Sowing

The following methods can be used for sowing. They are described further below.

• Seeds can be sown direct into the container in which they will be grown on for up to a year. Suitable containers include rootrainers, cells and other narrow but deep pots. Acorns and other large seed can be sown singly. For medium or small seed, sow 4-5 or a small ‘pinch’ of seed and then thin as necessary to leave one seedling. Thinnings can be transplanted into pots where none have germinated. Do not sow direct into large containers as this is a waste of space, and the compost will go ‘sour ’ before the roots have filled it.
• Medium or small sized seeds can be sown into trays or pots, from which they are transplanted or ‘pricked out’ shortly after germination, and grown on singly in containers, or spaced in trays for later planting out.
• All types of seed can be sown into outdoor seedbeds, and then thinned, undercut or transplanted.
• Seeds can be sown direct into the place where you want the tree to grow. The ground must be clear of weeds, with the soil a good, friable texture. Sow several seeds at each position, and protect them from birds by covering the area with a fine wire or plastic mesh. in the spring, check frequently for germination. Mark the seedlings with a cane or similar so they don’t get trodden on. Keep a 1m diameter area around the seedlings weed-free for at least three years. Thin as necessary to leave one tree by the end of the first growing season.

General sowing information

• Most seeds should be covered after sowing with a light covering of sand, fine grit or compost. This excludes light and helps maintain moisture around the seed. As a guide, the seed should be covered to the same depth as the size of the seed, measured on its longest axis. The smallest seeds are not covered.
• The pots or trays can then be covered with white polythene in order to keep them moist. If sown in spring or after stratification, check daily for germination. if outdoors, white polythene can be kept on for a few days to maintain high levels of moisture for the young seedlings, but keep a close look out for fungal diseases. In a polytunnel or greenhouse, remove the white polythene as soon as germination starts.
• Most tree seeds germinate best at temperatures below 20˚c. Warm temperatures may reimpose dormancy.
• Seed which has been stratified or subjected to temperature treatment may have ‘chitted’ at the time of sowing. This means that the seed has swollen, begun to split, and the tip of the root or radicle may be just visible. Place seed carefully on the compost with the radicle pointing down, and thinly cover with compost. Provided you handle and grow them on carefully, you have a good chance of successfully growing on every chitted seed sown.
• When transplanting or ‘pricking out’ tiny seedlings, handle them carefully by the seed leaves. Don’t pick them up by the stem, which is much too fragile for handling. Use the handle end of a spoon or fork to ease the young seedling out of the compost, and transplant immediately.
  
• Some species germinate erratically, so you may need to prick out seedlings as they appear, but then leave the pot or tray in cool moist conditions for other seeds to germinate.
• Germinating seed and young seedlings need protecting from birds, mice and other small animals. Outdoor beds are also vulnerable to disturbance by cats.

Sowing in containers

Rootrainers, cells or other deep, square sided pots are the best containers to grow seedling trees. This is because the roots are encouraged to grow downwards, rather than spiralling around the pot. Once spiralled, the central part of the rootball stays this way after planting out and for the lifetime of the tree, and may contribute to instability in strong winds. The sides of rootrainers have vertical grooves to further encourage downward root growth.

Various sizes of rootrainers are available, designed for different species and types of plants. For broadleaved trees, the recommended sizes are as follows:

• Sherwood. cell depth 12cm, cell volume 175cc. This is the smallest suitable size for most purposes. Most broadleaved species can be grown to a size for planting out in six months.
• Fleet A. cell depth 17cm, cell volume 300cc, or Fleet B. cell depth 20cm, cell volume 350cc. Suitable for growing seedlings on to larger sizes, for planting out on difficult sites. Also suitable for species which are sown in late summer, and therefore must spend a year in the container before they are planted out the following late summer.
  
• Rootrainers can be opened like a book, to allow inspection of the roots and to check that moisture is reaching through to the bottom of the plug.
  

The rootrainers are held firmly in special trays. As seedlings develop, empty books can be used as spacers to give more room for top growth. Rootrainers and trays can be washed and reused many times.

If rootrainers or other commercial pots are not available, other deep square sided containers can be used. Orange juice or milk cartons with the base removed are suitable.

Small quantities of tree seedlings can be grown indoors on windowsills, in cool conservatories or greenhouses. larger quantities should be grown in polytunnels. Consult a suitable reference book (Salt, undated) on the management of polytunnels.

All plants grown in protected conditions must be ‘hardened off’ before they are planted out.

Containers in greenhouses or polytunnels should be placed on a mesh rack or similar so there is an air space of at least 15cm (6”) below them. This ‘air-prunes’ the roots, which means that root tips are dried off as they protrude out of the pot. This is beneficial, as it stimulates lateral rooting within the pot. If containers are placed directly onto the ground, the roots will emerge from the base of the pots and grow together in a mat, which makes planting out difficult and damaging to the plants.

The simplest method is to raise the trays off the ground with timber battens. Simple benches can be made of concrete blocks and angle iron or similar.

Sowing medium

Seeds must not be sown into pots of ordinary garden soil, as this is too dense for young roots to easily penetrate, and becomes sticky or hard when watered. Sowing mediums or composts should have a high volume of air space, and be able to hold water without losing their structure. When removed from the cell prior to planting, the plug of roots and compost should hold together without crumbling, to avoid any damage to the roots.

Most commercial composts are based on peat, which is not an environmentally sound product, as it uses a non- renewable resource. Many alternatives are available, based on bark, wood fibre, coir, bracken and other materials. coir is the residue from the processing of coconut husks for their fibre, and is imported from countries around the indian Ocean. It can make good compost, but the environmental cost of transporting it should be taken into account. locally produced composts from recycled waste wood products, for example composted ground bark, are probably the best product to look for. They can be used on their own, or mixed with peat to reduce overall peat use. This is a rapidly changing market, with new products becoming available each year. Many local authorities are now producing compost from recycled garden waste. Water authorities are also producing composts from sewage sludge and other recycled material. contact them to find out what is available in your area. Compost suppliers are also listed in yellow Pages. Some national suppliers are listed here.

Many tree seedlings are sensitive to high alkalinity, and do best in a compost which has been formulated for ericaceous or acid-loving plants. a pH of 5.5 is recommended for broadleaved trees.

To mix your own compost, a recommended mix for the larger-size cells (volume over 175cc) is 70% medium coarse coir, with 30% ground composted bark. Alternatively, vermiculite can be substituted for bark, or a mixture of bark and vermiculite can be used. Coarse sand is often used in composts, but is not recommended for cell-grown trees, as more than about 2% sand will cause the plug to disintegrate when it is removed from the container.

Of home-produced composts, rotted leaf mould is the nearest to the sowing medium found in nature, and has the advantage of probably containing beneficial mycorrhiza. Preferably use leaf mould collected from beneath the same species of tree that you are growing. General garden compost may also be suitable. There should be no problems with weed seeds provided the compost has been well rotted at the high temperatures created in a compost heap. If weed seeds do emerge, they can be removed from the pot as soon as they germinate.

All composts should be moist, but not wet, before use. Rootrainers, cells or pots should be filled nearly to the brim, and then gently shaken to settle the compost into the bottom of the container. Do not fill to the brim, and brush off any surplus before sowing.

Additives

Fertiliser recommendations will vary between species, season and length of production. Commercial composts may already contain fertiliser and trace elements. If mixing your own compost, general recommendations for an NPK (nitrogen, phosphorus and potassium) fertiliser plus trace elements are as follows:

Mix with compost prior to sowing:
10%N + 10%P + 20%K – up to 2 litres/m3

During growing:
15%n + 15%p + 15%K – according to requirement

During hardening off:
10%N + 20%P + 20%K – according to requirement

In general, use less rather than more fertiliser. It is easy to scorch young plants with too much fertiliser, or too strong a solution. Slow release fertilisers, such as Osmocote, can be used but are not easy to spread evenly through small containers, because the granules are relatively large and sparsely distributed. Some cells may be missed out,resulting in uneven growth. Mix the granules very thoroughly, at the rate of 1-2g per litre of compost. Incorrect fertilising after mid-summer will keep the trees growing into autumn, and delay the onset of dormancy and the time of planting out.

Tree Growing Kits

These kits comprise 32 rootrainer cells with a holding tray and clear lid, and are supplied with coir, water retaining crystals, fertiliser, a growth chart and growing instructions. The lid doubles as a propagator lid and a tray for watering. They can be used to grow any species of trees or shrubs, and fit conveniently on a window sill for the early stages of growth. Watering from below, by using the tray or capillary matting, is recommended. Tree Growing Kits have been widely used by schools and other groups, especially in some of the Community Forest areas. With the addition of compost, the kit can be re-used many times.

Sowing in trays

Seed which has not chitted, and very small seed, can be sown into trays of compost. As the seeds germinate, they need to be pricked out and transplanted one to a pot or other container. To sow a quantity of seed for pricking out, use seed trays or large, shallow pots, plastic boxes or similar, with drainage holes.

For small seeds, such as birch or rowan, take a pinch of seed and scatter it over the surface. Do not press it in or cover it with compost. cover the seeds with a light covering of fine grit. Medium and large seeds can be placed on the surface of the compost, and then covered with a thin layer of compost. Cover the tray with white polythene, and proceed as described above.

Dunemann boxes

‘Dunemann’ boxes, ‘Kember ’ beds or similar containers are a useful method of propagating trees in small paved areas where no garden soil is available. Construct a bottomless, box-shaped structure about 1m (3’) wide, 30cm (1’) high, and of any length. Rough wood, old bricks or blocks can be used. It’s useful if boxes are accessible with a wheelbarrow. Site the box either on gently sloping paving, or on freely draining, weed free soil. Avoid frosty sites.

Cover the base with a layer of clinker (the residue from burnt coal) and ash, or with coarse gravel and grit. Fill the box with well broken-down leaf litter, preferably collected from beneath the type of trees you want to propagate. Consolidate the leaf litter by trampling at intervals while the frame is being filled. Top with either of the layers shown below, to give a level, firm surface. Before sowing, water the box well. cover the seeds with 5mm of silt-free sand, and protect with 6mm wire mesh secured with a few nails or anchored with bricks. Ensure that the bed does not dry out by watering it from time to time. Additional boxes can be used for transplanting.