Few flowers can top the amaryllis (Hippeastrum spp.) for showiness and bright colors. A newly planted amaryllis bulb benefits from fertilizing and, if you have a bulb that's already flowered, you can keep it growing and get it to bloom again by giving it a little extra care and fertilizing it at the right time.
For a New Bulb
If you've bought a new amaryllis bulb, it's important to wait until it has grown leaves before fertilizing, because feeding a leafless bulb can damage or kill its roots.
To grow the bulb in a pot, set it into a pot that has a drainage hole and a diameter a couple of inches larger than the widest part of the bulb. Once the first leaf appears, begin a regular regimen of fertilizer to boost flower production, feeding the plant every two weeks with high-phosphorus formula, such as a 10-30-20 formula, diluted at a rate of 1/2 teaspoon in 1 gallon of water. Continue this feeding schedule until flowers have faded and the leaves start to die back, a sign the plant's ready to enter dormancy.
An amaryllis can grow outdoors year-round in U.S. Department of Agriculture plant hardiness zones 8 through 10, so if you live within this range, you can plant a bulb directly into the ground. Fertilize the plant the same way you would potted bulb, waiting until leaves appear to begin feeding with the high-phosphorus fertilizer and continuing until flowering ends and leaves yellow and die back.
For Re-Blooming After Dormancy
If you want an amaryllis to bloom again after its flowers and leaves have died back, it's essential to give the bulb at least two months of dormancy, when the bulb rests and stops growing entirely.
If you grow the plant in the ground outdoors, this will happen naturally as cool weather arrives and you stop fertilizing and watering.
If it's a potted bulb, allow it to become dormant by withholding water and fertilizer for about two months, keeping the pot in a cool, dry place such as a basement or unused room. When you see the green top of a new flower bud emerge, move the plant into a brightly lit spot and resume watering and fertilizing, using the 10-30-20 formula. Feed every two weeks, diluting it at the same rate as for a new bulb.
If you only see leaves and no bud after dormancy ends, this signals that the plant didn't store enough nutrients in the previous growing season to produce a new flower right away. With time and regular fertilizing, you might still see a bud emerge after a few leaves have grown, or it might take another year for the bulb to flower again.