The many garden varieties of Hydrangea macrophylla make excellent pot plants which flower in spring. Plants are best raised annually from cuttings between March and May.
These are prepared from strong non-flowering shoots. Each should be 4 to 5 in. long and be cut cleanly just below a joint. The bottom pair of leaves should be removed and the base of the cutting dipped in hormone rooting powder. Insert them in equal parts of peat and sand and root them round the edge of a 3-1/2-in. pot. Place them in a propagating frame with a temperature of 16°C. (60°F.). or they can be enclosed in a polythene bag. Sealed with a rubber band. In this case, if the cuttings are well watered beforehand they will probably need no further attention until they have rooted in two to three weeks time.
The First Potting
The rooted cuttings should be potted individually in 3-in. pots of John Innes No. 1 Potting Compost. If blue rather than pink or redare desired chalk or limestone should be omitted from the compost and a proprietary blueing compound used instead. Note, however, that white varieties cannot be made either blue or red.
The Second Potting
When the plants have filled their pots with roots move them into 5-in. pots using John Innes No. 2 Potting Compost, but again minus chalk or limestone for blue flowers.
When well established in 5-in. pots each plant should have its growing tip removed to make it branch. This should not be done after mid-July. Cuttings rooted later than mid-May should not be stopped as side growths would be made too late to flower well the next spring. Instead, let them grow on a single stem and produce one good-sized flower truss per plant.
After stopping, move the plants to a cold frame, keeping them well watered. Watch for greenfly, and if they appear spray with derris. Remove any leaves which turn yellow or show signs of decay.
From late September keep the lights on the frames at night and a month later return the plants to the greenhouse.
The Final Potting
Well-grown plants will require a further move in October into 7-in. pots using John Innes No. 2 Potting Compost with the variation already referred to.
By November the terminal buds, from which next spring’s flowering shoots will be produced, should be well developed. It is vital that these should not be lost by cold or decay during the winter. Little water is needed now and just enough heat to maintain a temperature of 7°C. (45°F.). In January, if desired, growth can be speeded up by raising the temperature a little. A few weeks later clusters of flower buds will be seen and feeding should start with liquid or soluble fertiliser. Good plants should produce four to eight heads each.
It will be necessary to support the stems, and the stakes, which should come just below the flowers, should slope outwards to open up the plants.
When the flowers fade cut them off together with a little stem to keep the plants compact and encourage further branching. Carry out repotting now, using the same size or slightly larger pots, and a similar compost to the one used for the final potting. Place the plants out of doors for the summer in a sheltered position, or in a cold frame,the pots in sand or peat.