For all intents & purposes, the advice you received is accurate. An exception is if the plants have been in the ground for a short time - less than a year. In that case, the odds of lifting & transplanting are better, but still not ideal.

Al

You could certainly begin preparing for the project be root pruning, however.

As I hear it's been in the 90's for quite a few days now....that's hardly spring weather.
Root prune your trees and do it in the fall.....or next spring.
Linda C

Decide where you're going to translplant to, and prepare the site. Dig over the ground. Add any compost you have available.

Get your transport ready: barrow, or strong hauling tarpauline - and be sure the route to the new spot is clear. If the plant is taller than four feet - some practice with lifting gear is probably in order, and some assistance.

Root pruning: what you're trying to do is encourage the plant to make some new feeder roots closer to its trunk instead of out at its current root tips, so that when you move it, it can settle in more quickly.

In a soil with few rocks you can take a sharp spade and push it into the soil about 18" out from the trunk of the plant you want to move. Do that around only half of the circumference.

I'd leave it for a fortnight, then lightly feed with a liquid fertiliser every three weeks to a month.

Move it in autumn...It needs to go to its new place while the ground is still warm enough for the plant to make new growth in the root zone. I think I'd try extending the cut by a spade-width each month until I was right round the circle. (I'd usually do it over two years.)

When the time comes to actually move it you have to excavate enough to cut the anchor roots below - and reduce some of the branches above so there aren't more leaves to feed than there are roots to supply. Use the time now to work out what you'll do. Some summer pruning may be useful.

You might need to have stakes and ties on hand if they are biggish plants you're moving. Another 'ahead of time' task.

If your temperatures are already in the 90s, do not move anything unless it's an emergency. If you do, and you want to keep the plant, you'll be committed to watering and preventing water loss from the leaves for most of the summer. Two to three times a day. Autumn, after preparation, is MUCH better (says this lazy gardener!)

Our experience has been that you think you are going to water the transplant, but then you get busy in the summer and forget - - so transplanting in the fall is so much easier and safer for your trees. We've tried both - transplanting hollies - and the hollies survived when done in fall and looked ragged when transplanted while it was warm. Try and have patiences - it really does pay off.

I'm a little worried about some of the advice you've received. I suggest that you find some good resources regarding the processes of root pruning and transplanting and so some research first. We shouldn't be advising you, for example, to cut the roots 18 inches from the trunk when we don't know the size of the plants you are wanting to move! Pruning at the time of transplanting is also no long recommended, since it is the leaves (and only the leaves) the drive the root regeneration process!

If you do decide to root prune, I would mulch the root area afterwards and keep well watered! I would avoid fertilizing. You do not want to encourage a lot of top growth at this time which would be a stress to the plant after you've root pruned it. Rather, allow it to begin making lots of new roots inside that cut area. Water is the important thing here, especially at this time of year.

Here is a link that might be useful: transplanting techniques