23: Spring 2009

Author: Jeremy Spon

Trees and shrubs from seed

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Trees and shrubs from seedJeremy Spon

In my talk at the Seedling Swap, I tried to address two main questions: how to grow trees and shrubs from seed? And why? Of the two, I think the second is in a way more important. Although there are specific techniques that can help in germinating woody plant seeds, they are still just seeds, and most of the experience gained by anyone who has grown anything from seed still applies. However, I suspect, and the plants brought for the sales table at winter meetings tend to confirm this, that not many members think it is worth bothering with tree or shrub seed. So before I describe the ‘how?’ I will tackle the ‘why?’

I’m sure one thing that puts many people off is the thought of the time taken to get a decent-sized plant. This needn’t necessarily be that long, however. For example, I have several acers, such as A. grosseri, a beautiful snake-bark maple, in my garden, which are ten feet high and producing seeds, yet are only six years old. Sorbus (rowans) and Crataegus (hawthorns) can make equally impressive features in as short a time. Among evergreens, pines and cypresses can be effective garden plants when quite young, and eucalyptus can put on three feet a year. This speed of growth is particularly apparent when trees are planted small, which highlights an advantage of growing from seed. Many trees in garden centres are large standards, with relatively small, constricted root-balls, which may be difficult to establish, need staking for years, and can often be painfully slow to grow away. A one-foot seedling will often catch up a five-foot standard in just a few years; planting small is definitely preferable, and choosing the best size is much easier if you grow your own plants. There is also the question of cost – the difference between £25 for a garden centre tree and £2 for a packet of seeds will pay for a lot of soil preparation and aftercare. You can even afford to plant in groups, which can result in a more pleasing effect than planting singly, just as with perennials. Finally, (and admittedly, for me, perhaps the most important reason), seeds give access to a huge range of plants, many rare or hard to find, and experimentation is cheap, so you can afford to take chances with unusual species, or those of uncertain hardiness.

So, having persuaded you (I hope) to have a try, how do you go about it? Seed catalogues and specialist society seed distributions (including HPS’s own) will provide a wide range of seed ready to sow, but not necessarily fresh. If at all possible, it is worth trying to collect your own seed, as this will be fresh, and available at the best time to sow (usually late autumn for trees and shrubs). It will, however, need to be cleaned. Any seed contained in berries or other fleshy fruit will need to have the flesh removed, as this often contains germination inhibitors. Once the seed is separated, washing is also often beneficial. The types of berries, pods, and other fruit are too numerous to allow further detail here, but an excellent reference source is the RHS handbook ‘Propagating Plants’ edited by Alan Toogood. I will pass on a couple of useful and interesting tips gleaned from a very useful article on seed dormancy by Spence Gunn in The Plantsman of September 2005. Washing ‘in dilute lemon juice or cola drinks can simulate the passage through the digestive tract’ of birds and animals, and germination inhibitors can be leached away by suspending ‘de-pulped seeds in a stocking or net bag in the toilet cistern for a few days, allowing the repeated flushings to do the job’!

Having cleaned your seeds, the principles of sowing are the same as for most plants – use a well-drained seed compost, and try and reproduce the natural conditions. Most hardy trees and shrubs respond well to sowing in the autumn, if you can get hold of the seed then, otherwise sow as soon as received. Little or no protection from the elements is needed, and no artificial heat. Do watch out for mice, though, which survive largely on seeds in winter, and particularly love fleshy seeds such as magnolias and maples. I protect all my pots with fine-gauge reinforcing mesh, which is easily available in DIY stores and can be cut to any shape with wire clippers.

Proprietary potting composts usually need extra drainage, which I achieve by adding perlite and sharp sand. (That sold in DIY stores is fine, but don’t get ‘builders’, or mortaring sand by mistake, as this is too fine and dusty.) Be patient – some seeds take two or three years to germinate. I find the biggest problem with pots of seeds needing to be kept over months or even years is keeping them evenly moist. Once wet, many seeds resent drying out again, and either go into deep dormancy or die altogether. One way round this is to use a capillary bed, a tray of sand in which the pots of seeds are stood, kept moist by an upturned bottle of water. Surprisingly, this approach is very successful with Australian plants, even though these often grow in very dry conditions in the wild. You must, however, remove any pot from the tray as soon as signs of germination are seen. A very useful tip to avoid damping off and the growth of liverworts (which can effectively seal the surface of compost kept permanently damp) is to water with a weak (barely coloured) solution of potassium permanganate.

Pricking out of seedlings, as with most plants, should generally be done at first true leaf stage. At least with most trees and shrubs both seeds and true leaves are relatively large, so handling is easy. Potting on should follow in the normal way, but don’t underestimate how quickly a tree seedling’s roots can fill its pot, and don’t ever let it become pot bound. It is also worth bearing in mind that many deciduous trees put on a burst of root growth just as they drop their leaves in autumn, so this can be a good time for both re-potting, and planting out.

If all the above seems complicated or technical, bear this in mind. I was asked to speak on growing trees and shrubs from seed because I have done quite a lot of it. I have grown quite a range of plants, some quite rare, but this doesn’t mean that I am an expert, just that I have been willing to try lots of different seeds. I would contend that the successes I have had just show how easy the whole process is, and if you try it, I hope you will agree. I think you will also find it is one of the most rewarding things in gardening to be able to look at a fine tree or shrub and say ‘I grew that from a seed’!

First published in the Kent Group Newsletter, Autumn 2006
and subsequently in Cornucopia Issue 23
© Copyright for this article: Jeremy Spon

This article was taken from a copy of Cornucopia that was published in 2009. You could be reading these articles as they are published to a national audience, by subscribing to Cornucopia.

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