How to Stop People from Stealing Plants, Flowers from Own Garden and Public Park Which Only I Maintain with My Funds?

Try planting more flowers that grow back easily, and place small signs asking people not to take them. If it's a public park, talk to the local authority about adding a notice.

Another idea is to make the garden feel shared by inviting people to help care for it. People may respect it more when they feel involved.

You can't watch the garden all day, so focus on making it harder to steal from and easier for others to care about.