Answer: Lantanas grown as a summer annual should bloom all summer, beginning once it is hot and sunny and continuing all season. These might be pinched lightly to encourage branching but would not usually need trimming per se.
A lantana that has been overwintered indoors could be trimmed back to encourage bushiness, but this would normally be done in early spring when it begins to come into active growth again in response to the seasonal increase in sunlight. If the plant is unattractive due to legginess, you could still trim it back in late May, but be aware that this will delay the start of bloom.
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