Selecting Cool Season Lawn Grasses
Figure 4.1. Kentucky bluegrass lawn area.
Kentucky bluegrass is the most widely planted cool season grass planted in northern portions of the United States. Kentucky bluegrass has a number of desirable adaptations over other lawn grasses. The strong rhizomatous nature of Kentucky bluegrass allows it to quickly establish an area and repair itself from damage due to pests and use. Kentucky bluegrass will form a relatively dense lawn of good green color that mows cleanly (Fig. 4.1).
Kentucky bluegrass is adapted to a wide range of environmental conditions but does best in soils of moderate to high fertility and soil pH of 6 to 8. When mowed at 2 to 3 inches, Kentucky bluegrass is very competitive against weed invasion. It is best suited to lawn areas receiving full sun to very light shade.
Two disadvantages of Kentucky bluegrass are its relatively shallow root system under mowed conditions and relatively high demand for water. Older, "common" types of Kentucky bluegrass have the ability to survive extended drought periods by going into dormancy. During summer dormancy, the plant can lose its leaf tissue (as evidenced by the plant turning brown) and most of its root system. However, the crown and rhizomes remain alive for much longer and are responsible for the recovery observed when watering or rainfall is resumed. During extended periods of very dry conditions and very high temperatures, some water may be necessary to prevent permanent injury to the crowns and rhizomes. Once these are permanently damaged, the plant will not recover even if watering is resumed or rainfall becomes more frequent. A more detailed discussion on managing lawns during summertime hot and dry periods as well as summer dormancy is discussed in greater detail in Chapter 9 - Watering Practices.
Many varieties of Kentucky bluegrass have been developed, each differing in aggressiveness, tolerance to low mowing heights, resistance to various diseases, shade tolerance, and level of management required. Although all the differences in individual varieties are beyond the scope of this program, it may help to group some of the varieties according to suitability for low or high maintenance lawns.
Kentucky bluegrass varieties are often divided into two groups: common types and improved or elite types. Common varieties or "public varieties" as they are known in the seed industry, are older varieties (or selections from older varieties) that have been around for many decades. Up until the late 1960s, common types of Kentucky bluegrass were the predominant types grown in home lawns (Fig. 4.2). Table 4.3 lists the major plant and use characteristics of common bluegrasses.
Figure 4.2. Visual comparison of common Kentucky bluegrass (L) and improved Kentucky bluegrass (R).
Table 4.3. Plant and Use Characteristics of Common Kentucky Bluegrass Varieties
Use Characteristics of Common Kentucky Bluegrass
For the most part, improved or elite Kentucky bluegrass varieties have been developed in the last 50 to 60 years. Since most of the introductions to date have been selected under systems of generally high fertility and ample moisture, many have limited use in low-maintenance landscapes. However, there is an increasing amount of research being conducted to improve the water and nutrient conservative nature of improved types. As this continues, it is likely that improved varieties, with their superior disease tolerance, will also have increasing levels of tolerance to environmental stresses and lower fertility levels. Table 4.4 lists the major plant and use characteristics of improved bluegrasses.
Table 4.4. Plant and Use Characteristics of Improved Kentucky Bluegrass Varieties
Figure 4.3. Kentucky bluegrass texture typical of improved types.
It should be noted that where the landscape situation calls for the maintenance of a green lawn surface throughout the growing season and ample moisture and fertility can be provided, then it would not be a sustainable turfgrass choice to select common types of bluegrass for that situation. Under those conditions, common types of bluegrass are more prone to disease and generally do not perform as well, thereby potentially increasing weed invasion and an increased need for fungicides and herbicides.
Those improved cultivars adapted to higher and lower maintenance levels are based on research at University of Minnesota (UMN) as well as other Universities across the country. For Minnesota, results of their cultivar evaluations can be found at UMN’s Turfgrass Science Program website, www.turf.umn.edu . To view cultivar trial results, click on Research, then Cultivar Evaluation, then the trial you would like to view. Trials are organized by variety (e.g., Kentucky bluegrass, fine-leaved fescue, etc.) and year.
Additional cultivar evaluation information can be found through the National Turfgrass Evaluation Program [http://ntep.org/] (NTEP). Directions for finding information and how to use the data provided to determine good varieties and/or species for particular characteristic(s) are provided at the site.
Figure 4.4. Rough bluegrass lawn.
Rough bluegrass is a lighter green, stoloniferous turfgrass that is quite well adapted to moist, shady conditions of this region.
Figure 4.5. Shiny color and finer texture of Rough bluegrass.
In fact, it is often included in grass seed mixtures for shady areas, particularly those advertising for use in ‘dense’ shade. Where conditions are favorable for its growth, it can make a very good, vigorous turfgrass cover in areas where Kentucky bluegrass and the fine fescues would have a difficult time surviving or at least being able to provide a healthy, dense grass cover.
When included in a shady grass seed mix, it is often combined with some fine fescues and shade ‘tolerant’ Kentucky bluegrasses. If the site is more moist, that will favor the survival of rough bluegrass. If the site is on the dry side, the fine fescues will be favored and rough bluegrass will likely disappear over time from that lawn area. Figure 4.6 illustrates the darker, more blue-green color of the Kentucky bluegrass along with its somewhat coarser texture.
Figure 4.6. Texture comparison of rough bluegrass and Kentucky bluegrass.
Because of its stoloniferous habit, it can tolerate lower mowing heights than Kentucky bluegrass so long as ample moisture is maintained. However, at the higher heights of cut, (>2.0-2.5inches), common to most home lawns, rough bluegrass stems tend to lay over flat along the ground giving a more mat like appearance which some people find unacceptable (Fig. 4.7).
Figure 4.7. Higher cut of rough bluegrass.
It is this more horizontal growth habit that sometimes pushes rough bluegrass into the weed category when it occurs in patches in an otherwise Kentucky bluegrass lawn. However, its survival is linked to the presence of a uniform, adequate supply of moisture as it has very poor drought tolerance. When moisture is inadequate, rough bluegrass will turn brown rather quickly causing folks to think they may have a disease or insect problem when the real cause of the problem is rough bluegrass’ inability to tolerate dry conditions.
Rough bluegrass will tolerate growing in the sun so long as adequate moisture is available. In fact, it sometimes will show up in patches in well watered, sunny Kentucky bluegrass lawns. When it does occur in these situations it can be a very difficult grass to remove. At present, the persistent use of non-selective herbicides combined with reseeding or resodding of the dead areas are the only approaches to rid one’s lawn of this grass. One can limit its spread by growing the lawn a little drier, raking out the brown rough bluegrass, and persistently reseeding those areas to try and reestablish Kentucky bluegrass as the dominant grass species. However, if consistently moist conditions return, the likelihood of this grass returning is also quite high.
Rough bluegrass has very poor wear tolerance when compared to Kentucky bluegrass. Hence, even though it has good shade tolerance under moist conditions, it will not provide a good surface for heavy wear and tear.
Figure 4.8. Fine-leaved fescue lawn appearance.
The term "fine-leaved fescue or fine fescue for short" is generally applied to three similar species commonly used in our lawn seed mixes. There are creeping red fescue (Festuca rubra subsp. rubra), chewings fescue (Festuca rubra var. commutata) and hard fescue (Festuca longifolia). Occasionally, sheep fescue (Festuca ovina) is utilized in mixes to be used in very low-maintenance areas. Creeping red fescue does spread by rhizomes but is not nearly as aggressive as Kentucky bluegrass. Chewings fescue and hard fescue are considered bunch-type grasses as they lack rhizomes or stolons and spread primarily by tillering. Table 4.5 lists the major plant and use characteristics of fine-leaved fescues.
Table 4.5. Plant and Use Characteristics of Fine-leaved Fescues
Fine-leaved fescue has a fine to very fine texture and a medium to dark green color (Fig. 4.9). Note the slender blades of fine-leaved fescue compared to Kentucky bluegrass (Fig. 4.10). A similar contrast exists between fine-leaved fescue and that of perennial ryegrass and turf-type tall fescue.
Figure 4.9. Fine-leaved fescue texture.
Figure 4.10.Texture comparison of Kentucky bluegrass and fine-leaved fescue.
Fine fescue species and their respective cultivars are well adapted to our northern climate. There should be little difficulty in successfully growing the fine-leaved fescues in our area. They are also good to include in a lawn seed mixture when introducing low-maintenance varieties into an existing lawn or establishing a new lower maintenance lawn. Fine fescues are not normally sold or grown as sod.
Fine-leaved fescues, particularly the hard fescues and sheep fescues are often primary components of no-mow turfgrass seed mixtures (Fig. 4.11). Creeping red fescue types and Chewings fescues are also frequently included but usually at smaller percentages of the mix.
Figure 4.11. Fine-leaved fescue used in a no-mow situation (within yellow line).
The term no-mow can be somewhat misleading. While these grasses are chosen for their slower growth rates and somewhat shorter stature, mowing them is still possible although not necessarily required; in fact, these species are excellent for lower-input lawns in Minnesota.
Oftentimes, no-mow areas are still mowed once or twice during the season to help keep plants at slightly shorter heights and encourage a little greater shoot density. Typically that mowing is done either in the fall (about mid-October) after growth has slowed or in the spring just prior to new growth beginning (about mid-April). When done in the fall, this will also help prevent excessive matting of the longer grass blades. In the spring, mowing will help chop and shred the brown and dead winter killed leaf tissue giving more exposure to the grass crowns and encouraging a little quicker start to the growing season. Nonetheless, mowing heights should be no shorter than four to six inches.
Excessive clippings (Fig. 4.12) resulting from the mowing should be removed from the grass surface or gone over with the mower a couple of times to help re-cut and disperse the clippings down into the grass canopy to the soil surface where they can be slowly decomposed. If a thick layer of clippings are left on the surface, too much shading of the plants below can result in slow, weak growth or even death of the underlying plants due to lack of light. Excessive vegetation lying on the turf during the winter can also result in increased incidence of snow mold disease.
Figure 4.12. Excessive clippings left on the lawn are detrimental to healthy turfgrass growth.
Sometimes mowing is done just after the plants have finished flowering (around mid-June). This helps eliminate the taller flower stalks that can reach 20 to 24 inches tall, which some find objectionable. Mowing heights at this time of year should be no shorter than four to six inches in order to avoid significantly stressing the grass plant.
Fig. 4.13 shows the fine fescue no-mow plot and flower stem heights at the Rosemount Research and Outreach Center in Rosemount, MN. Excessive clippings should either be raked off of the surface or re-cut with another pass or two of the mower to disperse them down to the soil surface. It is not a good idea to leave excessive amounts of clippings on the grass surface for the same reasons as mentioned above.
Figure 4.13. No-mow flower stem height can reach 20-24” tall.
Another characteristic that some may find objectionable is that these grasses tend to flop over as their leaf blades grow taller. This creates a swirling appearance to the grass stand and can be more difficult to mow (Fig. 4.14). In fact, plant heights do not have to be that tall to exhibit this effect as can be seen in Fig. 4.15 where a lawn dominated by the fine fescues has been maintained at a three to four inch mowing height.
Figure 4.14 Swirling appearance of no-mow fine-leaved fescue.
Figure 4.15. Fine-leaved fescue showing similar swirling pattern even when mowed at 3”-4”.
Fine-leaved fescue no-mow mixes can be very good choices in areas where it is difficult to mow or where a reduction in annual mowing expense is desired. When healthy and dense, they are very good at keeping invading weeds to a minimum. Because of their slower growth and spreading habits, they can also be used as a transition plant material between natural areas or gardens, such as prairies or prairie gardens, and regularly maintained lawn areas. While not necessarily required, an annual application of one pound of nitrogen fertilizer per 1000 square feet of area applied around Labor Day will help sustain the growth and vigor of these grasses when used in a no-mow situation.
Figure 4.16. Bunch-type growth habit of perennial ryegrass.
Figure 4.17 Perennial ryegrass maintained at a mowing height of 2”-2 ½”.
Perennial ryegrass is a cool season, medium-textured, wear tolerant, bunch-type grass. It is a medium to high-maintenance grass well adapted to areas receiving high amounts of foot traffic such as athletic fields or intensively used backyards. The underside of the perennial ryegrass leaf is quite shiny and smooth (Fig. 4.18). This characteristic gives perennial ryegrass an almost shiny, glistening appearance compared to Kentucky bluegrass (Fig. 4.19) and fine fescue (Fig. 4.20).
Figure 4.18.Perennial Ryegrass
Figure 4.19. Kentucky bluegrass
Figure 4.20. Fine Fescue
One of the biggest drawbacks of perennial ryegrass usage for some parts of the northern plains region is its poor cold tolerance. It is considered the least hardy of our cool season lawn grasses and can thin out significantly or be completely killed during cold, open winters. Table 4.6 discusses the plant and use characteristics of perennial ryegrass.
Table 4.6. Plant and Use Characteristics of Perennial Ryegrass
Perennial ryegrass is known for its rapid germination and establishment and is therefore useful where quick repair and establishment of a turf cover is needed. A related drawback of this characteristic is that it can quickly shade and overpower slower germinating grasses such as Kentucky bluegrass and fine fescue seeded at the same time. To the unobservant, the bluegrasses and fine-leaved fescues may be totally lost from the stand due to the excessive competition from perennial ryegrass.
Because of its bunch-type growth habit, it does not spread rapidly. When seeding areas with mixes containing more than 50 percent perennial ryegrass, increased seeding rates will be necessary to quickly cover an area. If perennial ryegrass is allowed to thin out due to improper maintenance, the turf becomes bunchy and loses quality. If the degree of soil exposure is increased on sloped areas due to thinning out, there is an increase in the potential for erosion and thereby the potential for pollution of nearby lakes, streams, and rivers. Remember, grasses are one of the very best stabilizers of soil when they are maintained in good health and completely cover the soil surface.
Figure 4.21. Turf-type tall fescue lawn.
Up until the last decade or so, tall fescues were generally not recommended for Minnesota lawns, primarily due to hardiness issues. In addition, older varieties of tall fescue such as Kentucky-31 had very wide leaf blades and a medium to lighter green color that did not match very well with other lawn grasses. However, recent breeding efforts have created turf-type tall fescues with darker green color and significantly thinner leaf blades that do match much more closely with improved Kentucky bluegrasses and perennial ryegrasses. Figures 4.22 and 4.23 compare a leaf blade width of a turf-type tall fescue with that of an older tall fescue type Kentucky-31 as well as those of other lawn grasses (Figs. 4.24 and 4.25).
Figure 4.22. Turf-type tall fescue and Kentucky-31 tall fescue texture comparison.
Figure 4.23. Turf-type tall fescue and fine fescue texture comparison.
Figure 4.24. Kentucky bluegrass and turf-type tall fescue texture comparison.
Figure 4.25. Turf-type tall fescue and Kentucky bluegrass mix.
Newer cultivars have also demonstrated hardiness levels that may be sufficient for Minnesota. It is worth noting that even the modern day varieties are not tolerant to a solid ice cover especially during the late winter and early spring freeze/thaw periods. Extensive dieback can still be experienced following those conditions. In a practical sense, areas along curbs, driveways or sidewalks where snow melt water may collect and refreeze would be areas very likely to experience dieback of our tall fescues (and in some instances our other lawn grasses as well).
A second potential hardiness problem with tall fescue is that young stands of the species do not survive as well through the winter as do mature stands; therefore, it is recommended that any tall fescue lawn be seeded in the spring rather than the fall.
However, the newer turf-type tall fescues are being used more often and do seem to be performing well in athletic fields, home lawns and park areas. Tall fescues are very tolerant of wear and traffic and with their much deeper rooting; they are very good at surviving the often hot dry conditions of our Midwestern summers.
Tall fescue is observed to perform better and maintain good lawn density when mowed at heights greater than 2.5 inches. While able to get by on lower nutrient requirements, at least one to two pounds of N applied annually over the course of the season is still considered beneficial for the average home lawn. If two applications are done annually, an early to mid-May and an early to mid-August application of one pound of N per 1000 square feet of lawn area would be good timing of those applications. Additional N may be needed where the tall fescue is subjected to greater wear and traffic conditions.