While direct high temperature damage is often suspected in the loss of creeping bentgrass on golf course greens in the summer, little data is available on this subject. It is known that as supraoptimal temperatures are reached, grasses become more susceptible to biotic- or abiotic- stresses. High soil temperatures inhibit many physiological activities of roots cells and disturb the normal functions of roots such as water uptake, nutrients uptake, hormonal transportation, and photosynthates distribution. High temperatures also cause leaf injuries and accelerate leaf senescence (DiPaola & Beard 1992; Huang, et al., 2000, 2001).

Sometimes, soil temperatures can be so high that direct injury occurs to the plant. This type of injury is classified as "direct heat stress" ( Minner, 1981; Wehner & Watschke, 1981).

The exact scenario under which direct heat stress happens is not clear. It has been documented that a temperature gradient can be established above a turfgrass cover and that the surface temperature of the mat can be significantly higher than the air temperature just above the surface (Waterhouse, 1950). The objectives of this study were to investigate whether "direct heat stress" occurs on creeping bentgrass maintained under green conditions in central Iowa, to identify the forms of damage that occur, and to describe quantitatively the conditions that lead to the damage.

The experiment was conducted at the horticultural research station north of Ames, IA. "Crenshaw" creeping bentgrass was established at a rate of 2 lbs seed/1000ft² on Sept. 14, 2000, on a sand-based green constructed with 30 cm of sand on top of a 10 cm gravel layer. The green has a drainage system consisting of 10-cm-diam. drainage pipes placed at 10 m intervals. Starter fertilizer 1, 0.5, and 0.5 lbs/1000 ft² of N, P₂O₅, and K₂O, respectively, was applied at the time of seeding.

At the time of seeding, six 5’ by 5’ plots were established on the area by topdressing a 0.5-cm layer of Profile, Quick dry; Zeolite, Axis, mason sand, and a mixture of sand and peat (90% /10% v/v) over the surface. This was done to create conditions of different thermal properties on the surface of the green. The experiment was a randomized complete block design with three replications. Temperatures at three locations in each plot were measured at 0, 1 and 6 inches with thermal couples every 15 minutes for three consecutive days (Aug 11 to Aug 15). Water content was measured with a time domain refelectometer (TDR) in the top 5 cm of the soil media.

Mat temperature was 23.4 to 27.5 ° F higher than the air temperature (Table 1). When the air temperature reached 86.9 ° F° , the mat temperature reached 115 ° F. We closely observed the creeping bentgrass shoots that were directly in contact with the soil surface and found that a banding of injured tissue occurred to many of the plants at this temperature. We refer to this phenomenon as "heat girdling". Some of this type of heat stress penetrated through surrounding leaves and sheaths, deeper into the center of the shoots and when the new leaf emerged, the girdling was observed on the emerging leaf blades (Fig. 1).

Further investigation on turf plugs taken from these field plots and maintained in the greenhouse showed that heat girdling generally happens when soil mat temperature reached 118 ° F (48 ° C). Exact air temperature at which such soil mat temperature happens depends on the soil water content and soil thermal diffusivity. However, once the soil mat temperature gets to the threshold temperature, it takes less than one hour to cause heat girdling to the bentgrass shoots.

Further research is needed to correlate air temperature and soil temperature for different root zone materials with different thermal properties and water holding capacities in order to predict threshold air temperatures and the amount of time required for direct heat stress. At this point we may conclude that direct heat stress such as heat girdling could occur during high temperature periods in central Iowa. We also believe that soil properties can affect the degree of injury. In the next phase of this work, we will address practical measures that can be taken to reduce heat stress damage on greens.

Table 1. Root-zone surface temperatures observed when air temperature reached 86.9 ° F (30.5 ° C) on August 12, 2001.

Fig. Heat girdled band on youngest leaf shown 3 days after injury occurred on Axis treatment. Leaf continued to grow and injured area became visible.