Which vegetable seeds like to be planted in very warm soil: cucumber, carrots...
Throughout southern Ontario there is great variability in planting dates because of several factors including hardiness, weather conditions and micro-climates in your own garden. You are wise to use soil temperature since it is a much more reliable indicator for sowing vegetable seeds. Soil thermometers can be purchased from most any garden centre for just a few dollars. The file at the following link gives the optimum soil temperatures for various crops:
The soil can be warmed by using raised beds, cold frames, or colaches that cover individual plants. (These are also handy to protect vegetables from late frosts.) I suggest waiting until the soil reaches at least at the mid-point of the recommended temperatures for any given crop. This will speed them along in early development, and help prevent fungal diseases caused by cooler or wet soils.
If you have any other questions, please feel free to write again.