How do you make Magnolia Grandiflora in containers come into flower?

I have noticed that these do well in containers -- large ones obviously -- and last year I planted three of them (var. Little Gem) as an experiment. They're in an ericacious compost and they get some Osmocote type fertilzer and regular watering.

To my great surprise they've all burst into flower!

Now I remember that about 20 years I planted the same variety in someone's garden, it came with a flower bud which duly opened and . . . it has never flowered since.

I guess that commercial growers are doing something to encourage the production of buds.

But what?

 

I've never seen one in a pot, apart from in a garden centre/ nursery, most seem to be pretty big trees.
Root restriction might just force earlier flowering.

 

In fact they're often seen in pots in London, but I've never seen one in flower. They get quite big (they're in big pots), maybe 6 feet or so, and seem happy -- always lots of shiny green leaves. Mine are only about 24 inches at the moment!

 

Feeding them with a high potash food makes good sense, and I'll do it now since I think they make flower buds the year before they open -- can someone confirm that?

The flowers in my potted plants are the perfect complement for white water lilies, they are near a pond!

 

The ones I've seen flower sporadically all summer.
From last month right into Sept.

 

Just given mine a good dose of slow release high potash fertiliser, fingers crossed.