has this happened? Japanese Indigo seeds only stay fertile for about a year, unless you freeze them, right? Well, clearly not, as my seedlings testify. I moved the seed tray indoors and yet more germinations. What is going on?

Although the label on the tray is long missing, I believe that this is a tray of seeds planted during late summer (September) of 2019. I wanted to see how old seeds could and still germinate. I admit my documentation was poor, but when it comes to plants my memory is quite good (more than can be said for my ability to remember anyone’s birthday). Germinations at the time were very poor. Only a handful of plants germinated and eventually I decided there would be no more so moved the tray outside. Outside, the tray has been exposed to all weathers including some hard frosts. Moss and even a Common Rush (Juncus effuses) had grown during the late Autumn and winter. But in all this time the seeds (from 2018) were not dead, but dormant.

Newly germinated Japanese Indigo seedlings growing in moss covered seed tray.

Seed dormancy is a fascinating subject as plants use all sorts of strategies. Some seeds are dormant from the moment they become ripe and need a period of vernalisation or their tough seed coats need to be scarified i.e. damaged before germination. Some seeds will stay viable for decades and others only last a year. I had thought that Japanese Indigo fell into the latter category, as germination rates steadily decline from age 6 months onwards. At 12 months of age few if any seeds will germinate even when sown in good conditions with warmth, light and moisture.

Persicaria tinctoria seed

Japanese Indigo seed.

I asked myself a question. What happens to an annual plant whose seed only survives a year, when there is a serious drought lasting for months? The plant dies out of course. But thinking about it, that’s crazy – no plant would use such a strategy unless it had evolved in a world where there were never droughts or forest fires etc. Japanese Indigo seed with its hard shiny black protective coat actually looks like it’s designed to last a considerable time.

I have a distant background in plant biology and this is what I think is going on:-

Fresh seed readily germinates. But if the environment is harsh and the seed is prevented from germinating, dormancy-inducing chemicals slowly accumulate within the seed. If the seed is exposed to soil bacteria and moisture the protective seed coat slowly breaks down until dormancy is broken. Alternatively the seeds might need to be vernalised to break dormancy. My seed tray was exposed to low temperatures and frost which may have done the job too. As soon as the conditions are suitable the seed then germinates.

So how can the small grower take advantage of this new finding? At the current time I do not know. You can certainly try sowing old seed into trays and leaving them outside during the winter. But, until I know exactly what mechanism was involved (scarification of the seed coat or vernalisation) I cannot say with any confidence how you can break dormancy quickly. More experimentation is needed.

If anyone out there has any further information I’d love to hear it.

Seedlings of Japanese Indigo break dormancy

12 thoughts on “Japanese Indigo and seed dormancy

  1. susan dye Post author

    Hi Jan
    Yes of course you are right. I should (as mentioned in the post) have used a control. The whole experiment was really just a quick and dirty to see if there was any point in going for a more rigorous approach. From what i see posted in social media the great majority of people fail to germinate second or third year seed so the occasional post suggesting that cold treatment could be a quick and easy method of increasing success caught my eye as I had never tried it. Fresh seed is pretty bomb proof and should germinate very readily but in my experience older seed is very hit and miss with germination being tediously long if it occurs at all so I was keen to try and see if cold treatment could work. I’ve used seed that has been stored frozen for long periods (months/years) in the past but found germination was poor and the subsequent seedlings weak so i was skeptical and decided to add in the stratification experiment as being more likely to work. What clinched it for me was the speed of germination after removal from the refridgerator – exactly the same as fresh seed. Also the fact that two seeds germinated at temperatures below 5 degrees C is significant as it shows that there really is something about the stratification process that encourages germination.
    Well, clearly more work needs to be done before we can conclude with certainty that stratification is as good as it appears from this experiment. I hope it will inspire others to carry out their own experiments.
    Best Regards

  2. Jan Miller

    I have not read all the messages in detail, but from what I read about Sue’s method of trying freezing the seed for different periods I immediately thought ‘but was there a control?’ My own experience of germinating Persicaria tinctoria over many years showed that it was not always true that seed older than one or two years had poor germination. Certainly I happily used 2 year old seed with good results. So I think for your freezing experiment you should have to include sowing a batch of the same seed in a propagator normally (bottom heat always helps) at the same time as sowing the frozen seed, and compare the germination rates of the unfrozen aswell.

  3. susan dye Post author

    Hi Kathy. Your investigation of different varieties of Japanese Indigo is exciting. I hope you will be able to share these. In the years I have been growing the plants I have found the lack of information on varieties and the plant in general to be dissapointing and perplexing given its so popular. I did see a post by a big grower in the USA (via Brittany Boles Indigo extraction methods facebook page which confirmed some of my observations. This grower had also recently obtained a new variety of Japanese Indigo from the highlands of Japan which he called an early flowering variety. The advantage of this would be it’s early production of seeds in a northerly climate. A botanist friend of ours had also done some research into Japanese Indigo in the archives at Kew Gardens in London. He found evidence of a number of both cultivated and wild varieties of what appeared to be related plants that were also known for their indigo content.
    The slow germination of seeds over a year old is interesting. The seeds become more dormant the older they are (also if they have been frozen). I still have a few seeds of the white flowered variety and if you were interested could send you some, though the season is rapidly moving on. I think that white flowered and pink flowered specimens can be found within specific varieties (well I have been told that but not come accross this in my own growing. The varieties I grow appear to breed true despite very often being only a metre or two away from each other.
    Also can I ask if you have been able to attach the Japanese variety names to any of your 4 varieties. I tentatively think that the rounded leaf variety may be Senbon and the White flowered pointy leaf variety Kojoko but this is difficult to substantiate. Japanese indigo seeds have been swapped around craft growers for so long that the original sources have been long lost.
    Best Regards and please give us an update on researches.

  4. Kathy Nelson

    Hello Susan Dye,
    Thank you for your wonderful post on comparison of indigo content of 3 strains of indigo. we are doing a similar experiment here in south central Oregon, USA, involving 4 seed sources and 3 different growers/ conditions/ treatments. We have been having the same questions of pointed vs. round leaved varieties (don’t have white flowered).
    But my comment is on seeds. I have saved and successfully planted and raised plants from 2 year old seeds kept in a container (room temp.) in the house – they were the round leaved variety. No cold treatment necessary. Also my seeds seem to germinate over an extended period of time (1-2 months?) started under grow light in the house for about 1.5 months, then planted out after frosts in June.

  5. charlotte

    Hi Ashley,

    I have a question about germinating Japanese indigo, do they need direct sunlight to germinate? I live in northern Spain, and have just planted my seeds and put them in my ‘Galeria’ which is similar to a conservatory, so is warm and has sunlight all day.I have read that they need a lot of sunlight, but someone has told me they shouldn’t be in direct sun because it will dry out the soil (they have experience germinating many plants, but not indigo)


  6. Cristina

    Thanks Ashley for your reply Yes will certainly let you know what happens. Im afraid Im not very scientific with my seeds I simply rub the dry seed heads gently through my fingers and sprinkle over the tray and then cover all the seeds and bits with a very light sprinkling of potting compost. I usually keep my trays and pots of seeds on the sunny upstairs windowsills until well established. I then pot them up and place in the greenhouse with some plants going outside when its warmer. Im lucky to have a really protected sunny veg garden and they did well there last year. Thank you again for your advice.

  7. susan dye Post author

    Hi Sara
    We are located in Hitchin in North Hertfordshire in the UK (About 30 miles North from London). You can get dye plant seed from many places but usually there is no single supplier where you can buy all the seed you may want. If you are in the UK Try Wild Colours first We do sell seeds but usually only at events.

  8. susan dye Post author

    Hi Merri
    I’ve corresponded with other Australians growing Japanese Indigo mainly about the problem of premature flowering. This appears to be a result of not sowing the seed at the right time of year. The Australian climate being a lot warmer is confusing as the seasons are not clearly defined particularly in northern regions. I believe self seeding of Indigo is quite common in parts of the world where the climate is a lot warmer. My seed tray was outside when the seeds started to germinate and have survived because we have had no serious frosts since before Christmas (very unusual weather for the UK). No doubt that self seeding does normally occur here too but the frosts (and no doubt slugs) usually kill the seedlings off pretty quick so we don’t really notice.
    Allowing plants to self seed is a favourate strategy of mine. Saves a lot of work! Best of luck with your indigo.

  9. susan dye Post author

    Hi Christina
    Thanks very much for your comments and observations. We have known a number of people who have managed to get their Japanese Indigo plants through the winter – usually by bringing indoors and giving them plenty of heat and light. One person had access to a heated greenhouse and managed to get a lot of plants through. There are two types of annual (hardy and tender). The Hardy annuals usually germinate in Autumn and go through the winter flowering in the Spring or Summer. The tender annuals germinate in Spring/Summer and flower in the same year. Japanese indigo is usually classed as a tender annual. But of course the official definitions do not admit to all the grey areas in between. Some tender annuals will just keep growing and flowering provided the weather is good and only die when frosted or subjected to cold wet dark winters so its possible that these annuals could stay alive for many years. I think that Japanese indigo falls into this catagory. I need to do some more research. Once the plant has started flowering it may not stop in which case a second year plant may not be much good for growing foliage (so not so good for dyeing) though it would be good for getting more seed. Please let me know how your plants do and if they start flowering early. With our continued mild winters the survival of the plant may become a lot more common. For dyeing purposes I still think it’s best to grow from seed each year. The new seedlings grow very quickly and easily compete with plants that do manage to get through the winter.
    Also may I ask what method you use for seperating out the seeds?
    Best Regards

  10. Merri

    I’m in Melbourne, Australia and my Japanese indigo germinated in June. The temperature over winter doesn’t get over about 15 ° and we get occasional frost over winter. The seeds were those shed that autumn and came up in the pot of dead indigo. I had tried over two years to get seed to germinate and finally got a few plants late in summer. I was so surprised to get plants without trying!

  11. Cristina Fotheringham

    Hello Ashley and Susan Im a new follower of your site and thank you for sharing your findings and expertise. I live in West Wales near the coast and we seem to be a little warmer than inland. Last summer I grew indigo plants in my greenhouse and outside. I cut all the plants down to about 15 cm in October having saved as much seed as possible. The outside bed plants lost all their leaves very quickly and seem to be dead but one container I moved to protection underneath a bush has new tiny green shoots. I had previously thought indigo was an annual plant so can you let me know if in your experience they can be perennial? How exciting that your seed tray has produced seedlings.

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