A better and more flexile idea would be to load up the wagon with pots of any of you favorite perennials (or annuals), so you can rotate them in or out as needed. I don't really see the limitation growing flowers using the wagon. Someone shared a wonderful picture of her radio-flyer wagon load of sunflowers (all tall and annuals I believe) grown in recycled laundry detergent containers and milk jugs. Because the simplicity and unity of the display, the eyes naturally go toward the sunflowers and not the jugs. The setup looks very nice (recyling, too) but doesn't fit your requirements. The height inside the radio-flyer wagon should be taller than 5", so your shallow root requirement (4-5") isn't necessary unless you are planning for some major drainage layer. If you grow your flowers in regular 6-10" ht med or lg containers (even if way over the wood slats okay), you could plant anything you want. If necessary and for aesthetic reasons, you can just plant some trailers with your flowers to conceal the exposed pots. If you must go with the shallow root requirement, then consider the sun exposure where you will be positioning the wagon first and most (full or partial sun) and then look for the flowers (even herbs/plants) that interest you that are under 12-20" in height. There are many low-growing perennials to choose from: Rudbeckia, Salvia, daisies, Forget-Me-Not, or Goblin Gaillardia. An efficient way is to go to the garden centers or where you'd be buying flowers and seeds from and look through the flowers or the catalog or seed packets that are available to you for ideas that make the most sense for your growing conditions and style requirements. Of course, your growing choices increase if you add annuals or are willing to grow in 6" or taller planters. For example, Viola is only 6" tall, but it's an annual.

I'd go with the Bristly Locust too. It's a very small tree when full grown; kind of gangly looking but with pretty flowers. Guess it depends on what you like in your yard whether you keep it or not. I think it's kind of decorative (but then, there aren't too many plants I don't find decorative). WW Here is a link that might be useful: Robinia hispida

I feel rather dim witted right now. I never thought about the water being the culprit in making the leaves brown. I had the same problem with my "lucky bamboo" and had to start letting my water sit out for a day or two before I watered it to get the chemicals to evaporate before watering it. Makes sense if they are in the same family. For some reason it seems to love the south window. It gets plenty of light in the morning but shaded in the evening. But sadly if it is going to get much taller I will have to move it. It is in a hanging pot I made for it. And I don't think I have to worry about over watering it, it is a very thirsty plant! But thanks for the watering idea, I never thought about it.

hey most shrubs... generically speaking.. will outgrow that space very fast ... so you will be wanting to look for dwarf versions.. and even those will do the same.. but in many more years ... shrubs is a rather common ... generic term ... including all sorts of things.. including conifers ... conifers appropriate to your are would provide some nice back bones to your garden ... and provide year around color, shape and form ... visit the conifer forum and look for KC dave's posts ... as you can probably grow what he grows .... i would also double the width of the bed.. and leave at least 2 feet at the fence ... almost straight across from that tree but for my hatred of straight lines.. lol .. so that you can weed from back there.. or paint the fence in the future ... i would also use some bird netting.. and use some annual vines to breakup the boredom of the fence itself ... all this said.. dont get confused and think you can go all retentive and plan out the perfect.. forever .. garden.. just buy whatever strikes your fancy.. and stick it in the ground.. and if it doesnt work out.. get rid of it ... just do it ... i would start removing the grass.. and turning over the soil ... since it doesnt look frozen like mine is.. in MI ... prep ASAP ... and be ready for the buying season ... also.. have a soil test done.. and if you can and want to amend the soil.. you can amend the whole bed at the get go .... what is your base soil type??? sand.. clay.. etc. ... describe it for us ... ken ps: whats in the corner??? garbage cans or utilities .... if utilities.. have miss dig mark such ... and be careful .... when jumping on your shovel ...