When it comes to subshrubs, however, it’s a good idea not to cut them back to the ground since this can result in the removal of their growing points and can ultimately lead to their demise. Similarly, the worst time to prune is during the cool (or potentially fluctuating) temperatures of fall and winter. The tender new growth induced by pruning is likely to be damaged or killed when cold winter temperatures arrive. The loss of the energy expended on the new growth can exhaust the plant, causing it to lose its vigor or die. Depending on the type of subshrub, the best times to prune are in the spring and, in some cases, after the first bloom. During the cooler seasons, limit your pruning to the removal of spent blossoms and dead branches and avoid cutting into live woody stems.
As for all pruning, it is important to know your plant. Learn its life cycle: When does it initiate growth in the spring? When does it bloom? When does it rest? Just remember that if you make a mistake—like pruning off all the flowers or cutting too much off and killing your plant—there is always next year to experiment again.