Article by Adopt a Tree managing arborist Shannon Adams

How to grow Eucalyptus trees from seed

Seed propagation is the most common way to raise hardy Eucalypts and is the preferred method for raising seedlings for carbon capture operations.   When a Eucalyptus tree flowers, seed capsules will develop in place of the flowers.  These capsules containing the seed take a few months to become ripe and for the seed to become viable.  You can tell when the capsule is ready because scars become visible on the face of the capsule.  The capsule contains the seed, which is very fine and may consist of several different colours of seed all within the same capsule.  It is this very fine internal seed that forms the basis of new trees, and simply planting the seed capsule will not usually result in a new tree.  This seed inside is what needs to be harvested.  Seed capsules from large healthy parent trees with a known provenance of strong growth characteristics will usually yield the best seed to grow new Eucalyptus trees.

The seed capsules open on their own – they should not be cut or opened manually except as a last resort.  The capsules will normally open if they have warm, very dry conditions for a few consecutive days, as happens during natural propagation in a bushland setting.  If the seed capsules don’t open, it is possible to gently heat the pods and encourage the seeds out.  Once the seed is released from the capsule, it can be sown immediately or stored somewhere cool.

Many of the hardiest species of Eucalyptus must have their seed kept cold and wet for a period of time before they will germinate well.  This process is known as cold-stratification.  Depending on the species, some seeds will provide a much better rate of germination after being stratified, and some in fact will not germinate at all without being stratified.

Stratification involves chemical processes within the seed that mimics what would occur in native bushland conditions.  Essentially, the seed is kept cold and wet so that it will “believe” it is winter.  After a suitable period of time in these cold and wet conditions, the seed is brought back into a warm environment, simulating Spring.  Spring is the time of year when plants are hardwired to grow, and the Eucalyptus seed will be more inclined to germinate because it will “think” it is now Spring.  The reason why plants grow in Spring is because it gives them the best chance of long-term survival, with the longest possible time to grow steadily before winter comes around again, when their growth rate slows.  Without cold-stratification, the chemical inhibitors in the seed will not allow it to germinate unless the environmental conditions are just right.  In general, simulating winter conditions by cold-stratification appears to help most species of Eucalyptus germinate.  If you want to grow Eucalyptus trees yourself from seed, I would advise seeking expert advice from a qualified arborist before deciding if cold-stratification is necessary, as it may depend on the species of tree and the local climate.  The process of simulating winter by cold-stratification can be as easy as putting the seed in your refrigerator, making sure it is kept moist, and then returning the seed to warmer air for germination.

Once the seed is ready to germinate, it should be placed in a seed-tray (or a pot works fine if you are only looking to grow a few trees) and sown on top of the soil, and then covered in a fine layer of sand.  The seeds need to be moist but not wet to germinate.  Once the temperature hits about 22 degrees Celsius, the seed will germinate, turning into seed sprouts.  They are ready to harvest and put into pots once the first set of true leaves have expanded and the second set of new leaves begins to show.  These baby trees should be taken out of their seed tray taking care to include all the roots without any breaking off.  The baby trees can then be potted using a light organic potting mix that is relatively pH neutral.  It is important to use a light potting mix to avoid a crust forming on top.  As soon as the seedlings have germinated, they should be immediately moved to an area with plenty of bright light.  The area the seedlings are kept should be outdoors from when they begin to sprout, unless using a specialist grow-house or commercial nursery.  At this stage, the young trees should be potted without being staked.  They should only be staked or caged once they are in their permanent position and strong enough to withstand some lateral forces.

After 12-14 weeks in their pots, the plants are ready for planting out.  For best results, the little trees should be about 30cm tall when planted.  It is not advisable to grow Eucalyptus trees in a pot much bigger than this because their growth and development can be set back.  While it may be tempting to get a ‘ready-made’ larger tree, they rarely do well once planted if they are over 60cm tall.  Beware cheap large trees at nurseries – smaller is better.  Another important thing to remember is that although native Eucalypts are very drought tolerant after being planted, they are not very drought tolerant while they are in their pots.  If they are allowed to dry out in their pots to the point of beginning to wilt, they will most likely die. 

Once they reach about 60cm tall, they need to be planted immediately or their growth can be set back.  The water and temperature need of these trees is not especially demanding.  These native species are tough, hardy, and can survive in most conditions including drought.  As long as the little trees are not frozen solid or dried out in their pots, they can withstand a wide range of temperatures.  Very cold temperatures will slow the growth rate, and temperature adjustment is how we can control the size of the plants in their pots to make sure they do not get too big before being planted.

Late Spring or early summer is the ideal time to plant, but the baby trees can actually be safely planted from early Spring through to Autumn depending on the particular species and the local climate.  Trees should be double-staked and caged if needed.  We do not plant out urban-woodland areas unless there is enough time to let the trees get established in the ground before winter sets in, and trees planted in Autumn have a slower initial growth rate.  As long as they have enough time to found their root system, enough water and some organic fertiliser or compost, these hardy little natives will carry on happily over the winter months and start growing strongly again as soon as Spring arrives.  In a mild winter, the hardy local species actually grow quite well.

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