Hi Tom, Kimberly here from landscape-solutions-for-you.
Thank you for asking the question on suggestions for your planter rail box. The first thing you need to decide is if you want annuals or perennials in the planter. Here are some suggestions for each:
Annuals -
Geraniums
Impatiens Marigolds
Pansies
Dwarf Ornamental Grass
Petunias
Portulaca
Some Varieties of Verbena - smaller versions
Vincas
Violas
Zinnia
Perennials -
Carnartions - some varieties will be annuals
Aguilegia - Columbine
Dianthus
Hardy Candytuft
Any Variety of Daisies
Dwarf Perennial Ornamental Grass
Philox
These are just a few suggestions for you. If you need any other help with your planters let me know.
One thing you need to do is make sure you have a light weight potting soil when planting with some humus and vermiculite. Also make sure you keep the planter watered, especially during the warmer months. Good luck to you on your planter and let us know what you decide. Have a great day and thanks again for asking a question here at landscape solutions for you.
Kimberly