Search Results for: Birch

Downy birch is a pioneer tree. That means it is one of the first trees to colonise new (or cleared) land.

Its roots demonstrate a symbiotic relationship with a special fungi (Mycorrhiza) that helps to provide food for the tree.

Leaves

The downy birch has oval leaves that are rounded or triangular at the base. The edge of the leaf is coarsely toothed.

Flowers

Downy birch has both male and female flowers (catkins) on the same tree.

The male catkins are formed in the autumn and will remain on the tree all winter – only opening in April or May. The female catkins appear in spring with the new shoots.

Fruits

The seeds are dispersed by the wind. They are tiny nutlets (only 2-3mm across) with two wings, helping them to travel up to one mile from the parent tree.

The female catkins, where the seeds develop, eventually disintegrate in September and October, releasing the seeds. One tree can produce many thousands of seeds each year.

Bark

The bark is greyish-white (not unlike the silver birch) and has horizontal grooves. It contains a chemical called Betulin that gives it the white colour.

The young twigs are characteristically hairy, which is where the tree earns its name. The fine hairs give the twigs a soft edge, especially when seen with the light behind them.

Habitat

Downy birch is the most commonly seen birch in Northern Ireland. It tolerates the cold well and thrives in damp soil, often peaty and acidic.

Areas of poor soil are quickly colonised by this tree. As successive generations live, shed leaves and die, a richer soil underneath the trees builds up. This makes it possible for other, more demanding trees to move in. These may eventually overshadow the downy birch and replace it as the dominant tree. This is how north-western Europe was recolonised by trees following the end of the last ice age 12,000 years ago.

Picking

Collect the “strobiles” from the trees from the end of August onwards while they are still intact (a strobile is a multiple fruit containing many seeds). They can be collected just before they start to turn brown, but take care to avoid heating of the seeds – they have a high moisture content.

Extraction and storage

The intact strobiles should be laid out somewhere warm in thin layers (about 10cm) and allowed to dry. Once the strobiles are dry, the individual seeds can be separated and stored in a cool, dry place in plastic or hessian bags.

Pretreatment

Four weeks prior to sowing, place your seeds in a cotton bag and soak them in clean, cold water for 24-48 hours. The seeds should then be surface dried. This can be achieved by placing the full bag of seeds (tied tightly) in a spin drier and running this for a minute or so.

Alternatively, an old pair of tights can be used, the bag of seed placed in the ‘foot’ and spun carefully around the head.

Mix the seeds with a little moist horticultural sand and place in loosely-tied plastic bags in the bottom of a fridge (not the freezer!). Four weeks at low temperatures for moist birch seed improves the speed and evenness of germination.

Sowing

Downy birch is a fast growing tree and can, therefore, be sown later in the year and still produce good growth in one season. Aim to sow during April on seedbeds or in a pot (5-10 seeds per pot and remove all but the strongest seedling).

Cover the seed with only 1-2mm coarse horticultural sand or grit. It is important that the seed is not covered too thickly as this could prevent germination. Firm the seeds gently.

Watering

Never let your seeds or young seedlings dry out! Because these seeds are very small, they will quickly suffer without water. Stand them in a shady, sheltered spot outside and water, from the top, until the seedlings emerge. Keep moist at all times.

Growing

Following pretreatment in the fridge, downy birch seeds should germinate rapidly. Feed regularly with a liquid fertiliser until August to encourage growth. It would not be unreasonable to expect up to 60cm growth in the first year, but the plants can be left in the pots for another year, provided they are fed regularly whilst they are actively growing (April-September).

Planting

When your downy birch is tall enough, it can be planted into its new home. Prepare the site well by clearing away any weeds or grass and make a hole big enough to accommodate the root ball. Plant carefully in the hole, to the same depth as it was in the pot, and firm back the soil.

Now comes the culmination of all our efforts so far!

Sowing seeds is probably the most critical part of growing trees – get it wrong now and up to two years of work and patience can be wasted.

Get it right, however, and we will be rewarded with a new generation of trees for replanting into our landscape.

When to sow

During late-winter or early-spring carefully check your seeds for signs of germination. The ideal time to sow any seed is when the root is just beginning to emerge.

Why do we wait for signs of germination? Well, because of the dormancy that exists in most tree seeds, it is important for us to be sure that we have overcome this and that the seed is ready to grow. The only way to do this for sure (without any sophisticated laboratory equipment) is to wait for germination.

If seeds show no signs of germination after the recommended period of pretreatment, we can leave them for another year rather than wasting valuable space or compost.

If you are growing only a small number of trees in containers, then you can sow only those seeds that have begun to germinate. In seedbeds, we should wait until at least 10% of our seeds have germinated before sowing.

Sowing in containers

The correct size of container to use will depend on the species of tree you are growing and on how long you intend it to remain in the container. As a general guide, the faster-growing trees (alder, birch, hawthorn and so on) should only need to spend one year in a 1- or 2-litre pot.

1-litre milk cartons have proved successful as long as we make adequate drainage holes in the bottom. Slower-growing species (the oaks for example) may need two years before they can be planted out. In this case, it is best to use a 2-litre pot from the start.

Rootrainers can be used if available, but you will need to use the larger sizes for our deciduous trees.

Another very important consideration when growing in containers is the compost. It should be free-draining, but retain moisture and remain ‘open’ (maintain the air pockets essential for the roots to breathe).

Adding horticultural grit or coarse sand can help this. There are some very good peat-alternative composts available in most garden centres. Do not be tempted to use ordinary garden soil in containers – it is heavy and will not drain very well.

Sowing in seedbeds

Seedbeds can be used for larger-scale seed sowing. The soil should be forked over before or during the winter to a depth of 10-15cm removing all weeds and weed roots as you go.

Rake over the soil in early spring to remove any debris and large stones, breaking up any clods of soil. The seedbed must be firm, so walk over it on your heels (only when the soil is reasonably dry) and rake again.

Check to see if it is flat, even and neat. You are now ready to start sowing.

How to sow

As a general rule, seeds are sown to the same depth as their diameter. This means that an acorn (1-2cm diameter) is sown about 2cm deep, whereas a birch seed (1-2mm diameter) is barely covered by soil.

This is essential to ensure good germination. Too deep and the seeds won’t have enough energy to emerge through the soil!

In containers we can simply sieve grit to cover seeds. Sow a small number of seeds per pot. These are covered with compost, firmed gently, labelled and watered. Pots should then be placed somewhere warm, but away from direct sun and kept moist.

There is a slightly different approach to be taken when sowing in seedbeds. Seeds are ‘broadcast’. This means scattering the right number of seeds over an area of the seedbed.

It takes many years to determine exactly how many seeds to sow to obtain the best density of seedlings for best growth. If you are only sowing over a small area, aim for something like 5-20mm between each seed (depending on size – the bigger the seed, the bigger the distance).

Firm the soil with a roller or board (this ensures the seeds are in good contact with the soil) and cover with more soil or, preferably, horticultural grit to the required depth.

Protect any seeds from mice and birds by using netting (oak, hazel, hawthorn and blackthorn are the most susceptible – don’t bother covering alder or birch seeds).

Seed processing is essential to:

• extract seed from fruits for storage and sowing
• prevent seed going mouldy during pretreatment
• prevent damping-off (fungal contamination) after germination
• improve the regulation of seed moisture content during storage and pretreatment

Seed groups

There are five different groups of seeds produced by our native trees and shrubs and each group requires processing in a different way.

The five groups are:

• fleshy fruits (berries, cherries, apples)
• nuts
• wings
• pods
• cones/bracts

Fleshy fruits

These require the most processing to extract clean seed. Fruits can first be soaked to soften them, without letting them ferment, before being separated from the pulp by:

• Mashing: for large-scale operations
• Popping by hand: useful for small quantities of finger-sized fruits and seeds, e.g. hawthorn
• Pressing through a sieve: useful for small seeds that are easily damaged, e.g. rowan

Nuts

Nuts can be removed from their husks, but this is not necessary unless a viability test will be carried out.

The viability of hazelnuts can be tested by placing the nuts (without their husks) in water. The ones that contain a lot of air will float. These are either empty or have only a small seed inside, which might not grow well. The full nuts will sink. These contain a healthy seed just waiting to grow.

It should be noted though, that not all nuts that float are non-viable, so it may be worth keeping them all.

Wings

Separate individual fruits from bunches and remove stalks and twigs. The outer fruit layer (the wing itself) is not normally removed from winged seeds. These should then be pre-treated immediately in the usual way.

Brown ash keys will not germinate until the second spring after collection. The seeds of wych elm should be collected in late May or early June and sown immediately in prepared beds or containers.

Pods

When these seeds are ripe they are almost fully dry and should be collected before the pods rupture. The pods can be laid out in layers or placed in paper bags and allowed to rupture in a warm, dry place.

The seeds can then be stored cool and dry until time for treatment.

Cones/Bracts

Alder and birch are collected just as the cones or bracts begin to turn from green to brown. At this time the seeds are still firmly held within the cones or bracts.

Take care not to leave them in the collecting bags too long as they will begin to heat up and the seeds will start to die.

As they dry and turn brown, the seeds are released.

Place the cones or bracts in an open box (or large paper bag) and keep in a warm, dry place. As they dry, stir or shake them in the box or bag to dislodge the seeds.

When completely dry, store in bags in a cool, dry place.