Both of Weigela and Deutzia bloom on old wood. In the New York City area, pruning too late after flowering, or in the spring before they bloom, may destroy the buds for the coming season. They are among a sizeable group of plants that should be pruned right after they finish flowering, within one month to be safe. 

Here are other shrubs that fall in that same category and should be pruned within one month following flowering:

Prune after flowering (early blooming or flowers on year-old wood):

Berberis (barberry)

Calycanthus (Carolina allspice)

Chaenomeles (flowering quince)

Chimonathus (wintersweet)

Deutzia (deutzia)

Forsythia (forsythia)

Fothergilla (witch alder)

Hydrangea macrophylla, unless it is an everblooming cultivar (big leaf hydrangea); for everblooming varieties, see our Guide to Hydrangea for information on pruning

Hydrangea quercifolia (oak leaf hydrangea)

Itea virginica (Virginia sweetspire - if desired, hard prune in late winter, when you can see what you're doing)

Kolkwitzia (beautybush)

Philadelphus (mock orange)

Rhododendron (rhododendrons and azaleas)

Salix (pussy willows)

Spirea x arguta 'Bridal Wreath', S. thunbergii & S. veitchii (early blooming spirea)

Syringa (lilac)

Viburnum (viburnum)

Weigela (weigela)

You can find more information about pruning in our guide Introduction to Pruning. More information on some of these shrubs can be found in our Guides to Lilac, Viburnum, Hydrangea and Rhododendron.

Courtesy of NYBG Plant Information Service