The yobs threw 800 plants, including Geranium, Verbena and Cordyline, into a fountain in Marine Park Gardens in Aldwick, Bognor Regis.
Workers from contractor ISS had almost completed the gardens when the incident occurred, said contracts manager Dave O'Hare of the two 50m-long beds that were damaged.
"All the hard work by the gardeners in preparing the beds for this year's Britain in Bloom competition was destroyed," he explained, adding that the team subsequently had just one day to replant the beds.
"We have careful budgets and pre-order nursery plants. We will struggle and have to switch things around. It's a lot of plants to lose, especially at the season's start."
O'Hare said there was no closed-circuit television in the park and patrols may now be stepped up. Last autumn vandals were cautioned by police.
He added: "More patrols are needed but we are virtually powerless to do anything. It is difficult to get a prosecution, which is extremely frustrating for our guys working flat out. However, we're not going to let them beat us. My team worked very hard over the next couple of days tidying up the area and completely replanting the beds."
Arun District Council parks manager Oliver Handson said: "The vandals may think this is a victimless crime, but it is soul-destroying for the gardeners."
Marine Park Gardens, which is near the seafront, has used green and seasonal bedding in its planting scheme. The park won silver gilt in Britain in Bloom last year.