I have something similar, in size, about 3' tall. There is no way I would fill them up with soil. I use large square plastic pots as inserts. They have a 'lip' so they sit right on top of planter. My planters are black and pots/inserts are also black, so it looks good. I have few perennials potted in pots and few bulbs, and usually add some annuals in Spring - I like when plants hang down. There is some Lysimachia nummularia 'Aurea' there too. Evergreens will grow well, I would look for dwarf varieties and make sure inserts are deep enough for roots. Or boxwood.

I did put 3-4 large rocks or bricks in the bottom of each, but that is just a ballast - to make sure pots don't tip over if it is windy. They have drainage holes in the bottom.

No way would I put rocks either, or your gonna make it so heavy if you ever want to move. I have used everything from water bottles to large coffee containers (plastic) Milk jugs anything really to take up space.

I don't try to fill up whole container - the insert is approx. 18"deep ( it is 18" square pot) and that is plenty for roots of many plants. So I do not need any filler in order to save soil mix. But I do agree that filling planters with rocks would make them extremely heavy. As I already mentioned, few bricks are at the bottom for ballast, I can still move planter if needed with some effort :). My planters are on each side of front steps, but had one blown over by wind. So I am not taking more chances :)

Just slip an appropriately sized plastic nursery pot or container into the decorative planter. You can use another inverted pot (or two) to lift the planted container to the proper height. This is how I plant up my taller containers.

If in a high wind area, I might place a brick or two at the bottom of the decorative container for ballast. But if they are placed right next to the house on your porch as shown, probably no need :-)