Grow plants that need good pollination close to
aromatic plants that attract pollinating creatures.
          
Plant tomatoes by your asparagus. A substance called asparagin
in asparagus repels tomato pests.
          
Don't plant carrots near dill. They will be smaller and fewer
the closer they are to it. 
          
Radishes planted near leaf lettuce are tenderer. 
          
Garlic and yarrow enhance the production of oils in herbs. 
          
When starting seeds indoors water with periodically with
a weak solution of chamomile tea to help avoid damping off.
          
For hotter radishes plant chervil nearby. 
          
When planting carrots do not sow seeds too thickly.