In no particular order, transplanting tips and techniques from some of our Mentors:
Author of Arboriculture: Care of Trees, Shrubs, and Vines in the Landscape
Harris stresses that transplanting happens in all seasons, and that timing is not driven primarily by plant need but: "In most cases... the time of transplanting is determined by plant availability and the schedule of landscape installation."
In "Season of Transplanting" (page 229), he lists pros and cons of transplanting general categories of plants, and working in various seasons:
"...deciduous plants are most easily transplanted in the fall after the leaves turn color or drop but before the soil freezes, or in spring before growth begins.
Winter planting may be desirable for species that can withstand being moved with a frozen root ball.
Late summer and fall have the advantage of warm soil to encourage root growth...
Spring planting before top growth begins will avoid most damaging cold weather, allow some root growth before top growth resumes, and ensure ample soil moisture.
Most plants... should not be transplanted in late spring or summer, while they are still making rapid top growth."
...and E.B. Himelick, authors of the comprehensive and very practical reference, Principles and Practice of Planting Trees and Shrubs, tell us:
From "Factors that can Influence Planting* Time" (pages 52-53)
"We continue to learn from experience and experimentation as various plants are planted under different environmental conditions. ...Many plants are transplanted most easily when dormant. ...Many plants are moved more readily after terminal buds have matured. ...These species are best transplanted in spring:..."
From "Transplantability" (pages 29-30)
"Because field-grown plants are often transplanted at least once during production, they usually have a more compact root system and transplant more readily than uncultivated trees. ...
The so-called difficult-to-transplant species can often be moved very successfully if they are moved at the proper time and given the proper preparation and after-care."
If you pick up a copy of Watson's book, don't miss the appendix, Common Problems of Recently Planted Trees
*In this and many text books, planting = transplanting, since so much of what is sold has been dug from fields to be transplanted into the landscape.
Owner and for 30 years the chief propagator at Specialty Growers in Howell, Michigan. (We're privileged to have Bovio as a Forum Moderator, too.)
"...A common mistake indeed, to water before transplanting or dividing. When we transplant seedlings here, we carefully monitor soil (media) moisture in our seed trays and never attempt to transplant when the trays are wet. That's certain death, due to the extremely brittle nature of wet roots, full of moisture; they'll break right off when trying to tease them apart. Better to let them dry out a little, and they'll be much more pliant, therefore a lot easier to separate. I have never seen this important information in print anywhere else, nearly everything you read says 'water well before transplanting.'"
More of this story in What's Up 33 and What's Up 32.