Answer: Use one that will retain moisture but have adequate drainage. Commercial mixes usually contain milled peat moss and vermiculite/perlite (for drainage). Some are enriched with soil or compost as well. If you want to mix your own, try this recipie: 1 part each rich soil, moistened peat moss, and vermiculite/perlite. Mix in some slow-release fertilizer (such as Vitalize, available from Burpee) according to label instructions. You can vary the mix based on the needs of various flowers - some may need more moisture retention or drainage than others in order to perform well. Terra-Sorb (also sold by Burpee) is a soil additive that absorbs water for later use by plants - many gardeners find it useful for planters, since they tend to dry out so quickly in droughty weather. Hope this helps!
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