- Why Grass Type and Height of Cut Affect Your turfRad Readings

turfRad measures with microwaves, and microwaves see everything, also above the soil

turfRad uses an L-band microwave radiometer. At this frequency (~1.4 GHz), the sensor measures naturally emitted energy (spinning and vibrating electrons) from the surface beneath it. The soil moisture signal is real and physically meaningful, water significantly "calms down" the electrons (it has a very high dielectric constant) compared to dry soil, and L-band microwaves are uniquely sensitive to this water-specific contrast.

But, “the surface” is not bare dirt. It is a canopy of grass.

The microwave emission the sensor captures is a combined signal from:

• The soil itself
• Water held in the soil pore space
• The grass plant tissue (which contains water)
• The thatch and mat layer
• Air gaps within the canopy
• A fairway polygon (tall Bermuda, native soil) uses different correction constants than a green polygon (short bentgrass, sand construction).
• If you reseed a green or change your height of cut (HOC) significantly, the Customer Success team can update that polygon’s parameters without affecting any other surface.
• Adjustments are traceable and surface-specific — not a blanket offset applied to the whole map. This is applied in the cloud, not on the sensor hardware itself, no updates needed, and we can re-process back in time. 

The turfRad processing model separates the grass contribution from the soil signal. To do that accurately, it needs to know what kind of canopy it is looking through.

Why grass type matters, even though you’re measuring soil

Different grass species have fundamentally different physical structures. A dense, water-rich canopy (like well-irrigated Poa annua) emits more microwave radiation from the vegetation layer itself. If the model assumes a thinner, drier canopy, it will incorrectly attribute some of that vegetation signal to soil moisture — and read artificially low. The inverse is also true.

In short: the grass is not transparent to microwaves. It is a semi-opaque medium, and its optical depth at L-band depends on species and condition.

Why height of cut matters

Height of cut (HOC) directly controls canopy volume and biomass above the sensor’s footprint. The relationship is not linear, going from 12 mm (0.5 in) to 4 mm (0.15 in ) on a bentgrass green removes a disproportionate amount of water-bearing tissue and changes the canopy’s microwave opacity. Some turfRad users may have seen the "red egde" effect on the boundary of fairway polygons that include some measurements of the nearby rough. This is exactly this HOC effect. 

turfRad is often used across multiple turf surfaces, greens, tees, fairways, and eventually rough. Each has a different HOC, often a different grass species, and sometimes a different soil construction profile underneath. A single fixed model cannot accurately represent all of them simultaneously.

How polygons are used as mapping constants

When your Customer Success team sets up your turfRad Portal, they define spatial polygons for each turf surface, greens, tees, fairways, etc. These polygons do more than visually organize your map.

Each polygon carries a set of configurable parameters, fixed values applied to all measurements collected within that boundary. These include the vegetation correction coefficients for the grass type and HOC of that surface.

When the raw sensor reading is processed, the system applies the polygon’s parameters to separate the soil moisture signal from the vegetation contribution. The result is a volumetric water content (% VWC) value that is physically appropriate for that specific surface.

This means:

• A fairway polygon (tall Bermuda, native soil) uses different correction constants than a green polygon (short bentgrass, sand construction).
• If you reseed a green or change your HOC significantly (~20% or more), the Customer Success team can update that polygon’s parameters without affecting any other surface.
• Adjustments are traceable and surface-specific, not a blanket offset applied to the whole map.

Why this requires collaboration with the Customer Success team

The physical parameters that govern vegetation correction are not something a superintendent enters directly, they are derived from the radiometric model and validated against your course’s actual conditions. Getting them right requires:

1. Knowing your grass species, HOC, and overall soil makeup per surface: The team will ask for this during onboarding.
2. Reviewing an initial scan together: To confirm that values are plausible given your agronomic intuition and site knowledge.
3. Iterating if conditions change: Overseeding, renovation, or significant HOC changes may warrant a parameter update.

This builds on the tuning process described in the Tuning Your turfRad Sensor article. Tuning is the fine calibration; grass type, HOC configuration, and soil type is the foundation it sits on.

What to tell your Customer Success team

When you contact the team, have the following ready for each distinct turf surface:

• Grass species (and any overseeding mix in use)
• Current HOC (in mm)
• Soil construction type (sand-based, native, or blend)
• Any planned changes in the near term

Schedule your meeting with Dave Wilber - here or Miranda Robinson - here