I'm going to grow stuff roughly 2/3 the recommended planting distance for non-sprawling plants, as I need to cram in the plants. But help is appreciated in knowing what can be grown through.
Geranium-wise, I'm going for a few, Rozzane, Splish Splash, Versicolour, Pratense Purple Haze, Rise & Shine, Buxton Blue & Oxonianum Pink Lace (obviously not all in the same border).
Papavers - Pink Fizz by T&M, Orientale Royal Wedding & Grape.
Irsis' - Haven't thought of any yet.
Gladioli - Yet to have a look, but figured they could be squeezed in due to their upright habit, though I realise they will look a little dis-jointed if dotted rather than grown en-masse.