A reader asked, "When is the best time to to prune a Japonica?"
I did a quick internet search and found at least a dozen plants with Japonica in their name, including: Japonica Lonicera - Japanese Honeysuckle, Flowering Quince Japonica, Cryptotaenia Japonica, Euonymous Japonica and Caradina Japonica. Fatsia Japonica is commonly known as aralia. The common names for Spirea Japonica is Japanese Spirea.
So, the answer is, "It depends on which plant you have."
Here is the shorthand answer - Many spring blooming shrubs and vines are pruned just after flowering so they can make the flower buds for next year on the wood they grow this summer. Other spring flowering plants are pruned in the late winter.
If your Japonica is a Honeysuckle, prune it late winter because they bloom on new growth. If it is the Spirea Japonica, prune it next February according to The Missouri Botanical Garden's home gardening site.
Pruning can be a confusing topic. For example, Butterfly Bushes (Budlia) are pruned depending on their variety, confusing gardeners even more. According to the Tulsa Master Gardeners website, Budlia alternifolia blooms on the previous season's wood (so cut after flowering) and Budlia davidii blooms on new wood so it is cut to the ground in spring before flowering.
Could your Japonica be a flowering Quince with pink to red flowers in the spring? Its Latin name is Chaenomeles speciosa. The University of Arkansas Extension Service has an online pruning guide that recommends pruning flowering quince immediately after blooming. See page 4 at that link for a list of shrubs and when to prune them.
No matter which plant you have, pruning dead, diseased and damaged growth can be done at any time. Up to one-third of any shrub can be removed for shaping purposes. If you decide to remove entire branches of a shrub, cut out a few of the largest and oldest first, leaving the youngest branches in tact. Stand back and look, then cut more.
If this does not help, can you describe the plant? Maybe I can help identify it. Is your Japonica a shrub or a vine? When does it flower and what do the flowers look like? Once we know more about it, the advice can be more specific.