Healthy grass roots in the home garden
Turf grass is one of the most popular choices for outdoor surfacing. Grass is attractive, cool underfoot, stops dust and mud and is readily added or removed from an area should its use change. In addition, it has the benefit of reducing the surface temperature in the surrounding area through its natural evaporative cooling effect, a by-product of transpiration.
In turf, what is seen on top more often than not reflects what is not readily visible – the root system. In grass, roots are specialised plant structures whose main function is to supply anchorage, water, nutrients and some plant hormones to the rest of the plant. In addition, green couch (Cynodon dactylon and its hybrids) and kikuyu (Pennnisetum clandestinum) have specialised underground stems, called rhizomes, which are capable of forming new shoots and roots and contribute what lies beneath the soil. The health and vigour of the root zone is a primary determinant of the quality and vigour of what grows on the surface, which consists of stems, leaves and stolons (above-ground stems capable of forming shoots and roots).
It surprises many people that in unrestricted conditions, grass roots can easily reach a depth of 20 to 30 cm. This is most commonly seen in well-watered sands. However, many domestic garden soils in South East Queensland are clay-based and hard and the turf is shallow rooted. Unfortunately, on some new estates, developers remove the rich surface soil, exposing less desirable and much harder sub-surface soils. A shallow surface layer of topdressing soil may have been applied before laying the turf in rolls. This results in the root zone of the new grass being restricted to the shallow surface layer. These roots find it difficult to maintain adequate water and nutrients for good growth. It is similar to trying to grow grass over rock! Not surprisingly, the shallow rooting depth gives disappointing results for the homeowner.
In areas with shallow soils, rock and recently cleared trees and shrubs, early preparation prior to laying turf is highly desirable. Gypsum can be used to break up the soil, organic manures can be added, and the pH should be corrected. The area should be rotary-hoed and raked level prior to planting. If necessary, additional topsoil should be brought in and added.
In established but poorly performing lawns where soil depth is restricted, the strategies are similar, though the process is slower. To begin with, the lawn should be aerated. This will improve the penetration of water into the profile, carrying with it lighter dressings of gypsum, lime and organic matter. In time, the rooting depth should improve, giving the grass greater resilience.
Soil compaction is one of the biggest, but unpublicised, problems in South East Queensland lawns. In heavily trafficked areas, the best option is often to introduce some hard surfacing, creating pathways which subtly direct people away from adjacent areas of grass. In addition to making it difficult for roots to anchor, compaction reduces root zone aeration, while growth above ground is slower.
The root system requires a continuous supply of oxygen for good health. Unlike above-ground growth, roots are incapable of producing their own oxygen. Vital oxygen supplies are normally sourced from pores in the soil and other spaces such as worm holes and decayed root channels. Essentially, in a compacted soil, the root system is unable to breathe properly.
Aeration restores oxygen supplies to the roots. This is commonly done using a drum or belt of 8-10 cm long spikes or corers driven into and withdrawn from the soil at 10-14 cm intervals (hence the term ‘coring’). Light work can be made of this job by hiring a motorised lawn aerator or outsourcing the job to a professional contractor. After aeration, roots are able to spread more freely through the more friable soil profile without the previous physical restriction.
In poorly drained soils, water replaces air in the spaces, again depriving the plant of its vital oxygen supply. Where waterlogging is likely, subsurface drainage can be installed. Species that are more tolerant of wet soils can be used, such as broadleaf carpetgrass (Axonopus compressus).
Thatch (Figure 1) is the ‘cushioning’ layer of bare stem and other organic matter between the grass leaves and the soil. Some thatch is beneficial, providing mulch, retaining soil moisture and protecting the roots from extremes of heat and cold.
A thick puffy layer of thatch can become a problem by preventing oxygen, water and fertilisers from entering the root zone and creating an environment conducive to disease development.
Thatch is easily removed by routine manual raking or scarification, which encourages healthy new growth. In severe cases, a 5 mm layer of quality loam topdressing may be required.
Water repellent soils
Sand to sandy clay loam soils with a high humus content can develop a problem known as water repellence. If severe, this will prevent moisture from entering the root zone. This results from sand grains becoming coated with organic residues and is exacerbated by dry weather. The problem is readily detected as drops of water stay on (rather than soak into) the soil surface, leaving the soil below dry. If a droplet remains on the soil surface for 10-60 seconds, the soil has moderate water repellence, but above 60 seconds the problem is severe.
The challenge with water repellent soils is to enable them to wet up more readily. For example, incorporating clay into the soil will help to draw moisture into the profile. Proprietary soil wetters, designed for lawns and professional turf use, can also be effective.