I would say earlier the better before the temperatures get too hot. lantana is a hardy perennial that can take drier soils and hotter tems but the best time to put new plants in is before the temps get too hot. Make sure you water the plants well each day there is no rain. Don't use any type of fertilizer the first year to prevent buring the roots. Let the plants get settled and then in the fall, when temps cool somewhat, you can use a weak solution of fertilizer once a week until the weather turns colder.