Steady state cardio (aerobic) is a far cry from high intensity (anaerobic), and gardening is a cake walk compared to HIIT.
Do not do HIIT on days where you are strength or resistance training. That's a double anaerobic blast to your body and your CNS won't appreciate being abused like that (especially if you are doing HIIT properly -- the fact is, most people don't do HIIT or Tabata Protocol properly).
You should be absolutely exhausted after 15 minutes of HIIT or you aren't doing it properly. I wouldn't do HIIT more than 2 times a week and I used to be a sprinter.
So my advice to you is read the sticky, find more information, and see whether or not your doctor says it's OK to pursue.