Q. When should I plant hydrangeas?
The hydrangeas that you can buy in flower in grocery stores and elsewhere at this time of the year are usually mophead hydrangeas (Hydrangea macrophylla). These plants have been forced to produce flowers well ahead of their usual flowering time (August). Nevertheless you may be able to keep them going and transplant to the garden or grow in pots. After they have finished flowering prune back the branches until 2-3 leaves remain on each. If you want to continue to grow it as a pot plant, then repot it into a pot that is 2-4 inches larger. Use an equal parts of potting soil and peat moss. Keep the soil moist but not too wet. Place the plant in bright light until the weather gets warmer (early May). The plant may be then moved outdoors into a spot that ideally gets morning sun and late afternoon shade, which is important to keep the plant from getting sun scorched. The plant can be moved back to the house in late Fall.
Mophead hydrangeas are hardy to USDA Zone 5 (not colder than 10 F), so if you live in such an area then you could try growing the plant outdoors. These hydrangeas flower on old wood (i.e. the buds are formed the previous year) occasionally when a very cold winter occurs, it could prevent flowering during the following year. After flowering, cut back the plant as discussed above and when the danger of frost has passed plant the hydrangea in a partly shaded spot (morning sun, afternoon shade is best). The soil should be kept moist, particularly for the first season. Hydrangeas can grow quite large, so leave plenty of room around your plant. Mulch added, away for the trunk and on the roots, helps keep it moist.
Unless you are particularly keen to get a flowering hydrangea right now, you may be better off waiting until late spring when non-forced hydrangeas will be available in your local nursery.
You may want to check out LibGuide for further information on hydrangeas - http://libguides.nybg.org/howtosforhydrangeas