Generally speaking the size of the root ball depends on the drip line which is where you start digging. I'd wait until the fall. The roots are fairly shallow or at least they were when I had a bulldozer pop out a 8' multiple stemmed holly last spring and replanted into a bulldozer dug hole and watered and it is performing great this year. You shouldn't have to go too deep.

I am going to wait till September and then pull it out with a rope tied to the back of my truck - after vertical spade work around the drip line.......that should work! I will have to figure out where to plant this ratty thing. Is the Jap Holly is shade tolerant?

I cut this bush back and then pulled it out - it had shallow roots so it wasnt as bad as I expected. I didnt need the rope and truck. At least I dont have to look at this ratty Jap Holly everyday anymore.

Just a week ago because many homes along our main road were purchased from county to be torn down everyone has been going nuts transplanting from these torn down homes. I found a home with very mature Japanese Holly. 6 in total, I transplanted. The only way we could successfully get these out with more of the roots was a tow strap hooked to my truck. The roots were very shallow and what I believe to be the tap actually ran only 6inchea below the soil line and ran not deep below the plant but along the side. Long as hell these roots were. But because they are in the evergreen family for a long time they can look alive. If you have ever trimmed one and its trimming which tend to appear alive you'll know what I mean. Again the tap didn't run straight down like most taps go these ran horizontal along the ground. We got it though. It's new homes hole was dug 3 times it's original size and the tap was laid out just like the original. Again like in my rose of Sharon post in another forum I will be back to inform everyone of its progress. It was transplanted this July. Not much of a choice though. Heavy equipment is due to come through and plow down everything without prejudice. I love to hear everyone's input here on their own experiences with transplants. I tend to get good ideas and advice here with folks who have done these things already. Blindly doing so can be a big waste of time. I have a neighbor who refuses to ask for help and tends to lose the battle each and every time. You know those guys who think they (from birth) just know everything! He got some Hollis as well last month and already they are showing signs of death. Poor guy!

So to help this topic along, as mentioned above I have transplanted 6 holly all approx 4foot wide by 4 foot tall on average. I have seen the majority of the root balls to be quite extensive. We had a tap root actually running horizontally and on one holly actually was 7 foot long while other taps were 4-5 foot in length. The ball again extensive but majority of the ball was under the drip line. Or in other words the ball extended only 3 foot from the trunck. Now this could be because of our mountainous, Rocky, red clay and concrete like soil properties. Tough as nails to dig! I would assume with more loose and far better soils the tap would do its typical vertical run below the ball. We saw bigger root balls when after the 2nd transplant we pulled them out rather than digging out. At first we dug at least 2-3 foot outside the drip line and found after we pulled the rest there was a whole lot more to the root ball. So the remainder of the Japanese Holly we used my truck, wrapped the trunks in a slick and thick black plastic so as not to tear the bark and ended up with far more of the root ball using this pulling process than the digging process. But that's just the way we did it. Not everyone will chose this method. We found this to be far less time consuming and far more effective in getting more of the root. So, using a tow strap we did a slip knot with plastic wrapping the trunck then put the truck in low gear and with ease (1/2 ton F150) slowly creeping pulled 95% of the ball right out. So if possible I would try to pull out rather than dig out with a spade. But that's just how we did it. If you can't get a truck in that area and or the tow strap won't reach them your only bet is to use a spade, yes. But keep in mind like we have witnessed, the tap can sometimes run horizontal and be 3 times longer then the holly is wide or tall. Our holly was 4 foot wide by 4-5 foot tall and its tap was 6 foot and some were even 7 foot long. But I live in western North Carolina. Soil content is different here than most. I hope this helps some.