Growing garlic for the first time

Sorry I can't give you a definitive answer Can, garlic needs a cold spell of under 10c for 30 nights which helps the cloves develop properly that's why your friend is a bit critical about you growing in a tunnel as for the leaves yellowing it could be the watering, which is another reason for growing outside their growing requirements of pH 4.5 to 8.5 which gives quit a wide range, but if you want to drop the pH a bit add a small amount of vinigar to a full watering can of water and soak the soil