Propagation by Cuttings
As most Osteospermums can only be considered half hardy, it's always best to propagate new plants by taking cuttings each year. This is preferable to over-wintering large plants.
We normally take our 5cm stem tip cuttings in autumn, although they can be rooted at any time of year given the right conditions. Always choose non-flowering shoots. It's advisable to use a hormone rooting powder or liquid, containing a fungicide to prevent basal rotting. The cuttings should root within 3-4 weeks.
As we propagate a lot of plants we like to use a tray with 84 segments that have a depth of about 6cm. If you only want to take a few cuttings you can also use 7cm square pots. You should be able to fit 6-7 cuttings in each pot.
You can use a variety of mediums for rooting your cuttings in. A well-drained mixture is preferable, although we've also had good results in the cheapest of peat based multi-purpose compost. Some garden centres sell their own seed and cutting compost. This is often an ideal mixture as it will be relatively low in nutrients, encouraging root formation/growth. If you choose to root your cuttings in pots or trays you can also try other combinations. We have had good results with a 50-50 mix of perlite and vermiculite and 50-50 mix of peat and perlite. Horticultural grit can also be mixed with perlite or peat/compost to give excellent drainage.
When using segments as mentioned above, we have found that it's best to fill the compost in quite firmly. Make a small hole in each segment with whatever tools you have available. We use the top end of a thin paintbrush. Prepare your cuttings by carefully removing the lower leaves and then cut the stem just below a leaf node with a sharp knife. Then dip the end of the cutting into hormone rooting powder and place the cutting into the segment, firming it in gently. Make sure you label the cuttings as you go along. When you have finished, water the cuttings thoroughly and place them in a propagator or on a heated sand bench, with the thermostat set at 15-20° C. Although the cuttings do not need to be totally enclosed, some protection is recommended. Avoid direct sunlight or they will rapidly wilt and die! Although you mustn't allow the cuttings to totally dry out, you also shouldn't over-water them as this will lead to rotting. If any cuttings should rot you should remove them immediately.
When the cuttings are well rooted we set the thermostat to 5° C. You can then either pot up these cuttings immediately into 9cm or 1 litre pots using a multi-purpose compost, or leave the cuttings late winter, feeding them every 2-4 weeks. High-potash feeds are often recommended to promote strong and healthy roots. However, we prefer to use a general feed.
If you decide to pot up, you should pinch out the tips of the cuttings two weeks later to promote a bushy growth. Rooted cuttings may also be pinched out an an early stage. All plants should be hardened off 1-2 weeks before planting out in May (after any threat of frost has past).