The nursery where you get the flower boxes will be able to advise you and suggest plants they have in stock so you won't have to make multiple trips. If you don't have confidence in the first person you talk to, though, ask them for a really experienced staff person. Sometimes you can get marginal advice, though most reputable nurseries have knowledgeable staff. Good luck with the sale of your home!

I assume that you are in MN since you came to this forum. Where in MN someone here would probably be able to tell you about a good nursery and or what plants will do well where you are. You will want to use both plants that are in bloom and some showy foliage plants.

Keep them watered. That may mean daily in hot weather depending on the size of the container.

Another tip I've found useful when picking out plants for containers is to browse what nurserys are selling in containers. Most times if you look close they will have the plant tags in the containers. Then you can buy the same plants (cheaper) to put in your own container.

The biggest key is going to be keeping them watered.

Are you putting your house on the market right away this spring? Or later in the summer? There are some specific plants that bloom great in the spring but then run out of steam when it gets hot. Pansies and viola come to mind.

It might also help to know what style home you have for recommending container plants.

I do not accept it that you have a black thumb! There is just something or other about keeping plants happy that you could learn. Ask someone who is an experienced gardener to plant the window boxes with you and I bet you will find out what the problem has been. I thought I was planting spring bulbs according to the directions, but when a friend and former Master Gardener watched what I was doing, I quickly discovered why my bulbs were rotting in the ground. Similarly, when I helped a novice but very enthusiastic gardener with her perennials, I found that she was just sprinkling them rather than watering them deeply, planting sun plants in the shade and vice versa, and trying to grow plants in the thin old dirt that had been under her grass instead of in good garden soil. So, if you want to, you can learn to garden!

I agree with crocosmia, Gardeners are made , not born.

I agree with all that was said, enthusiasticly, I would only add that not enough of us can stress the importance of watering and watering deeply, that means till water comes out the bottom of the container. Too many people wonder why their plants dry out when they water them every day...a little sprinkle on the top of the soil will do them no good at all, and with containers, rain is often of little use, especially when the plants get bigger, you need to put the hose or watering can into the container and soak them. Flowers will make a home look beautiful, but not if they are dead or suffering. Good luck to you, and maybe if you have fun with these flower boxes, you can visit us again when you are in your new home and want to take up our (little hobby) Helen.