If you mean you want rocks in the ground to heat up the soil, this reminds me of some modern greenhouses which run hot water pipes about 18 inches under the growing beds to heat it up in the early spring, which gets plants growing much earlier (soil has enough thermal mass it is not as useful for extending into a late season). This is very energy-intensive though, and only useful if energy is extremely cheap, and the greenhouse itself trapping warmer air is overwhelmingly more important, though the amount of sunlight is still the primary factor controlling the growing season (unless you get down to near-freezing temperatures, or are trying to grow inappropriate crops like tropical fruit, sunlight is overwhelmingly important).
A historical example of trying to do something similar to what you want might be the fruit walls which were used in northern Europe from the 16th century to grow fruit father north than their typical range.
Basically a brick wall soaks up the sun during the day and radiates it during the night, which creates a micro-climate right next to the wall, allowing a fruit tree planted up against the wall to be grown much farther north than their natural range.
This is very limited in range of effect, so only helpful for that which is trellised against the warmer stone, and even magically warmed stone would be much the same. Warm air rises so you're not going to be able to keep a whole field warm this way. This also only works to keep the night chill off of the leaves, and prevent a frost, so this approach might not be useful for what you are trying to accomplish, but it might give you ideas of how people have approached applying warm stones to growing crops.
The amount of energy which would need to be produced for hot rocks to provide a consistent mass of warm air over an entire field in a cold climate sufficient to significantly affect the length of the growing season, would have a wide range of applications if so cheap as to be spent on keeping farm fields warm.
This kind of magic would cause enormous changes in the world as it would change innumerable industries, housing, living habits... basically such cheap easy access to energy would have far better uses than trying to keep a farm field warm. The implications for other aspects of your world might be much farther reaching than you anticipated, especially if you are going for a lower-technology civilization (assumed from the presence of magic).