In March and early April, it’s time to wake up most of the plants that you put into dormancy in the basement for the winter, including tender bulbs like cannas, callas, tuberous begonias, caladiums, dahlias, gladioli, and alocasias. It’s also time to start any pelargoniums (geraniums) that were likewise stored dormant.
What to Do
If you stored your bulbs in pots, simply place the container in a sunny and relatively warm location (normal room temperatures are fine) and start watering. Gently at first, as the plant has no roots at this point: therefore, barely moisten the soil. But as growth resumes, increase your water quantity and frequency. Once the plant is in full growth, and that may only take a month, start water it according to the golden rule of watering : water deeply enough to moisten the entire root ball, then allow the soil to dry before watering again.
If the bulb was stored without soil, pot it up in a slightly moist planting mix. Position it so that the young sprouts (probably visible by now) are directed upwards. Begonia tubers prefer to be just barely covered at first or not at all: plant them so they are covered with no more than ½ inch (1 cm) of soil. Cover other bulbs with 2 inches (5 cm) of mix. (When you plant them outdoors in late May or June, you can adjust the depth according to the table above). Again, little watering will be necessary at first, because the bulb has no roots, but as the plant begins to grow, the soil will dry out more and more rapidly and you’ll have to increase the frequency of watering in consequence, according to the golden rule of watering above.
The zonal pelargonium (Pelargonium x hortorum), also known as zonal geranium, is not a bulb, of course, but many people still put insist on forcing it into dormancy in the fall, often hanging the plant upside down without soil in a cool, dark room. Quite honestly, pelargoniums hate that kind of treatment (their true nature is to keep growing all year long), but if your dormant plant still shows signs of life, repot it now, prune to remove all the dead parts and start watering gently, as explained above, until growth resumes.
When Exactly Should You Start Your Plants?
Some of the plants mentioned above need a long period of growth indoors in order to be ready to bloom fairly early in the summer, so should be started indoors in March or early April. This is the case of alocasias, caladiums, cannas, tuberous begonias, and pelargoniums. But others are faster growing: only pot them up 4 to 6 weeks before the last frost date, otherwise they tend become too tall indoors and produce weak stems likely to fall over. That is the case of dahlias and gladiolus. And if you start callas (Zantedeschia) too early, they’ll often come into bloom on the window ledge, probably not what you wanted. A 4 to 6 week head start is plenty for them!