Continual, continuous, constant, incessant, unremitting, perpetual, perennial are comparable when meaning characterized by continued occurrence or recurrence over a relatively long period of time.
Continual implies a close or unceasing succession or recurrence; continuous, an uninterrupted continuity or union (as of objects, events, or parts). As applied to a single object, continual also stresses frequently the idea of going on indefinitely, though not without interruptions, in time rather than (like continuous) that of unbroken connection or substance; thus, continual industry implies that one is always at it; continuous labor, that the work itself is performed at a stretch.
Continuous refers to both time and space, continual only to time; thus, one may speak of a continuous (not continual) expanse, but of a continual (or continuous) noise.
Constant implies uniform, steady, or persistent occurrence or recurrence and usually connotes lack of change or variation (as in character, degree, or rate.
Incessant implies ceaseless or uninterrupted activity; unremitting, unceasing activity without slackening or halting; and perpetual (see also LASTING), unfailing repetition or lasting duration.
Perennial carries the implication of existence over an indeterminate number of years; especially in older use this idea may be stressed and that of exhaustlessness often connoted.
In current use, probably because of the application of the term to plants that die down to the roots and spring up again seasonally over a number of years, the implication of continual recurrence or constant renewal is perhaps more common.