Bulbs will grow well in a stump that is rotted out. Hollow out the rotted center with a spade. Use a small axe to remove the denser portions until you have enough room to meet the planting depth of your bulbs. Fill the hollowed out stump with gardening soil and plant narcissus and tulip varieties according to plant instructions. Water and fertilize your bulbs as you would with any container.
If your stump is in a shady area of the yard, such as around other trees, ferns can grow well inside of it. Ferns prefer soil similar to their environment, so be sure to add some peat moss and sand when you add garden soil to the stump. Keep fern plants moist. Ferns will grow as they do in the wild and look natural in the landscape.
Fill a rotted stump with annuals that have a lot of foliage to help the stump blend more naturally into the landscape. Impatiens and begonias are annuals that produce lush greenery in addition to colorful blooms. These plant varieties do well in containers and will thrive even if your stump is in the shade. Plant more annuals, bulbs or ferns in the ground around the stump to make it appear as part of the landscape.
If the stump is too young to easily hollow out, you can try using a small axe to chip away at the trunk. An alternative to hollowing out the tree stump is to set containers on top of the flat area. Fill the containers with plants that trail, such as ferns or trailing annuals and allow the plants to drape over the sides of the stump. Over time, the stump will begin to rot and you will eventually be able to plant inside of it.