if we would like to do a picnic there do we need to reserve it? Don't know how many people would like to join! We are going there and we would like to invite friends!

So are the fields reserved?

You do need a reservation for the pavilions (https://www.howardcountymd.gov/pavilions), but I don't believe you need anything for a simple picnic in a green space.

Only to avoid showing up to find that the place is reserved.

if you have a large event planned you would definitely want to reserve a pavilion.

I am not sure