Question
I have two box planters attached to my patio railing. They will be in direct sun.  My box is about three feet long.
What annuals can I use that will bloom all year and that will
look good together in the planters?

Answer
Carole,
 Do you mean bloom all summer?  If you live in a place where it is warm enough for plants to live in boxes all year, you might try the "Red Riding Hood" Mandevilla - a pretty constant bloomer except for the winter months. You can combine that with trailing verbena, which will also live from year to year.

But if you don't live in a tropical climate, and you mean bloom all summer, you might try Scaevola (Blue Fan Flower) mixed with Geraniums or Marigolds. If you want you could add either the lime green foliage or dark foliage Sweet Potato Vine that will cascade over and grow down long.

Wave Petunias cascade down and bloom well although you need to prune one or two stems back per week to keep them blooming top to bottom.

If you can find Purple Heart Setcresia in your area it has purple foliage and combines well with other annuals.  

Just remember to use fresh potting soil and mix some time release fertilizer such as Osmocote Indoor Outdoor into the mix so that the plants will get fertilized automatically.

I hope this helps!

C.L.