Hi Laura29
I used a high phosphorus fertilizer on my dahlia's actually on all my
flowers. Which is the middle number on a fertilizer, So you'd need i.e. a 10 30 20 The middle number is important for blooming also the last number is good , the last number is potash which is good to have as well. Generally found at garden centers or your local nursery. It's always good to have some high phosphorus fertilizer on hand through out the
growing season, I sometimes just sprinkle it around the soil and water in as an extra boost. Make sure your dahlias gets plenty of sunshine
.