A microclimate is a climate that differs from its surrounding area. A microclimate can be as small as a garden bed or as encompassing as many square miles. Many factors can go into creating a microclimate such as elevation, water sources (such as the two Miami Rivers), or even urban settings.
Though a majority of Ohio is listed as Zone 6A, due to these various topographical and environmental differences, the Miami Valley is more realistically classified as zone 5B. Within the area itself, there are also minor differences. Due to the urban heat island effect, the City of Dayton itself will be slightly warmer than the surrounding farmlands. The Bellefontaine Ridges are higher in elevation and will have slightly cooler and drier weather conditions.
Though it is tempting to plant your annuals, vegetables, and tender perennials in April, the frost-free date is more accurately May 15th.
For frost-tolerant annuals, such as Cole vegetables, pansies, and snapdragons you can start to plant those when the chance of a hard freeze has passed. This is usually at the beginning of April, though weather patterns can change every year.
For perennials and cold tolerant annuals, like petunias, you can start planting those outside at the end of April or begging of May. However, petunias do not like nighttime temperatures lower than 39 F. Anytime the nighttime drops below 45 F you will want to cover any newly planted perennials or cold-tolerant annuals.
For all other annuals and vegetables, you will want to wait to plant outside until closer to May 15th. Starting around the first full week of May, keep an eye on the extended forecast. Once the nighttime lows stay consistent about 40-45F you should be ok to plant. Of course, always keep an eye on the weather, and if the temperatures drop below 45 F, bring in your hanging baskets and cover your annuals.