The botanical garden is always open, and the entrance is free! Sundays in the period 9 june - 29 sept from 12 o'clock - 3 p.m. there is a free barbecue, that is the charcoal is lit and you can bring whatever you want to grill, and members of "Botanisk hages venner" (Friends of the botanical garden) sell icecream, coffee, herb tea, waffles, softdrinks and seeds of rare plants. A nice place for a walk it is, too :-)
(Text freely translated from the linked page, which contains pictures from the garden, including one taken from the air.)
Knowing the Botanical garden is open all year round, we decided to visit it. Unfortunately, at this time of year (in November) the plants are not in their best period and the garden does not have greenhouses.
But anyway, it is a nice place to walk:)