Re: My beautiful Agapanthus
That's a shame puddles.
I think where some people go wrong is taking Monty too literally.
I would say they like their roots 'snug,' rather than tightly constricted so much they can't grow new roots.
When mine get too constricted and have no more growing space, I tip them out the tubs and split them, making more plants. That also gives me the chance to check the roots for health.
I replace the top couple of inches of soil in their tubs every spring with fresh compost and feed. Then feed every 2 or 3 weeks with either a liquid general purpose or a tomato feed throughout the growing season.
They also need winter protection, but not kept overly warm, else they won't flower next year.
I would try tipping yours out and giving it fresh compost - about half and half mixture of John Innes No.3 and multi purpose - and check the roots for any unwanted livestock eating them.
Here is some advice from the expert himself, a Mr. Hickman - breeder and show judge. :
"All Agapanthus grown in containers are best given some protection in the winter, a cold glass house, conservatory or cold frame is ideal. Plants can also be placed next to the house wall well wrapped in fleece. Water the plants well mid-November and give very little water until early March. Evergreen varieties need light through the winter and can also be put in a garage or shed near the window for protection. Deciduous varieties can be kept under a bench in the glasshouse or anywhere in a shed/garage as light is unimportant until early spring when grown commences.
Agapanthus grow well in a sunny position, although partial shade a few hours a day has no detrimental effect. They are not sensitive to acid or alkaline soils.
Agapanthus do grow well in containers but they need a well-drained compost, so always add 1-part horticultural grit or sand to your compost. John Innes No.3 is ideal and gives stability. Remember to water regularly and feed with a high potash feed every 2-3 weeks April to September.
Agapanthus do not suffer from any major pests or diseases, slugs, snails, rabbits, deer & weevils do not trouble Agapanthus. Occasional mealy bug can be a problem, but a systemic insecticide normally offers a quick cure for these.
Agapanthus are easy to propagate by division to obtain plants identical to the parent plant, this is best done between spring and early summer. Avoid splitting plants too often as this will reduce flowering. Large clumps in the border should be lifted in spring every 4 - 6 years and replanted. Plants in containers should be potted on every 2nd year. Pot into a containers 10cm (4") larger for a further 2 years. Once the plant fills a 30-40cm (12-15") pot it can be split into 4. Pot the divisions back into individual smaller pots and start the process again. When plants are in large pots i.e. 60cm (24") they can be left for years until the number of flowers declines".