Sometimes, your plants can have a hard time pulling phosphorus from cool soil. It's hard to notice early on, but phosphorus deficiency can slow plant growth, and as it progresses, you'll see leaf discoloration (yellowing leaves and purple veins in severe cases) starting with the lowest leaves and moving upwards. Problems generally occur early in the season and are generally corrected as warmer temperatures arrive, but if you want to help out, try digging down to your roots and adding bone meal, rock phosphate or superphosphate directly.