Growing trees from seed

At this time of year (December) I like to grow from seed, just to give me something green to watch.

Ginkgo & dawn redwood seeds growing
Ginkgos & Dawn Redwood growing on a desk in my study (Dec 2022)

To many people the very idea of growing an actual tree from seed to the point that it can be used for bonsai seems completely ridiculous. If, like me, you’ve come to the hobby as a ‘mature adult’ (ahem), you might be doing calculations to work out if anything you grow from seed now will ever make it to a decent size in your lifetime.

But you don’t actually need that long to grow some species to a size that’s suitable for bonsai, and if you are a fan of mame (very small bonsai) it’s eminently achievable. You will need at least 5 years up your sleeve realistically but it’s a nice project to have on the boil while you’re working away at your desk dreaming of retirement. If you choose a so-called pioneer species (trees which establish first in clearings), it will grow very quickly. Such species in Europe include birch, alder, willow, poplar and rowanref, in the Pacific north-west these would include Douglas fir, western hemlock, western redcedar and Pacific silver firref. From personal experience, larch grow extremely quickly as well, particularly if they’re in the ground. More info on thickening the trunk of a tree as quickly as possible is in Thickening the Trunk.

This cedar was started from seed about 4 years ago – it’s about 15cm high now. It’s not going to win any bonsai competitions but I like having it in my garden and the nebari is gnarly. Cedar have quite dense short-needled foliage so they don’t need too much work to look decent.

Another way to use seed-grown trees is in small bonsai forests – when they are seedlings they can be positioned very close together in groups without disturbing their roots. When they grow their roots intertwine and their trunks become close which makes for a nice aesthetic. Japanese maple and Ginkgo are good for this.

The advantage of growing from seed is obviously that you can influence the shape of the tree from the very beginning – and in the case of coniferous species which don’t readily backbud (particularly Pinus), this helps get the branch structure right and allows the introduction of trunk movement. The key is not putting the tree in a bonsai pot until its trunk is at the desired thickness as this will basically stop the tree growing. Grow it on in a normal pot or in the ground until it gets the trunk size you want, shape the trunk as you go, then proceed as per the instructions for shaping bonsai.

A key point to understand when growing from seed is that seeds come from sexual reproduction between male and female gametes (in plants defining this is more complicated than in animals due to the two generations involved in reproduction but that’s a subject for another post). Sexual reproduction mixes up the genes of both parents in the offspring (actually it mixes up the genes of all four grandparentsref), which means that you cannot grow named varieties from seed – as named varieties always have specific genetics.

You probably don’t want to grow from seed if your goal is to have flowering or fruiting trees, or trees with cones. Flowers only develop when a plant reaches sexual maturity which can take decades in some species. This means seed growing realistically is best for foliage trees. Clonal propagation is better for flowering trees (cuttings or micropropagation) since it uses the parent plant (including its age) as the starting point.

The other thing I have noticed is that seed packets and advice online appears to vary widely and can’t all be correct. There are many factors which have evolved to help seeds be more successful in germinating and it’s beyond the scope of this post to cover them all. But whenever trying to germinate a seed, always take a look at Google Scholar to see if any research has been done on the seed in question. One also needs to consider where the tree lives – for example Swamp or Bald Cypress lives in warm, humid areas of the Americas where the temperature doesn’t go below 5 degrees C – why on earth would they need cold stratification for several months? And in fact – they don’t.ref

Many seeds exhibit dormancy once they have dried out, this is a state of suspended animation which “improves survival by optimizing the distribution of germination over time”.ref The plant growth regulator abscisic acid (“ABA”) is involved in initiating and maintaining this state – according to one study, it prevents seeds from absorbing water which is a key mechanism required to break dormancy.ref The seed coat is known to be a site of ABA synthesisref and the removal of it can enable germination to proceed, in the presence of water (I recently did this with some wisteria seeds and they immediately germinated).

Some hard-coated seeds use their coat as the barrier to water entry, and can be helped along by nicking the coat with a knife, sanding down the end of it or otherwise allowing water to enter (known as scarification). For really hard seeds, acid is recommended! In this study seeds soaked in concentrated sulphuric acid for 3h showed the highest germination (but 4h was too long).ref On the other hand, gibberellic acid is also known to induce germination. It is used in in-vitro micropropagation to break seed dormancy so a seed can be used as an explant (the material from which to propagate new plants) (Johnson, 2020). Gibberellic acid can be purchased from suppliers to the hydroponics/plant micropropagation trade. Some seeds have embryonic dormancy (ie. they are prevented from germinating due to substances occurring within the embryo or seed itself)ref – this is harder to get around, without destroying the seed.

It’s also useful to find out if your particular seed requires light – in one study all 8 taxa included had significantly improved germination when exposed to white light, and in some red light was sufficient.ref

The next section is not based on science at all, but reflects my personal experience of growing trees from seed:

  • Ginkgo – a very easy tree to grow! The hardest part is finding a female tree – they are unpopular because of the stink that the ginkgo fruit creates as it rots. The fruit ripens all summer and then falls from the tree apparently ready to germinate – although the later they fall, the better they will germinateref. I put them in the fridge for a couple of months and then plant them in some potting compost, this results in quite a high germination rate. I’ve had similar success just putting them into a pot over the winter and they eventually emerged in the late spring.
  • Cedar (cedrus) – also very easy once you’ve managed to get plump seeds from a cone. Find a semi-falling-apart cone and extract the seeds – you’ll see which ones look healthy and which don’t. Put them in the fridge in a bag with a bit of moist paper towel, wait a month or so and they will start germinating. Then you can pot them into little pots to grow on. My interest in cedar came from seeing this amazing video by George Omi, wiring a blue atlas cedar as he was taught by his father in the 1950s:
  • Oak – oak trees are the easiest plant to grow, they have all the food they need in their acorn, they tolerate heat, drought, frost and being treated badly and they backbud fantastically when cut back. I make oak groups planted on mounds & rocks which are really easy and look nice even after just a couple of years. To germinate – take your acorn, stick it in a pot over winter, keep giving it some water occasionally and hey presto, an oak tree will grow.
  • Horse Chestnut – the same as oak. Stick in a pot, wait. A tree will emerge. A bit more tricky to make good bonsai but I have a couple of nice mame.
  • Crab Apple – a common choice for bonsai but note that seeds will reflect genetic variation and will probably be different from the tree that the seed came from. Extract the seeds, pop them in the fridge and plant out a month or so later.
  • Japanese Maple – also easy to grow. Gather fresh seeds from the tree in autumn, put them in the fridge in a bag with some moisture for 3-4 months. They should start to germinate in the fridge and at this point bring them out, plant them up and let them grow. Note as per above you cannot be guaranteed of similar characteristics as the parent tree due to the seed being a mix of it’s grandparents’ genes.
  • Dawn Redwood – I’ve grown dawn redwood straight from a fresh seed pod I collected in Richmond Park in London – no refrigeration was required (although the germination rate was quite low).
  • Hinoki Cypress – if you’re lucky enough to find one with a seed cone, give it a shot, I found they germinated surprisingly easily.

Anyway, if you have the winter blues, get some green in your life and grow some trees from seed!