Alex, you are so right on the ajuga and grass. I have the same problem with lamium that got invaded. My daughter's boyfriend is coming today to dig out the two portions of beds where just that has happened. No other solution but the dig them out, clean and separate roots and replant while vigilantly watching for any grass missed to be pulled, roots and all, at the first sign. I always rim my beds with professional edging but somehow it gets through anyway. I even use duct tape over the joints where two rolls meet but the dratted grass finds a way regardless. Drives me nuts.
I have found tiny grass slumps growing in my front bed and can only speculate that someone was spreading grass seed and it drifted into my bed. I am digging it as fast as I can before it develops deep roots. I am tempted to dig that bed up also, replant then Preen like mad to prevent germination of new seeds. Course then I cannot spread my poppy seeds. So I have to start them indoors in 6 paks and just plant each pod without trying to separate the tiny seedlings. I have a tiny pair of scissors that I use to thin the plants as trying to pull the excess seedlings just disrupts all the seedlings.
