The long, brutally cold and windy winters in Minnesota limits the numbers and kinds of rhododendrons (especially evergreen types) that will survive in gardens. As long as the soil is acidic and evenly moist, many deciduous rhododendrons (which include plants commonly called azaleas) grow in woodland gardens in the southern half of the Gopher State. The semi-evergreen P.J.M. rhododendron, Korean rhododendron and University of Minnesota's Northern Lights series of azaleas are the best choices to survive the Minnesota climate. Choose nicely structured plants from the nursery so the need for pruning is diminished, and give the plants ample growing room so constant trimming to fit the space isn't necessary.
Prune out any dead or broken branches from the rhododendron shrub in early to mid-spring. These branches can be removed any time of year when encountered, but in mid-March to mid-April as snows melt, they are best removed so as not to spoil the upcoming flowering display. Make the pruning cuts 1/2 inch above a lower-living, healthy branch junction or dormant leaf or flower bud.
Wait until after the spring flowering display before any further pruning. Generally, rhododendrons don't need pruning since they have a naturalistic, attractive shape. Occasional errant or uneven branches may warrant trimming to make a more nicely shaped plant.
Prune off any spent (old) flower clusters from the tips of branches immediately after the flowering season ends. Carefully clip off the flowers so that only the flowers are removed, not leaves or their dormant buds. This process, called "dead-heading," may not be needed or practical if there are lots of flowers or you don't mind if some seeds are formed on the plant.
Trim back any plant branches or twigs lightly that tend to visually spoil the symmetry of the rhododendron shrub. Make the pruning cut 1/2 inch above a lower branch junction or a dormant bud or opening leaf. Use restraint, since there's no need to hack at the shrub. Never reduce the size of a rhododendron shrub more than one-third its original height since Minnesota's growing season is so short and the winter severe, according to Gerten's Nursery of Inver Grove Heights.
Allow the rhododendron to grow the rest of the growing season with no further pruning. Pruning after June 15 isn't recommended, as ample time is needed for new growth to sprout and mature. Next spring's flower buds are formed in mid- to late summer, so pruning too late actually diminishes the flowering next April or May.