Warm and Cool

The unique seed heads of warm-season Bouteloua curtipendula.

The unique seed heads of native warm-season Bouteloua curtipendula,Sideoats grama.

Fall Is The Best Time For Planting…Except

Each year in the fall, as lawns finally perk up from summer drought, we are asked to provide flats of our native grasses for fall plantings. Each autumn we disappoint and frustrate customers by suggesting they reconsider planting certain grasses in the fall. Why not? Turns out, fall is the best time for planting many plants but warm-season grasses aren’t one of them.

Grasses are divided into two types – cool season (our typical turf grasses) and warm season (many of our native grasses). They are different, reacting differently to weather changes and the seasons and we need to treat them differently.

The red tinged leaves of Panicum virgatum 'Shenandoah' a native warm-season grass.

The burgundy-tinged leaves of Panicum virgatum ‘Shenandoah’ a native warm-season grass.

No Amount of Pampering

The warm-season and cool-season grasses both create energy from the sun in the process we all learned a long time ago, called photosynthesis. The process in which these two types of plants do this is different, resulting in differences in growth at various times of the seasons.

Cool-season grasses are also known as C3 plants. These plants are notoriously inefficient and lose somewhere between 15% and 40% of the energy made during photosynthesis due to photorespiration (a process where the plant starts fixing oxygen instead of carbon which happens on hot dry days and is extremely inefficient, resulting in the plant’s need to close pores to conserve energy loss). C3 plants fix carbon dioxide most efficiently in cool weather, loss of energy increases as temperatures increase. When temperatures get too high, the plant cannot regulate and goes dormant. (Think brown lawns in summer) Of course this is a very basic explanation – for a more in-depth explanation check out this link. 

Cool-season Bottlebrush Grass (Elymus hysteria) stays true to type by thriving in cool shady woodlands.

Cool-season Bottlebrush Grass (Elymus hystrix) stays true to type by thriving in cool shady woodlands.

Warm-season (C4) grasses on the other hand are more efficient at creating energy than cool-season grasses. When these plants create energy from light very little is lost through phoriespiration. These plants also use much less water in the creation of leaves, roots, stems and flowers than C3 plants. Because of this they are much more drought tolerant, and do not go dormant in warm dry weather. Warm-season grasses are also prompted specifically by day length and are most productive during the warmer months of summer, this means if you plant them in the short days of fall, they are not triggered to grow.

Pink seed heads of Muhlenbergia capillaris, a warm-season native grass

Pink seed heads of Muhlenbergia capillaris, a warm-season native grass

So no amount of pampering these plants will result in plants as healthy and established as those you plant at the correct time.

If Not Fall, When?

Warm season grasses grow best when day temperatures are above 70 degrees. Planting warm season grass plugs should happen in the spring or summer allowing for the longest period of time with good growing conditions. Your plugs need time to develop a good root system before taking on the harsh conditions our winters have to offer.

Warm-season Prairie dropseed (Sporobolis heterolepis)

Warm-season Prairie dropseed (Sporobolis heterolepis)

Ideally you should wait until soil temperatures are above 65 degrees and all danger of frost and freezing has past, if you plant too late in the fall, your plants will go dormant and all growth will cease until the conditions are right again in the spring. Typical around these parts it is best to plant warm season grasses between March and May.

Planting warm season grasses in the fall is risky and may result in high mortality rates, which is why we discourage using warm season grass plugs for fall planting.

The showy and unique seed heads of Blue Grama (Bouteloua gracilis)

The showy and unique seed heads of warm-season Blue Grama (Bouteloua gracilis)

Warm Season grasses are drought tolerant establishing an extensive root system (the images below were captured from the Exposed: The Secret Life of Roots exhibit at the United State Botanic Garden) to tap into ground water not available to the relatively shorter and less extensive roots of cool season grasses that thrive when precipitation is most abundant. The surprisingly extensive root systems of warm season grasses create plants that are not only drought tolerant but need little additional fertilization and tolerate poor soils. These pictures are photos taken of the exhibit photos by Jim Richardson and featured at the exhibit. See more of his excellent photos.

Planting in Fall? There are Cool Season Native Grasses for That

But do not fear, if you want to get that late planting of grasses in, there are some native cool-season grasses you can try. Native cool-season grasses tend to be shorter than their warm-season cousins and may go dormant in a dry summer without the addition of irrigation and are available as an option for you. Calamagrostis, Danthonia and Deschampsia are all cool-season grasses you can plant in fall or early spring, before the time is right for the warm season grasses to thrive.

In fact, according to information in their Grass Roots exhibit, the U.S. National Arboretum scientists are working with Poverty Oat Grass (Danthonia spicata) as a potential native cool season turf grass option. And yes, we have that!

Here Danthonia specter is on display at the US National Arboretum where it is being studied as a native cool season turf alternative.

Here Danthonia spicata is on display at the US National Arboretum where it is being studied as a native cool season turf alternative.

Of course, diversity is the key to everything – so why not try a nice mix of cool and warm in your plans? If you are dealing with cool, shady, damp sites – chances are a cool-season grass will do best for you. Have a hot, dry, sunny site to contend with – try some warm-season grasses…just don’t plant them in the fall.


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  2. […] Bouteloua curtipendula – Sideoats grama […]

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