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Like most perennials, one way to propagate lupins is to divide and split them. However, this is usually not at all successful due to their long and deep tap root. Lupins don’t like to be disturbed at all once they are established in the ground. They really don’t like to be transplanted. There is an exception to this, though; if your Lupin develops several crowns, you can divide these in early spring and they usually grow well. Just make sure they get plenty of water until they get their roots out.
Traditional, I usually divide most perennials every few years once the centre starts to die back. However, this usually doesn’t seem to happen with Lupins the same. Lupins seem to last around 6 years, and they are best replaced, so a better way of propagating lupins is to either take basal cuttings, which is quicker or simply sow seeds taken from established plants and have them growing on in pots so they can be used to replace older plants in beds and borders.
For seed, I like to sow them in early to mid spring, I like to knick the seeds with a sharp knife and then soak them in seed for 24 hours before planting as I get a better germination rate doing this. They usually germinate in around 10–14 days at 10–15°C (50–59°F). A little tip is to sow the seeds in individual pots or cells if grown in trays. This way, you minimise root disturbance which is what seems to kill lupin seedlings when you try to transplant them into pots. They really do not like their roots being disturbed.
As for taking basal cuttings, take these in spring when new shoots from ground level are about 3-4 inches tall in mid-spring. I also recommend planting the cuttings in individual pots.