Does this mean new growth is bursting out? Or does it mean just Spring time?
There may be some misunderstanding of what is actually going on here. In a citrus tree, xylem sap flows throughout the day, but stops (or nearly stops) at night, year-round. Xylem sap is the water supply coming up from the roots, carrying fertilizer nutrients with it. Phloem sap, on the other hand, moves more or less continuously in the tree, carrying sugars, amino acids, hormones, and other plant metabolic products around. There is nothing that happens at any time of year in a citrus tree that would be analogous to the "sap flow" in something like a sugar maple tree, in which massive amounts of stored starch in the roots are converted to sugars and transported upward for use by the spring growth flush.
Some methods of grafting work about as well on any day of the year -- chip budding and cleft grafting, for example.
However, there are grafting methods that rely on the bark of the rootstock plant "slipping," meaning that it peels cleanly away from the wood (T-budding especially, the most popular of all methods of propagating citrus). In this case, there are definite "good" and "not so good" times to do the operation, and in that case, it generally happens when the tree is in a major flush of new growth. It's not really related to "sap;" rather, it is the fact that the vascular cambium, which is a very thin layer of cells between the bark (phloem) and wood (xylem) is actively dividing new cells, in the process of making new wood and bark, as the stem thickens during the growth flush. Those new cells are softer and not as tightly "glued" to their neighbors, and so when you cut into the bark and pry it back, the bark "slips" as the cambium layer tears. Growth of the vascular cambium is stimulated by auxin hormone (3-indoleacetic acid) flowing downward from the new growth, through particular cells in the phloem tissue. So it occurs in a wave, somewhat later than the beginning of the top flush, and ending just after the end of the top flush. It's easy to test for -- you simply try to cut and peel a bit of bark. Either it will peel as easily as a ripe banana, or it will hang on tightly and you'll just be scraping splinters.
Malcolm M. Manners, Ph.D.
John and Ruth Tyndall Professor of Citrus Science
Florida Southern College
Oh my! Thanks for the detailed information!
Malcolm, your explanation of 'sap flowing' and 'bark slipping' is very valuable to me. It explains why my cleft grafts are more successful than my T-budding.
How does one encourage sap flow, or new growth, in rootstock so that budding is more successful?
John, You can just wait until the tree naturally flushes. But in a commercial nursery situation, they tend to carefully give entire blocks of seedlings the same care -- same fertilizing schedule, same irrigation, same environmental conditions in the greenhouse, etc., and that tends to synchronize their growth. Even then, there will be a percentage of the seedlings that are not in synchrony, and so they are either set aside, or more often, chip budded (which does not require slipping bark, but does take a bit longer to do)