I often get asked about those 'miffy' plants that can sulk or drop dead just as soon as you approach them bearing secateurs. I would include plants like Acers, ornamental and edible vines, lavender and Gaura.
Lavenders should be pruned every year to keep them compact, otherwise they become woody and sprawling. Pruning lavender is confusing because it flowers in mid-summer, so it doesn't obviously fall into the early summer/late summer classification, and it is evergreen, so is technically not dormant in winter. Many experts, like Monty, recommend pruning is undertaken in late summer immediately after flowering. On established plants in August, use secateurs to remove flower stalks and about 2.5cm of the current year's growth, making sure that some green growth remains. Next years flowers will be produced on these stem tips and on new growth made in the season.
I like to leave the flower stems on my lavenders, to benefit from chance seedlings that pop up in the gravel where they are planted, giving me a constant supply of replacement plants.
Some books recommend spring pruning - I have found this to be successful if timed correctly. Wait until mid spring when the weather is properly settled into warmer days and nights, and there is just a sign of growth at the stem tips. Using secateurs, trim shoots leaving 1.5-2.5cm of the previous year's growth. I also have a hunch that leaving the old stems to overwinter offers some protection from winter cold in a bad year.
Gaura lindheimeri is a bushy perennial with slender erect stems bearing small and starry white or pink-tinged flowers in loose racemes in summer and autumn. It flowers for months when properly established so getting it through winter is really worthwhile. Gaura is classified as H4 on the hardiness scale, so can succumb to cold wet winters. The trick is to leave it unpruned all through the winter and early spring - looking sad, messy and scruffy. I only prune it in late spring when I can't bear to look at it any longer! Cut hard back to produce strong bushy growth.
Japanese maples are best left unpruned unless absolutely necessary. The most graceful shape comes from a tree that has been allowed to develop naturally. Acer palmatum is best pruned when fully dormant (November to January), as maples bleed sap from pruning cuts at other times, weakening the tree. However, pruning is still best kept to a minimum - just remove badly-placed or crossing shoots to encourage a good framework of branches to form. And cut out any terminal shoots that have suffered die-back.
If you must reduce height and width, follow long stems back to a side branch and cut out at this point. I have also seen mature garden specimens which have been pruned regularly to control the size and shape - little and often, but this can only be done in summer when the foliage is established but still soft, to minimise risk of bleeding. A bit like bonsai pruning but without the miniaturisation of that discipline.
This one is simple - a bit like Acers, prune in early winter before the end of December or not at all! Vines are so vigorous that sap rises at enormous pressure in spring. Pruning too late could lead to bleeding of sap from the pruning cut, and weakening or death of the whole plant.