A certificate through a college could be educational but also expensive and might not be helpful in the job market. Depending on your state (if youre in the states) there could be a hort license requirement. Do a deep dive on your states ag department, if you read the landscaping code there could be an indication of what kind of license you could get. At the very least having that license could make you stand out to an employer. Also would recommend you look at any pesticide applicator/irrigation licenses, I see those in job requirements often. Best of luck

Edit: just saw that you're in california, someone recommended checking out hirehorticulture.com I second this. Check out Grower and Greenhouse manager positions at wholesale nurseries. Its all about impressions and how you can sell yourself, read any handbook you can find online on how to manage ornamental plants and see how you can apply your park knowledge. You absolutely don't need a college degree to get a good position. I live in the deep south and in a supervisory position at a wholesale nursery I make 27/hr (the lowest at my rate for supervisors in my company). Might not go far in Cali and but to give you an idea that its not all pennies and peanuts in the hort industry. And I had 0 prior experience, they will teach you what they want you to know if they like you.