Ask Mr. Smarty Plants

I'm not sure what the tall clover you describe is, but I'm pretty sure the shorter plant is bur clover. This obnoxious import is having a banner growth year here in central Texas and many areas are overrun with it.

Both bluebonnets and bur clover (Medicago spp.) are in the same family (Fabaceae) so they have a lot in common. You've already observed that they like to grow in the same area and that when one is having a good growth year, so is the other. This also means that a herbicide that is effective on bur clover will probably also get your bluebonnets.

Mr. Smarty Plants can't offer a nice, neat solution to your problem, but here are a couple of paths for you to think about:

Go ahead and apply a preemergent herbicide in late summer or early fall, then reseed bluebonnets a month or so later in the hope that the bur clover will be killed and that a new cop of bluebonnets will come up.

Be patient. Bluebonnets have been blooming aound here for thousands of years. Bur clover is a newcomer. My bet is that the bluebonnets will win in the long run.

If you decide to go the preemergent pesticide route, here are a couple of ideas

Here is an article from the Texas A&M Extension service that lists several preemergent herbicides that are effective in controlling clovers including bur clover.

Many gardeners, who prefer not to use synthetic herbicides, use corn glutem meal to control annual weeds. Here is an article from the University of Minnesota Extension Service about the use of corn meal gluten.