Lavender cannot be divided; it is actually a small shrub. In my experience, except for small plants, it transplants very poorly. Good luck.

I have also heard it doesn't transplant well. I got away with it this year with a plant that was about a year old, but it wasn't happy.

On the bright side, they seem to grow very quickly. This is the second year on my Hidcotes, and they are full-sized from a 4" pot planted last spring. Maybe you could try to dig as much of the root ball as possible, and replace them if it doesn't work out?

I have no problem transplanting them. When I put in a lavender garden I purposely plant too close together for the first year. The second year as they fill out in the spring I remove every other one to a different location. I have never lost any plants doing this and no pruning is needed. If they were in bloom I would cut them back, however. Al

My experience with transplanting or dividing lavender is the same as lacyvail's - it doesn't like it and perhaps even less so at this time of year. Very young plants may fair better but "bushel" sized will sulk at best if not just give out. They are inexpensive plants and grow fast. I'd consider just replacing them.

Any Lavender 5 or more years old should be replaced, rather than transplanted. They get too woody and are not attractive at that point. Our neighbor was in the lavender products business with four acres of lavender and replaced her plants every 5 years. Al

A friend told me different species of them behave differently, but my experience is not much luck. I bought one labeled just 'lavender' in a 3" pot last year, didn't plant it, didn't baby it over winter (sat on porch) and then this spring, to my surprise IT CAME BACK. Left it alone to establish, it came back in full force, finally planted it -- and now it's dead.

Point being. It could take all the neglect and abuse it got from being forced to live it a 3" pot over winter and sitll survive, yet it could not handle being put into the ground a few weeks ago, BEFORE it got really hot (and I kept it watered). So that's my experience.

Can't tell you the issue, but if Birds & Blooms has a search on their site, try that. Probably 2009. I remember an excellent article about growing lavender, how to properly plant, etc. Maybe it has some hints????

Good Luck !!!