Artemesia
(Silvermound)

Cut back by half or two thirds as soon as the flower buds form, or when the clump is just starting to open up.

Asters & Mums
(Fall flowering)

Pinching back several times before July 4th will ensure shorter, bushier plants.

 
Coreopsis grandiflora
(Tickseed)

Varieties like 'Sunray' need to be pruned back by half in
mid summer to prevent flopping.  Consider planting
shorter varieties like 'Baby Sun' or 'Little Sundial'.
 

Coreopsis verticillata
(Tickseed)

"Moonbeam" tends to get floppy as the season progresses. Cut it back by at least
half, and you will have nice, full plants and lots of buds and blooms a few weeks
later. This also works well with 'Heaven's Gate', and 'Sweet Dreams'. 'Creme Brulee'
has a more compact habit, and will only need an occasional tipping back.

 
Eupatorium (Joe Pye Weed)

This wonderful tall growing variety is great for the back of the border.  You can cut it back by one half in early June.

Grasses

Tall varieties may be cut back to 12-18" high during late June or into early July.  This will reduce the final height.  This includes many Miscanthus varieties as well as some varieties of Panicum that tend to flop.

Heliopsis
(False Sunflower)

Great impact! Trim back by at least one half in early June.

Hibiscus
(Rose Mallow)

Pinch shoots when stems are 8-10" tall, or cut back by one half in early June.

Leucanthemum
(Shasta Daisy)

 Prune tall varieties like "Becky" back by one half in early June.

Lobelia
(Cardinal Flower)

Cut back in early June before the plants flower.

Lythrum
(Purple Loosestrife)

Cut back by half in late May or early June. Cutting back by half again immediately after flowering will promote neater plants and rebloom.

Monarda
(Bee Balm)

Vigorous plant with large blooms.  Cut back by half in early June.

Nepeta

Cut back by half or slightly more early or mid summer.

Perovskia
(Russian Sage)

Prune in early June to about 6-8" high. This will help ensure nice, full plants later.

Phlox paniculata
(Garden Phlox)

Cutting back to about 4-6" late May to early June will result in shorter plants, and more blooms, although the blooms will be smaller.  Dead heading right after bloom will encourage rebloom.
 

Platycodon
(Balloon Flower)

The taller varieties tend to flop if not pruned by half during early June.  Another alternative is to plant shorter varieties like 'Sentimental Blue' or 'Fairy Snow'.

Rudbeckia triloba
(Three Leaved Coneflower)

Rudbeckia triloba is a taller variety, this one needs cutting back by a third to one half in early June to prevent flopping when it flowers.
 

Salvias
(Sage)

After the first flowering, prune back to the basal foliage. This will encourage new flowers. Compact varieties like 'Marcus' tend to flop less.
 

Saponaria tends to sprawl all over the place. After flowering, prune back by half into a neat mound.

Sedum spectabilis
(Stonecrop)

For Sedum spectabilis, cut back to about half. Low growing varieties like 'Dragon's Blood' just need an occasional tipping back to keep them full.

Larger Sedums like 'Autumn Joy', 'Matrona', and 'Purple Emperor' must be trimmed back in early June to ensure a nice, full upright plant when they flower in the fall.

Tend to get floppy after they flower, or when the temperatures warm up. Pruning back will encourage new foliage to grow.