Question: Can I plant annuals on top of tulips? When their foliage dies I have a big gap in my garden.
Answer: The problem with planting annuals over tulip bulbs is that tulips prefer to be kept totally dry over the summer. When you water the annuals, you increase the chance your tulip bulbs will rot.
Many gardeners treat tulips as annuals themselves—pulling up the bulbs after they flower, discarding them and planting fresh bulbs in the fall. With this method you can fill the space however you like over the summer.
If you want to reuse the tulip bulbs, you can lift and store them. See tip #5 in "Tulips That Come Back."
If you want to leave your tulips bulbs in the ground in hopes that they return next year, you could fill the gap with potted annuals. Wait until the tulip leaves die completely and clip them off. Then put pots of annuals on the bare patch where the tulips stood. You can move the pot aside when it needs to be watered, so the any water draining out doesn't reach the bulbs.
A must-read for bulb lovers: Bulb by Anna Pavord.
Follow the adventures of Chris Wiesinger in Heirloom Bulbs as he rustles up the best flowering bulbs for the South.