I am almost done creating an app that allows people to create a planting calendar based on garden journals created by people who live within a certain radius of a user. In those garden journals people record how many days it took to reach maturity/harvest among many other things. The whole idea behind this planting calendar app is that people who live near each other share the same average monthly temperature and rainfall with each other. Not only that but they will often also share the same soil type and soil pH. In other words, if person A finished a garden journal about tomato variety X and then determined that planting the variety on date X takes 80 days, then person B living say 10 miles (16 km) away will likely harvest after a similar amount of days, giving that person sows/plants the tomato around the same date as person A did.
Now I know that growing season A can be a little bit hotter/colder than the current growing season where person B plans to start growing tomato X, it might even be a little bit dryer or rainier than it was during season A. But overall person B will still see a similar result after about 80 days.
My question is: At what distance will the (micro) climate be so different from the climate of person A that the results of the garden journal of person A will no longer be useful for person B?
I've looked at the average temperatures of two inland points in the Netherlands and I have determined that if those points are about 50 km apart from each other they hardly differ. But if I compare a location near the sea with a location 50 km inland they do differ by a few degrees. This difference becomes significant after a distance of 20 km inland.
This is only the Netherlands, perhaps there are places in the world where larger swathes of land share the same average monthly temperatures.
I am trying to determine what the maximum radius should be that I should offer to my web application users. Right now the maximum is set to 500 km or 500 miles. But I think this may be too much and not useful to anyone. However I could be mistaken. Perhaps in the center of Russia this is a large area that shares a similar climate with similar monthly temperatures.
I would like to hear your opinion about what the maximum should be.