Do I need a Contactor?
Contactors protect your timer from high inrushes. They're a must for anything that draws a lot of power upon start up.
For grow lights, you really do need one.
Some timers have one inbuilt, in which case, job's a good'un. Otherwise, you need to buy one.
Without one, your timer will likely fail, leading to your lights being left permanently on. If you notice early enough, your lighting cycle will be disrupted. I the problem goes unnoticed, you could end up with irreversible crop damage.