Answer: Unfortunately, it is difficult to grow bulbs in flower boxes because they tend to freeze to death during the winter in such an exposed location. For a good flower box bulb display, it is usually better to plant the bulbs in pots, chill and then force the bulbs in those pots, and then set the pots into the windowbox for display.
Bulb forcing is part art and part science, so you would need to start several batches with a target date in mind, some a bit earlier and some a bit later than predicted. The reason for this is that the bulbs can be a bit unpredictable as to their reaction to the temperatures and then the lighting conditions in the spring when they come into growth. At this late date your selection might be dependent on what you can find available for sale as well.
Spring blooms will be a bit of the same balancing act, with seedlings responding to weather conditions and if done indoors, all the variables of temperature, light and fertility in slightly different ways. They would also need to be conditioned to the outside growing conditions prior to being planted in the box.
Since you want a display for a particular date, in my experience it would be more reliable to shop for plants in bloom at that time and set them into the window boxes at least temporarily just for that date.
Since window boxes provide limited soil and therefor very limited space, most gardeners will remove early spring plantings and replace them with a summer planting so that they always look full and fresh. An early spring planting might include bulbs along with violas or pansies or sweet alyssum, a summer planting might include impatiens, caladiums or coleus for shade and, for sun, perhaps marigolds or small zinnias, blue salvia, or even an annual dianthus. A fall planting could include mums. There are so many choices for either set of conditions, you might take into account the owners' color preferences, but the growing conditions should be a top the of your list when making the selection.
Basic supplies would include a good quality basic potting soil, possibly a water holding polymer, some fertilizer (possibly a long release formulation for flowering annuals) according to label instructions, a watering can or hose or possibly an automated system including a timer and manual override, and an organic mulch such as shredded bark.
You might wish to check directly with suppliers as to pricing. Good luck with your project.
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