|I just planted a gardenia|
|Your gardenia is probably just going through a little transplant shock. Any change in exposure to sunshine, humidity or watering can make gardenias unhappy. The good news is that they generally pull out of it. So keep watering as usual. Feed with an acidified fertilizer (Miracid or similar) and just give your plant a chance to adjust to the growing conditions. I think it will rebound once the roots become established.
Pull the grass out before it spreads. A thick mulch will help keep weed seeds from sprouting so you may want to add another couple of inches of mulch to discourage weeds from sprouting. Best wishes with your garden!