Root cuttings of eryngium
Since it is almost time to take root cuttings, and the results of some I took in January are showing promise, it seems the right time to mention this strange way to propagate plants. Root cuttings are taken when the plants are dormant and used mostly for herbaceous plants. They are typically used for phlox, acanthus, eryngium, some primulas and anenomes and many others. The advantage of this method for phlox is that any stem eelworm in the parent plant will not be transferred to the new plants.
It does seem a strange way to propagate plants but we have all taken root cuttings when we dug up a dandelion and the remaining root grew into a new plant.
First, the basics. Wait till the plant is dormant, dig it up and shake or wash off most of the soil. Trim off a number of the thickest roots – books say they should be pencil thickness but this is not always possible. Trim into sections about 8cm long and trim these so the top is cut straight across and the base at an angle. This is to make sure you put them in the compost the right way up! Fill a pot with cuttings mix and push in the cuttings so the top is just below the surface. Keep them cool over winter and shoots should appear in spring.
The simplest way to take cuttings of an erygium or acanthus is to place a potted plant on the border, water it all season so it grows and roots through and then cut away the plant in the autumn – the roots in the border will sprout in spring!
I took cuttings of ‘Neptune’s Gold’ (above photo) in spring. This is a protected plant so cannot be propagated for sale but it is OK to do so for your own use. If you look closely at the photo below you will see that the cutting on the right is sprouting well but the one on the left was put in upside-down by this idiot. It has produced a long shoot that spread its leaves on the compost surface and was perfectly OK – but not good practice!
These were carefully potted separately and by July were large enough to be put in the garden.
And now they are quite strong young plants. Because I like yellow and blue I have put them with some agapanthus, including the blue and white ‘Twister’.
The Biking Gardener 
I would not have thought of doing that with acanthus, just because it is such a weed! However, root cuttings are a common propagation technique for a few perennials in the nursery industry.