Monday, 11 April 2011

Cutting Back Ornamental Grasses

It's that time of year when we cut back last years old growth of ornamental grass to tidy up our beds and allow the sun to bring on this year's growth.  We usually cut back the cool season grasses first, if we haven't already done so in the fall.
Calamagrostis can be cut down in late fall when heavy frosts have killed it off.  The first heavy snow usually flattens it so you lose the winter interest.  It is easier to cut back grasses before new foliage arises than to work around newly emerging shoots.  Leave at least 6" (15 cm) of old growth.   If you've left it late and there's new growth, just be sure to cut well above that growth.  The photo of Calamagrostis 'Overdam' taken on May 3 last year shows the new growth already covering the old stalks.

Calamagrostis acutiflora 'Overdam'

We cut back Deschampsia in early fall, usually September,once they start to look ragged.  Cut it back into short, neat mounds and it will send up lots of new growth in the fall.  In the Spring we trim them back again. (This is a HINT for making cool season grasses fill out faster but don't try it on warm season grasses)
Plants that remain somewhat evergreen here like Helictotrichon sempervirens (Blue Oat Grass) & Festuca often just need the dead grass removed by running your fingers through the plant.  If there is a lot of winter kill and there seems to be more brown than blue you can trim them back and they should send up new growth. 

Miscanthus giganteus
 We usually cut back warm season grasses in the spring to about 4 - 5"(10-12 cm).  They remain standing in winter longer than cool season grasses and Miscanthus Giganteus remains up all winter.  Dormant grass left standing in the winter can keep the garden alive with sound & movement.  Birds and wildlife will visit your garden seeking seeds and shelter.  The dormant foliage will also help to insulate the plant from the cold which is important for grasses that are borderline hardy for our climate.
We use long handled pruners to cut off Giganteus.  It has a tough stem which makes great stakes to tie plants to for support in your garden. (We're making instant fences with the dried stems by tying bunches onto 1 x 2's.  I'll post a photo later)  Other grasses can be trimmed back with long blade grass shears, a bread knife or just break them off with your hands.  If you have large plants, you can tie a bungee cord or string around them first & they'll be easier to dispose of in one bunch.  Miscanthus have sharp edges - best to wear gloves!

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