To get your plants off to a good start, the first thing to do is submerge the root ball even before removing it from the pot in a bucket of water, until air bubbles stop rising to the surface. Next, make a 1-2 inch layer of expanded clay pellets at the bottom of the flower box and top it with a porous, nonwoven (fleece) material, cut to size. Both can be found in garden supply centers.
This design allows excess water from watering or weather to drain completely – an essential point, since persistently damp conditions can cause root rot. Cover the drainage layer with high quality potting soil – the box should be about one-third filled – and place the unpotted plant into it. Then fill in with soil and gently firm it around the root ball. Finally, water it thoroughly, and, with that, you’re ready to start balcony season.