Introduction to lawn and turf

Lawns are a biological system comprising soil, fungi, bacteria, invertebrates and nutrients required to support a monoculture of a desired plant species, usually a grass. They provide an amenity area, a nitrogen-rich leaf litter for composting, and a firm area underfoot for safe playing.

This information provides an understanding of the principles of lawn care and information required to maintain lawns in a green, firm and hardwearing condition. Regular maintenance of established lawns is required to maintain a monoculture of one particular grass cultivar in the face of competition against weed species. Frequent tasks are mowing and watering. Annual tasks include fertilising, controlling moss and other weeds, pests and diseases.

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History

Development of turf ryegrass

New Zealand scientists have had considerable involvement in the development of turf-type perennial ryegrass. The first true turf-type perennial ryegrass cultivars were bred in the USA in 1975. At that time the New Zealand Government was concerned that wholesale importation of inferior turf varieties could result in contamination of our own turf seed industry, so they introduced an “Acceptable Cultivar List”. Only varieties that were tested and found to have merit were allowed into the country. There was a fear that the dwarf turf- type perennial ryegrass would spread their pollen and cross contaminate our important pasture ryegrass seed industry. After careful assessment it was realised that by using normal isolation practices different varieties could be kept from cross pollinating, so that now the production of turf ryegrass seed is an important crop in New Zealand.

The first use of a turf ryegrass on sports turf was in the late 1970s when the cultivar Manhattan was imported from the USA and sown in the Basin Reserve cricket pitch. Since then we have imported turf ryegrass mainly from breeders in USA and some from Europe. New Zealand breeders have been breeding successful varieties since 1985.

In recent years there has been a significant move away from the use of fine grass species such as browntop (Agrostis capillaries) and fine fescue (Festuca rubra subspecies rubra), to the use of turf-type perennial ryegrass for lawns and turf. The main reason for this change has been the dramatic improvement in the quality of turf-type perennial ryegrass varieties. The rapid germination and quick establishment of perennial ryegrass, even in cold conditions, means that homeowners and contractors find the new turf cultivars easy and reliable to establish. The lawn can often be used four to six weeks after sowing, most months of the year.

The earliest turf ryegrass varieties were only marginally finer than pasture types. Later, the varieties became finer, denser and cleaner cutting. Modern turf ryegrass can be mowed at 12 mm and from a distance look as fine as any lawn. The colour of perennial ryegrass is usually darker than fine grasses.

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The discovery of endophyte in ryegrass

Turf ryegrass did not gain immediate acceptance because it sometimes suffered from poor persistence. In drier regions, ryegrass could disappear completely over a single summer while in other areas it appeared to perform well. In the 1980s, New Zealand scientists, along with international collaborators, found that the surviving grasses contained a fungus that grew within the plant. Because of this, the fungus was called an endophyte (pronounced “end - oh - fight”) with the generic name of Neotyphodium. The association between the plant and fungus was synergistic, as both organisms benefited. The endophyte gave the grasses resistance against attack from insect pests, including argentine stem weevil (Listronotus bonariensis) and black beetle (Heteronychus arator), and contributed to their survival. The resistance was due to the release of a number of different alkaloids, including peramine. Infected plants did not have disease symptoms. The fungus could only be passed on to non-infected turf through the seed of infected grasses, and this helped explain why only some of the grasses in the turf contained the endophyte and others did not. Its entire life cycle takes place inside plant tissues. A plant does not become infected from its neighbours, nor can it infect other plants. Since it does not affect the appearance of the grass plant, its presence can be detected only by laboratory analysis. Although seed may decrease in endophyte over time, plants that are infected maintain their endophyte fungus. It is best to plant endophyte seed within two years of harvest because the fungus deteriorates gradually over time, becoming attenuated and non-viable in three-year-old seed.

Within a year of the pasture discoveries, New Zealand turf grass breeder, Alan Stewart, working at Pyne Gould Guiness Seeds in Canterbury determined that almost all of the overseas-bred turf varieties lacked endophyte. In collaboration with Dr Reed Funk, a plant breeder at Rutgers University, USA, he soon determined that the endophyte also gave resistance to sod webworm (Scoparia species) in turf in the USA and to a wider range of other pests, making it valuable in turf throughout the world. Endophyte presence in ryegrass is now known to affect over 40 invertebrate pests, mostly those insects feeding on the lower part of the tiller rather than root feeders such as grass grub (Costelytra zealandica).

Turf breeders in the United States continue to produce a large number of new turf ryegrasses with finer leaf texture and darker colour each year, now all containing endophyte. Future research is likely to lead to endophyte strains for turf, which will provide resistance to an even wider range of insect pests than at present. Endophyte-enhanced varieties also have increased growth and vigour, making the varieties more tolerant of drought stress, summer weed invasion, and other possible turf diseases. The advantages of endophyte are most obvious during the late summer and fall months. Breeders in New Zealand have concentrated on producing varieties adapted to our climate. In particular they have led the way in introducing active winter growth into turf ryegrasses so they perform better on our winter sports fields.

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Features of turf ryegrass

Perennial ryegrass is versatile. It has excellent wear resistance and is used on sports grounds and cricket pitches. Provided it is sown heavily and kept frequently mown, it can be mown at 5 mm in tennis courts, at 12 mm in golf course tees, at 15 mm on golf course fairways, at 12-15 mm in a home lawn, at 20 mm in a cricket outfield, and at 25 mm in a sports field.

Pasture ryegrass is nothing like turf ryegrass. It is coarse and open textured, and produces lots of vertical leaf growth. The leaf tips produce stringy white fibres making turf difficult to mow cleanly, even with a sharp mower, and causes a frayed looking white colour to the leaf tips. The stalky seed heads produced by the ryegrass over a prolonged period over summer are also difficult to mow. Conversely, turf types are fine, dense and compact growing, and mow cleanly. Turf ryegrass produces seed heads over a short period and vegetative growth resumes once it is cut.

Some of the new turf ryegrass varieties are so dark they are almost black. The extremely dark colour presents the buyer with a dilemma. While the very dark colour looks good when the lawn is newly sown. Over time it will inevitably become invaded with browntop, Poa annua and other grasses to form unsightly contrasting light-coloured patches in the turf. Choosing a medium green-coloured grass means that the lighter patches of the other grasses will be masked somewhat, and the turf will still look good.

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Turf ryegrass seed quality

Much of the perennial ryegrass seed sold to homeowners is of indifferent quality. Furthermore, seed that is stored for over 18-24 months under normal seed storage conditions will lose its endophyte. Buyers wanting to ensure they purchase seed of good quality should seek the following information.

1. Is it a named turf variety that has been bred in the last 10 years?
2. Is it a variety with high endophyte content?
3. Is the seed less than 18-24 months old?
4. Has it a high germination as evidenced by a recent germination test?
5. Does the purity and germination certificate state a high purity of cultivar?

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Alternative turfgrasses

The Australian grass micolaena (Microlaena stipoides) produces hardly any vertical growth and requires less water and mowing than conventional, warm-season exotic grasses.

Exotic, short-growing grasses that produce a small amount of clippings, such as a type of couch grass code-named C118, grows horizontally instead of vertically, and also requires less mowing than conventional grasses.

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Turf growth

Turf grasses are suitable plants for monoculture because they rejuvenate continuously. Turf species have a limited height, though the leaf blade grows after cutting from an intercalary node just above the soil (Figure 1). It produces new shoots from creeping horizontal stems, thereby replacing old shoots, which die over time. Turfgrass species have the ability to cover vast areas.

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Turfgrass physiology

Turfgrass plants are made up of two components, the shoot system and the root system (Figure 1). The shoot system is responsible for manufacturing the food supply that is used by the plant, while the root system is responsible for taking in water and nutrients, and as such is more important in maintaining healthy turf.

Figure 1: The components of a grass plant.

A turf grass seed is made up of the seed coat, the embryo, and the endosperm (Figure 2). The seed coat is the protective covering on the seed, but is not hard and does not prevent germination. The embryo inside the seed develops into the new turf grass plant. The endosperm provides the food supply for the developing embryo. Once the seed has started to germinate, enzymes are released to break down the food reserves (endosperm) in the seed to provide the nutrients for growth.

As the embryo grows, the number of cells and their size increase. The first part of the embryo to enlarge and break through the seed coat is the primary root or radicle. The primary root develops into a temporary (seminal) root system, which supplies the nutrients and water for the early stages of seedling growth. Just after the primary root emerges, the plumule, which is encased by the coleoptile, breaks through the seed coat and begins developing into the first shoot. The coleoptile is not a true leaf in that it does not have the ability to carry out photosynthesis but rather is a protective sheath for the plumule. Once exposed to light, the plumule photosynthesises and continues to grow.

The growth and emergence of the coleoptile and plumule are dependent on the food supply in the endosperm until photosynthesis begins. Thus, placement of grass seed at the proper depth (2-4 mm deep) is of utmost importance since the endosperm may be used up before the new shoot can reach the soil surface and start making its own food.

Figure 2: Hypogeal (cotyledons remain below the soil surface) germination patterns and structures of a typical grass seed.

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Root system

The seminal root system is active for only a few weeks and is composed of only a few roots. It decays two weeks after germination, and the plant develops an adventitious root system, which arises from stem tissue rather than from root tissue. The adventitious roots, located at the base of the shoot, serve the plant's needs for the rest of the plant's life. This system is characterised by extensive branching and is very fibrous.

Fact: Soil supporting Kentucky bluegrass turf can contain 1000 roots/cm³, including half a million roots hairs, with a combined length of 1.2 km and a surface area of 420 cm².

Turf grass species have either an annual system, regenerating their entire root system every year, e.g. perennial ryegrass, or a perennial root system, retaining a portion of their roots in subsequent years, e.g. Kentucky bluegrass.

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Shoot system

A turf grass plant has a compound shoot system made up of a single repeating unit called the phytomer, which comprises the leaf blade and sheath and a bud at the base of the leaf sheath. The shoot does not have elongated internodes so that the phytomers are stacked on top of each other.

Peeling back the leaves of a turf grass plant will reveal that the leaves that are pulled off consist of the blade and sheath. The bud located at the base of the sheath will either not be visible or will have developed into a tiller, rhizome or stolon.

The growth of the shoot system is similar to a collapsible telescope that is made up of a series of concentric rings where the eyepiece is the innermost and smallest ring. The youngest leaf of a turf grass plant can be compared to the eyepiece of the telescope. The oldest leaf of a turf grass plant corresponds to the outermost section of the telescope. As a new leaf begins to grow, it emerges from the leaf sheath of the next oldest leaf. The shoot system of a grass plant originates from the crown at the base of the shoot. This is where all leaves originate. The growth of the leaf is due to intercalary meristems located at the base of the leaf blade and leaf sheath, and away from the apical meristem. A meristem is an area of the plant where new cells are produced by division of older cells.

Turf grass plants withstand mowing because their intercalary meristem is located below the mowing height. Many broadleaf plants cannot withstand mowing because their apical meristem is removed during the mowing process. The leaf of a turf grass plant cannot grow indefinitely. Once it has reached full expansion it will stop growing and remain below the mowing height. The newest growth is always removed during mowing.

The bud at the base of the leaf sheath can either remain dormant or develop into an intravaginal tiller, where it remains inside the leaf sheath of the previous leaf, or an extravaginal tiller, where the new tiller penetrates the leaf sheath and develops into a stolon or rhizome. All turf grass species have intravaginal tillering, e.g. perennial ryegrass, while some species have extravaginal tillering in addition, e.g. Kentucky bluegrass.

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Processes in turfgrass metabolism

Photosynthesis

Plants have the unique ability to photosynthesise; the process of converting carbon dioxide and water to carbohydrates (sugars) and oxygen with energy supplied by the sun. This process accounts for 90% of the plant dry weight whereas only about 10% is derived from minerals through the soil. Photosynthesis is summarised in the following reaction:

CO2 + H2O = CHO + O2

The carbohydrates (CHO) are then used by the plant to make energy or stored for future use. Photosynthesis is highest in bright sunlight, but cool season turf grass plants such as creeping bentgrass or Kentucky bluegrass reach their maximum photosynthetic potential at about one-third full sun.

The end product of photosynthesis is the simple six-carbon sugar glucose. Cool season plants form two three carbon sugars, which are combined to form glucose. This is called the Calvin cycle and these plants are referred to as C3 plants. Warm season grasses like zoysia and crabgrass have a different photosynthetic cycle and form an intermediate four-carbon sugar prior to conversion to the six-carbon glucose. These plants are referred to as C4 plants and have marked differences in metabolism and response to environmental stresses compared to C3 plants.

Photosynthesis decreases as temperatures increase beyond a maximum. This is because the enzymes that attach to the CO2 molecule begin to lose their affinity for CO2 and begin to attach to O2, thereby decreasing the photosynthetic rate. Oxygen is isolated from the photosynthetic apparatus of C4 plants so these plants are more efficient at photosynthesis than C3 plants, especially at warm temperatures. But C4 plants are less efficient at photosynthesis in low light or cool temperatures than are C3 plants. This explains why a C4 weed like crabgrass is not a problem in the shade or during cool temperatures in spring or autumn.

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Respiration

Carbohydrates are broken down in the process of respiration to release energy, water and CO2:

CHO + O2 = H2O + CO2 + Energy

The energy is used to maintain life in processes such as protein synthesis, transpiration and mineral uptake. Respiration occurs constantly during the life of a plant. Like humans, respiration in plants is highest at warm temperatures; the higher the temperature, the higher the respiration. Respiration is very high in C3 plants in hot weather. In prolonged hot weather, respiration may deplete all of the carbohydrates in a C3 plant. C4 plants tend to have lower respiration at high temperatures than C3. Thus C3 plants are not vigorous during hot summer weather whereas C4 plants thrive in hot weather.

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Growth

Plants grow in two ways: cell division and cell elongation. Cell growth takes an enormous amount of energy, which is derived directly from photosynthesis or indirectly through stored carbohydrates.

Fact: For cool season plants, shoot growth is at a maximum between 21 and 27°C. It is minimal between 0 and 10°C, and drops off sharply above 27°C. Maximum root growth occurs when soil temperature is 16°C. Root growth occurs as long as soil is not frozen but is minimal until soil temperatures rise to 7°C. It ceases at temperatures above 21°C.

Growth is the first process inhibited by water stress. More growth occurs at night because there is no water stress, unlike during the day when plants are actively transpiring and creating a water deficit.

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Storage

In turf grass plants, carbohydrates and other products are stored primarily in the stem bases and crowns, but also in rhizomes and stolons. These products are utilised for respiration during periods of stress to help maintain the plant when photosynthesis is not occurring or occurring at a low level. Storage products are also used whenever the plant is forced to grow, such as after mowing and nitrogen applications, recovery from a stress, or breaking winter dormancy. The more carbohydrates that are stored in the plant, the stronger it is.

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Managing the process balance

These four processes can be combined into one equation to describe plant metabolism:

Photosynthesis – Respiration = Growth + Storage

The left side of the statement symbolises net inputs whereas the right side is net outputs. When photosynthesis is high, it provides enough energy to support the other three functions. If photosynthesis decreases for any reason, the storage products must provide for respiration and growth if it is occurring.

The easiest way to maintain the maximum photosynthetic potential of a plant, regardless of weather, is to maintain the turf at the highest possible mowing height for the species.

Scalping a turf removes much photosynthetic potential and the plant must "borrow" energy from storage to maintain respiration and regrow. Repeated scalping will deplete storage, leading to poor health and possibly death.

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Cutting height

There is a direct relationship between cutting height and the total storage volume of the root system, as the turfgrass plant develops a balance between its top parts and its root system. When the turfgrass plant is mowed, the plant no longer needs the same size of root system, and the root system is reduced to achieve balance. The lower the cutting height, the shallower the root system becomes (Figure 3). A shallow root system impairs the plant’s ability to withstand drought stress, and root pruning from grass grub (Costelytra zealandica), striped chafer (Odontria striata) and manuka beetle (Pyronota species).

Figure 3: Effects of mowing height on root depth of turfgrasses.

Furthermore, cutting height influences the ability of the plant to protect itself from summer heat. The temperature-sensitive growing points (crown) of the turfgrass plants are at or near the soil surface, and are insulated to a certain extent by the surfaces of more mature leaves. Reducing the cutting height subjects the plant to a greater likelihood of high-temperature injury. As a consequence, the plant may die, and the turf gradually thins out during summer.

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Seasonal effects from fertilising

Application of fertiliser stimulates turf grass photosynthesis, respiration, and growth, but storage reserves are used. Respiration increases because there is more plant material to keep alive. Fertiliser application has different effects on plant metabolism at different times of the year due to the varying physiology of the turf grass.

In spring, photosynthesis increases because it is cool and sunny, respiration is minimal because of cool temperatures, and growth is naturally increasing, which slightly depletes storage. By fertilising under these conditions, growth is forced, increasing respiration slightly and further depleting reserves and compromising the plant later in the summer.

During the heat of summer, photosynthesis is down, respiration is up, and as a consequence growth is reduced, and storage decreases because it is being used to supply respiration. An application of fertiliser forces growth, which increases respiration and further depletes storage. Regardless of the amount of fertiliser applied, photosynthesis is not increased because of high temperatures.

In autumn, cool season turfgrasses are under the least stress and are thriving. Photosynthesis increases to a high rate, respiration decreases to a minimal amount, and storage is occurring despite the growth. Fertilisers applied at this time will increase photosynthesis but since it is fairly cool, growth will not be stimulated significantly and more carbohydrates will be stored. With average temperatures of 10°C, fertilising increases photosynthesis and since temperatures limit growth and respiration, storage is increased dramatically.

Growth is stimulated by applications of nitrogen, especially at rates above 300 g N/m². Thus nitrogen should be applied in autumn to enhance turf density and rooting, rather than in spring or summer when it stimulates unnecessary leaf growth.
By maintaining a proper balance between these four processes in the plant, turf managers can maintain a healthy turf capable of tolerating stresses. Proper cultural management is essential for maintaining these processes in the plant.

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Nutrient requirements

The 16 nutrients required by turfgrasses for optimum growth are listed in Table 1. Usually, only N, P and K are not available in the soil in quantities sufficient for good growth, and must be periodically added as a fertiliser. In acidic or alkaline soils, iron or magnesium may be bound up in the soil particles, and may be required as a fertiliser.

Table 1: Primary sources of nutrients required by turfgrasses.

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Nitrogen (N)

The turfgrass plant requires more N than any other nutrients. Grasses usually contain 4-5% of their dry weight in N. Nitrogen is a component of chlorophyll, and is crucial to the growth and developmental processes in the plant. Since the addition of N increases shoot or leaf growth at the expense of root growth, it is possible to produce a turf with a large amount of green leaf growth but with a restricted root system. Therefore, reasonable amounts of N are generally desirable so that N that is surplus for top growth is channelled into the roots.

However, high N levels produce a turf grass plant with thin cell walls, making it susceptible to attack by fungi or insects, and a high water content in plant tissue, increasing the requirement for irrigation and making the plant more susceptible to heat and drought stress. Furthermore, N-stimulation of leaf growth can deplete nutrient reserves in the roots, reducing the ability of the plant to survive dormant periods. Therefore, for these reasons alone, application of N in summer is not recommended. A good fertility programme should produce a reasonable amount of top growth, but not at the expense of root growth.

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Phosphorus (P)

Phosphorus facilitates energy transfer and storage within the plant. The roots are a primary organ for energy storage and are dependent on P levels in the plant. The demand for P is greatest during formation and germination of seed. Since the turf plant is not maintained for its seed production, its need for P is low. Therefore, turf fertilisers for established lawns are low in P. Because germinating seed has a high requirement for P, along with its property of being fairly immobile in the soil, particularly clay soils where it may take years for P to move a few centimetres, a fertiliser containing a higher level of P, such as an N-P-K ratio of 1-2-2, is recommended.

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Potassium (K)

Potassium regulates water relations in the plant. The absence of adequate amounts of K and high N levels results in thin cell walls and high water content. Increasing the ratio of K to N in the plant induces thicker cell walls and higher cell moisture content, allowing the plant to be less susceptible to fungal or insect attack, and more tolerant of droughts. Furthermore, the rate of leaf growth is reduced, thereby reducing the demand for nutrients by the leaves and allowing the nutrients to be available for stolon, rhizome and root growth.

Potassium is easily leached in the soil, and may also be lost from the plant through its leaves during rain or irrigation. Thus, K should be applied to the soil at regular intervals at a constant rate.

Acknowledgements

Thanks to Bill Walmsley, New Zealand Sports Turf Institute, for his article which was abridged for the section ‘Perennial Ryegrass for Lawns and Turf’, and Sam Wakelin for the drawings in Figure 1, Figure 2 and 3. Thanks also to Robert Lamberts for the photographs of weeds.

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