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Flowering Lawn Grasses Create Curiosity and Concern Among Homeowners

Bob Mugaas, UMN Extension Educator - Horticulture

Minnesota lawn grasses are known as cool season grasses as their peak periods of growth and activity occur during the (usually) cooler seasons of spring and fall. These grasses include Kentucky bluegrass, fine fescue, perennial ryegrass and tall fescue. The middle of May through most of June is the prime flowering period for these cool season grasses in Minnesota. Kentucky bluegrasses tend to be the first of the grasses to begin flowering with the fine fescues, perennial ryegrasses and tall fescue coming on slightly later. See Picture 1 of Kentucky bluegrass flowering.

Picture 1. Kentucky bluegrass flowering
Kentucky bluegrass flowering
Bob Mugaas
Grass flowering is an entirely normal process whether observed in a mowed lawn or an unmowed area. However, the height of the flowering stem will usually be slightly to significantly taller in an unmowed situation than a mowed lawn. The initiation of the flowering process actually begins late the previous fall when the growing point (crown) of a mature grass shoot goes through a biological change from producing stems, roots and foliage during the late summer and early fall period to one that will produce a flowering stem the next spring. The crown remains in that condition until the following spring. As temperatures warm and day length gets longer during late April and May, these shoots begin to send up a flowering stem known as a 'culm' in grass terminology.

Picture 2.  Kentucky bluegrass inflorescence
Kentucky bluegrass inflorescence
Bob Mugaas
In most instances, even if we are regularly mowing the lawn, these shoots continue to elongate in an attempt to produce their flower cluster known as an 'inflorescence'. See picture 2. The result of mowing regularly is that we often do not see the fully elongated flowering stem and hence, the lawn appears normal and we observe little to no grass flowering. Whether mowed or unmowed, or whether a flower cluster was visible or not, these flowering shoots eventually die off during late June and into early July. This is often a time when folks express concern about their lawn looking a little thin and lots of brown stemmy material in the lawn. As it is entirely normal for these flowering shoots to die once flowering is completed, lawns can temporarily look a little thinner. The apparent increase in brown stemmy material is actually the remnants of those original flowering stems. Eventually, they will fall back into the turfgrass canopy and begin decomposing leaving little to no evidence of their prior function.

Of course, not all of the shoots present in a lawn will have gone through the biological changeover to a flowering 'bud' the previous fall. Hence, our lawns have enough growing shoots present, even though the lawn may be a little thinner, it still looks and functions like a lawn. Also, by the time we get to early August, a new round of grass shoots will be starting to form along with the production of new leaves, rhizomes, tillers and roots. This growth will continue through the fall period when once again grass shoots with sufficient biological maturity will make the changeover to flower buds that will again produce next spring's flowering shoots.

Because these flowering stems temporarily disrupt the otherwise uniform appearance of a healthy lawn surface, their presence is often viewed unfavorably. The important point here is that grass flowering is a normal, temporary condition common to most lawns. There really is little that we can control within this naturally occurring process. If desired, mowing slightly shorter for a couple of times to remove more of the inflorescence can make the flowering stems less apparent. Also, increasing mowing frequency for 2 to 4 weeks during peak flowering will help keep flowering stems from becoming too visible and disruptive. However, since flowering occurs just before the warmer and drier parts of the growing season, it will be important to raise mowing heights back up as soon as possible to encourage as much root growth and rooting depth as possible before those more stressful conditions settle in.

On the flip side of the grass flowering question is whether or not any of seed produced will actually provide some 'reseeding' back into the lawn. In other words, if one lets their lawn go to seed will they receive some benefit from the seed produced in terms thickening up the lawn. The short answer to that question is usually not. Since the process of mowing continually cuts off the developing flower cluster, any seed that starts to develop doesn't reach sufficient maturity to actually be viable. In an unmowed situation such as would be the case in a seed production field, the flower stems are allowed to fully ripen, turn brown and dry. The seed is then harvested just before it has a chance to naturally disperse from that dried flower cluster. That harvesting usually occurs from mid to late in June to perhaps early July.

Picture 3.  Kentucky bluegrass
Healthy Kentucky bluegrass
Bob Mugaas
While flowering grass stems may be an unsightly disruption to an 'attractive', uniform lawn surface, remember this is an entirely natural process of our grass plants. It is also important to keep in mind that this is a temporary process that runs its course over a few weeks in mid to late spring. Once that cycle is completed, those remaining shoots that did not flower as well as newly produced shoots continue to grow and fill-in those thinner, empty spots left behind by the dead flowering shoots. See Picture 3.

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