This section is from the book "The Gardener V3", by William Thomson. Also available from Amazon: The New Organic Grower: A Master's Manual of Tools and Techniques for the Home and Market Gardener.
Coniferous plants may be successfully grafted at any time between the end of August and beginning of April. We prefer the end of August, or as soon as the young wood is ripened. The best mode is what is known as " side grafting " - that is, the top of the stock is left on till such time as the union is effected. If the operation is performed in autumn, the plants should be kept close and cool till early in spring, when they should have a mild heat to start them into growth: if deferred till spring, they may have heat at once. During the whole process a close moist atmosphere is absolutely necessary, and in all cases it is of great importance to have the stocks well established in pots - if a whole year, so much the better.