Q: I filled a large flower bed with the new Wave petunias last year, and they made a terrific show at first. But near the end of the summer they just quit. The plants still looked healthy, but the flowering stopped completely. — Renee Bohm, Denver, CO
A: They ran out of gas or, rather, soil fertility. Petunias like to flower extravagantly and will go nonstop until frost. But in doing so, they consume a lot of soil nutrients, especially nitrogen. Your soil was fine at the start, but the plants exhausted it. If you had applied a liquid fertilizer when you noticed the flowering beginning to drop off, they would have rebounded in about a week. After flowering stops completely, it will take about two weeks to bring them back. This year, before you plant your petunias, mix some slow-release fertilizer into the soil. If the petunias are already planted, spread it on the ground between them. I recommend composted manure, soybean meal or alfalfa meal. Cover the ground with a thin layer (1/4 inch deep or less), then work it into the top few inches with a cultivator or hoe. Then your petunias won't poop out late in the season. Still, keep an eye on the bed, especially as the weather turns cool in fall. If you see signs of a drop-off, supplement your soil with some liquid fertilizer — weekly — and you'll have tons of petunias well past the first frosts.