You can walk in and pay for tickets to enter the garden, but you need to reserve a parking spot ahead of time as their lot is quite small.
You can buy your tickets on-line or on-site. On open days (Wednesday through Sunday), if arriving in a private vehicle before 3pm, you're not guaranteed to (very likely not to) find a parking spot unless you make a reservation ahead of your arrival. On the summer Fridays that they're open late, they want you to have parking reservations even after three. Check their site for details.
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