Growing trees from seed – part 1

Growing trees from seed is easy! This is a great time of the year to collect seeds from many local trees around you. So hopefully this article will encourage you to give it a go! In the first part, Steven Meyen, Teagasc Forestry Development Officer, will explain some of the basics to get you going.

Collecting tree seeds

Tree seed can be collected either directly from the tree or from the ground. Collect seeds from a mix of local and healthy trees collecting only firm, undamaged seed. Avoid collecting the first dropped seeds.

Paper shopping bags are great for collecting tree seed. Berries can also be collected using plastic bags. And don’t break off branches to get to seeds!

Be careful when collecting seeds: watch out for sharp thorns and don’t fall down steep banks. Make sure to ask the owner’s permission if it’s OK to collect seeds.

Now that you have collected some seeds, you must decide if you wish to Sow, Stratify or Store?

Seed storage

Seeds can be stored in a cool, dark place for a couple of weeks. Cones and catkins can be stored in their entirety shaking the seeds out just before sowing. When storing berries, leave the flesh on the berries to prevent them from drying out until you are ready to stratify, sow or store.

Tree seeds can also be stored very well for several years if well prepared. In that case, extract, clean and dry seeds from cones, berries, etc. Place them in an airtight plastic bag, squeeze out the air, seal, label and place in a fridge (2-5° C).

When you are ready to use the seeds, stratify as required aiming to sow in early spring.

Seed categories

Nuts

oak, hazel, beech, chestnut,…

Fleshy fruits

blackthorn, hawthorn, rowan, elder,…

Winged seeds

ash, sycamore, lime,…

Cones

alder, birch, conifers,…

Seed preparation

How seeds need to be prepared depends on the type of seed, see above.

In the case of nuts, remove cups, twigs and damaged nuts. Drop the nuts in a bucket of water and throw away all the seed that floats. The nuts that sink to the bottom of the bucket are much more likely to germinate.

Fleshy fruits need to be macerated, i.e. flesh removed. Half fill a bucket with berries you collected and add a small amount of water. Using a square piece of wood (e.g. 5x5x20 cm), bruise the berries very gently. Pour off the flesh, skin and seeds that float to the top and repeat until all the pulp (flesh and skin) has been removed from the berries and you're left with clean seeds.

Remove the stalks and other debris from winged seeds (wings don’t need to be removed).

Cones are gently dried indoors until they begin to open but don’t put them on the windowsill in direct sunlight or on a radiator. When the cones begin to open, place them in a paper bag and shake to release the tiny seeds.

Dormancy

For seeds to germinate successfully, their dormancy must be broken.

Tree seeds with a shallow dormancy can be sown directly. These tree species tend to produce large amounts of seed such as birch or alder.

Haws, spindle or holly on the other hand need to be stratified or go through controlled temperature treatments (see below) before they germinate successfully.

Regardless of the method used, aim for early spring germination (i.e. sow early March).Stratification

Stratification mimics our typical wintry conditions tree seeds have to endure. So how do you imitate this overwintering process?

Use a large plastic container with holes in base and large stones. Mix thoroughly one part seeds with one part builders sand and one part compost. Put this mix in the container, cover with builders sand and place outdoors in a shaded, damp (not wet) location and protect from mice and birds.

In the spring, check weekly for germination looking for swollen seeds. You should see the tip of the radicle, especially after a mild spell (>10° C).

Pick out the germinating seeds and sow immediately, continue checking dormant seeds weekly.

Controlled temperature treatment

A more ‘modern’, efficient way is to carry out cold or warm controlled temperature treatments.

Mix the seed with moist composted bark and sand (as above) and place the mix in a clear plastic bag and seal. Chill in the fridge at 2-5° degrees for a period of four to twenty weeks dependent on the tree species. Shake the bag periodically and sow immediately when germinating.

Some tree species are kept warm at 18-22° C up to 12 weeks and then followed by a cold treatment.

Sowing seeds

I find that tetrapak milk cartons make fantastic containers. Cut off the top, punch a couple of holes in the bottom of the container and fill with compost to 1-2 cm below the top.

Use potting compost or mix soil, sand and compost in equal proportions.

Small seeds (e.g. birch or alder) can be sown directly onto the surface of compost and covered with a thin layer of sand. Only put a small pinch of seeds in each container.

Acorns and other large seed can be sown one per container at a depth of 1.5 times their size.

Place the milk cartons in a sheltered location ensuring good drainage (lift the milk carton off the ground a few centimetres).

This is the bit that no one reads…

Prevent the compost from drying out, feed and weed. Protect from bird and animal damage and check regularly for pests and diseases.

When the saplings are about 50 cm tall, cut open the milk carton and plant them out in their final locations.

Holly and blackthorn are the exception as it is preferable to plant them out while still small.

Keep records!

As you gradually get into growing trees from seed, you will learn from your mistakes (I have!) and you'll discover that there are good and bad seed years.

It is a good idea to record the location and date of collection, what the tree looked like (for instance, was it straight or crooked, the fancy word for that is tree phenotype) and keep track what (and when) seed treatment you applied, how germination and growth went.

And finally…

Growing trees from seed is very satisfying and makes for fantastic personal gifts to family and friends. Why not celebrate the birth of a child or remember a loved one by planting a tree you grew yourself from seed?

In part 2, I will give you some more detail how to grow popular trees from seed successfully including oak and hawthorn.

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