sthuman has made a number of excellent points to consider. I don't have the Tamron, but my impression from what I've read is that it is generally an excellent lens. (I do own the Canon 100 mm f/2.8, but don't do a lot of really fancy macro work with it.)
While we know you want to shoot flowers, I would suggest you break the test into pieces that don't try to examine everything at once. As suggested above, if you are working outdoors, you have the possibility of motion from air currents. Combining that with flowers having a lot of curved surfaces to confuse auto-focus and the very short depth of field at high magnifications, you have quite a juggling act to deal with.
I would suggest doing some sort of table top setup indoors to check the basic lens/camera functions. Use a printed page of text or some tidbit of mechanical hardware with sharp edges and contrasty lines as a subject and experiment with focus and depth of field. Even some fairly awesome lenses get a bit soft wide open, so as suggested, targeting mid-range -- f/5.6, 8 or 11 would be a good idea. I don't see any comment, but I assume you are using a good solid tripod and cable release, as the camera needs to be really anchored for macro work at high magnification.
A way sometimes used to examine depth of field is to use a yardstick, ruler, or maybe a steel machinist's scale for a smaller target, mounted so the scale angles across the view of the camera, with the scale and numbers visible to the camera. Attempt to focus on some major division near the center of the scale and then see a) is that division indeed the center of focus, and b) how far forward or behind the scale divisions show acceptable focus to get a sense of depth of field. I've seen macros of insects where only about 1/4 of the critter's body is sharp -- it's tricky business.
After you get a handle on how the lens behaves on the camera, you can introduce more complex subjects -- like flowers!
Even for that you could start with 1 or 2 in a vase on the table top setup to eliminate some outdoor variables.
Good luck with it!
DaveT