How to grow Agapanthus
My first day off yesterday, in just short of a year, and I got to spend it ogling at pretty flowers with the marvellous Debora who lunches with me irregularly, and shares my passion for extremely bad television.
We were at the RHS Spring Show - that vast hall, with the concentrated scent of new fresh growth, punctuated by narcissi, hyacinth, mimosa...depending on where you are standing.
I am a sucker for spring...I cannot tell you how many bluer-than-blue corydalis, whiter-than-white anemones; how many tiny pots of scented pelargoniums I have bought at these shows - they totally see me coming. This year I was utterly sidetracked though, by Agapanthus. Hoyland Plant Centre, who hold the National Collection of Agapanthus, had a stand, complete with a lovely, geeky table explaining the different stages of growth, and (crucially) small plants in 9cm pots, and divided sections of larger plants, which I can actually afford.
We went upstairs and sat down to listen to Steve give a talk on Agapanthus, and, as usual, I learned more from this twenty minute question and answer session than I'd gleaned from years of reading books or internet. One person with a passion, who knows his subject inside out and is able to explain it in plain English - the whole audience was rapt.
Here, in a nutshell, is what I learned.
There are two different types of Agapanthus. Deciduous and Evergreen. Deciduous are hardy pretty much everywhere. Evergreen need to be in the south (or see below for methods of protection).
Compost.
Needs to be well-drained. Steve uses two parts ordinary multi-purpose mixed with one part sharp sand or grit. Treat them mean. Too many nutrients will produce leaves and no flowers. (see below for fertiliser)
Root restriction.
The rumours are true; these plants like their roots restricted at first. This helps the rhizome to form and bulk up, allowing for flowering. Obviously they therefore do well in pots. Here is the kind of pot-bound-ness that is perfect for an agapanthus. Don't re-pot until this level is achieved.
Here's a plant that is too pot-bound and needs re-potting:
If you want to plant in the ground and the roots are not yet congested enough, sink them inside a pot with the bottom removed.
Feeding.
Feed regularly with a high potash feed (that's the 'K', or Potassium in your 'NPK' fertiliser; their feed is 30% potash) from March to September. This will encourage flowering.
Winter care.
Give containers a really good soaking in November, buy a bag of bark chippings and mound them over the crown of the plant (in amongst the leaves if it's evergreen) and, if you can move them, bring them inside somewhere frost-free (evergreen will need light, deciduous won't) for the winter. If you can't move them, wrap the container with bubble wrap and then the whole thing with fleece. If you're in a sheltered area you'll get away without doing this, but if your plant is super-precious to you, then Steve says you should do it anyway.
In the border, just use bark chippings and mound them up over the buds.
Dividing
Use a knife that's an appropriate size for whatever you're chopping. Steve likes his meat-cleaver. He also uses one of those Nigella-type mezzaluna things for smaller plants. He looks rather better than Nigella wielding the thing. It is a sight to behold. Sorry I didn't get a picture.
Hack away...these plants are tough, but if you are using something serrated, then be sure to shave the cut with a knife afterwards so that the wound can heal cleanly (it's the difference between having a clean cut and a graze on your skin - clean cuts heal better and quicker. Leave the pieces to heal over for 24 hours before re-planting in the compost above. If you're re-planting in the ground then you don't need to do the plastic pot trick if the plant has flowered the previous year and has flower-buds on it - the rhizome is sufficiently bulky.
Here is the inimitable Steve with some of his cleavers. 
I have a container-grown agapanthus that I've neglected for years and hasn't flowered for the last two or three. It's so pot-bound that it's pushing itself out of the container. I was about it release it from its pot, chop it up and re-plant in the border, but Steve says I should re-pot it first in a larger pot, feed and mollycoddle it for another year, and then chop it up, once it's flowering again - invaluable advice, brilliantly delivered.
I bought a beautiful chunk of Agapanthus praecox from Hoyland - an evergreen that I'm going to put with all my other agapanthus in a special new bed I'm creating....updates soon.