"In the spring, gardeners are numerous."
Translation:Au printemps, les jardiniers sont nombreux.
With seasons, it's fairly easy to remember. To say "in" + season, use the following:
- En été
- En automne
- En hiver
- Au printemps
Note that the spring is the odd one out. If the action is habitual, you may also use the definite article: l'été, l'automne, or l'hiver. But au printemps remains the same. And no, "dans" doesn't work with any season.