"On occasions, bentgrass greens develop soft and puffy qualities that tend to ridge or buckle into a slightly higher position than established by the mower. The loose, dense, poorly rooted growth makes a poor putting surface. This slight unevenness occasionally enable the mower to grab chunks of the soft, puffy turf and its scalps or gouges the surface. This produces a poor green."
The above was written by the late Dr. Ralph Engel (Rutgers University) many years ago and published in a USGA Green Section Record article.
Credit to Dr. Karl Danneberger, The Ohio State University, who offered some of the following descriptions for morphological reason for "puffiness." This information is important for one to have a better understand of how grasses grow and what may cause a turf to become "puffy" and "scalp" during mowing.
Know that grass tillering (tillers develop into individual daughter plants) is an organized system that is well structured.
As a general guideline 2.7 to 3.1 live leaves are produced per tiller regardless of density. Individual leaves (as found on a grass tiller) live only three or four weeks during the growing season.
So, to maintain plant and leaf density in a turf, during the summer, grasses require a constant and orderly replacement of new plants (tillers) and new leaves.
The graphic below shows a creeping bentgrass stolon (horizontal stem), with a daughter plant starting to grow at the base, from a node on a stolon, on the left. This new plant, found at the base of the plant, on the left is an example of a lower order bentgrass tiller.
The plant on the right shows a higher order bentgrass tiller initiated from a node on a tiller, on the right. It is the higher order tillers that may cause a turf to become "puffy" and "scalp" while mowing.
Tillers appear in a specific sequence and may be influenced by the growth of a bentgrass variety.
The lower order tillers are considered primary tillers that emerge close to the base of the primary tiller, while the higher order tillers are produced from subsequent axis of later emerging leaves. Higher order tillers generally appear slightly higher in the canopy.
In a dense turf, higher tiller orders are typically smaller, more succulent, compete for light, and become dominant over lower order tillers.
If the higher tiller orders elongate, the leaves associated with these tillers become raised higher into the turf canopy creating a "puffy", "soft", and "spongy" turf that is prone to "scalping" during mowing.
The photo above is creeping bentgrass maintained at a higher height of cut. Under these conditions you can see a long stolon, with individual tillers growing from a node. From these tillers we find both lower and higher order tillers developing.
Unevenness observed in a putting green turf may result from internodal elongation of higher order tillers that may be found in a high density bentgrass turf.
Higher order tillers tend to be succulent and soft, easily pulled upon, with leaves from these tillers elevated above the canopy creating unevenness to a putting green surface (as described by Dr. Danneberger).
Photo above: Scalping on bentgrass plants and tillers, "raised" into the thatch, as commonly found when the thatch is excessive (greater than 1/2 inch).
Excessive thatch provides a "puffy" soft surface which allows mowers to sink in and scalp a bentgrass turf.
How best to manage greens so they do not develop a "puffy" surface.
1. Aerify greens sufficiently so thatch levels do not exceed 1/4" inch. At a minimum aerify greens with 3/8 inch - 1.0 cm hollow tines (pull a plug) in May and September. Aerify greens with 1.4 inch - .6 cm hollow tines (pull a plug) in June and October.
2. Apply light frequent fertility, spoon feed with Nitrogen weekly from May – September (0.2 pounds Nitrogen / 1,000 square feet – 1.0 grams / square meter). Total yearly Nitrogen applied with 20 applications over these five months.
3.Apply light sand topdressing on a weekly basis (topdress1 to 2 cubic feet of sand / 1,000 square feet – 305 to 610 cubic cm square meter).
4. Schedule light to moderate verticuting of greens (at 0.1 inch – 3.0 mm depth) two or three times each in the spring or fall months.
5. Plan to brush greens once a week with the exception of summer heat stress periods.