first article in this series<\/a>. Instead of simply correlating weather and clipping yield across all of those sites, each golf course was compared with itself at approximately the same time of year. That helped separate a weather response from the large differences in normal growth that already exist among golf courses. We also corrected for skipped mowing days, because the catch after a skipped day represents more than one day of growth.<\/p>\nFor rainfall, we were also able to use NOAA\u2019s Multi-Radar Multi-Sensor (MRMS) radar-derived precipitation estimates. Rain is much more spatially variable than temperature, and a summer thunderstorm can soak one golf course while missing another only a few miles away. The MRMS data gave us a much better estimate of when meaningful rainfall actually occurred at each golf course.<\/p>\n
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Rain increased growth, but not as much as we might think<\/h5>\n
The rainfall result was interesting because it partly confirmed the conventional wisdom, but the change was smaller than most people would expect.<\/p>\n
Following larger rainfall events, putting-green growth generally increased for several days. After an inch or more of rain, clipping yield typically increased by about 15 to 25% for one to three days compared with comparable periods without rain. Some of that increase persisted for roughly five days, and about 70% of courses saw clipping volume move in that same direction.<\/p>\n
[\/et_pb_text][et_pb_image src=”https:\/\/www.greenkeeperapp.com\/marketing\/wp-content\/uploads\/2026\/10\/growth-series-3-rain-by-amount.png” _builder_version=”4.21.0″ _module_preset=”default” hover_enabled=”0″ sticky_enabled=”0″ title_text=”growth-series-3-rain-by-amount” align=”center” max_width=”85%” box_shadow_style=”preset1″][\/et_pb_image][et_pb_text _builder_version=”4.21.0″ _module_preset=”default” module_alignment=”center” max_width=”85%” hover_enabled=”0″ sticky_enabled=”0″ text_font_size=”15px”]<\/p>\n
Figure 1.<\/strong><\/em> <\/strong>Daily clipping yield as a percent of each course’s normal clipping from five days before to five days after rain that ended a hot, dry spell (blue), versus dry dates at the same course and time of year (gray, dashed). Day 0 is the morning after the rain.<\/em><\/p>\n[\/et_pb_text][et_pb_text _builder_version=”4.21.0″ _module_preset=”default” hover_enabled=”0″ global_colors_info=”{}” sticky_enabled=”0″]<\/p>\n
That sounds substantial until we put it into context. The median golf course in this dataset typically sees clipping yield move by 28% from one day to the next. The average growth response following rain was therefore smaller than normal day-to-day variation.<\/p>\n
That distinction is important. The analysis says that rainfall has a measurable effect when we average many storms across many golf courses. It does not say that every one-inch rain should result in an obvious growth flush the next morning.\u00a0<\/p>\n
[\/et_pb_text][et_pb_text _builder_version=”4.21.0″ _module_preset=”default” hover_enabled=”0″ sticky_enabled=”0″]<\/p>\n
What about heavy rain after hot and dry weather?<\/h5>\n
After seeing that initial model result, my mind wandered to the next question: what happens when heavy rain follows a stretch of hot, dry weather? My expectation was that this combination might produce the largest response. We specifically identified dry weeks with unusually high temperature or evapotranspiration for that course and time of year, and then we looked at what ClipVol did after substantial rain ended that dry period.<\/p>\n
Surprisingly, those events did not stand out.<\/strong><\/em><\/p>\nRain following a hot, dry week produced approximately the same general response as rain following more ordinary conditions. We changed the length of the dry period, adjusted how hot or dry it needed to be, and looked at how long it had been since meaningful rain. None of those approaches produced convincing evidence of a much larger growth surge when rainfall ended the dry weather.\u00a0<\/p>\n
[\/et_pb_text][et_pb_image src=”https:\/\/www.greenkeeperapp.com\/marketing\/wp-content\/uploads\/2026\/10\/fig_rain_after_hot_dry.png” alt=”Chart of putting-green clipping yield after rain. Rain after a hot, dry spell overlaps rain after any other week.” title_text=”fig_rain_after_hot_dry” show_in_lightbox=”on” align=”center” _builder_version=”4.21.0″ _module_preset=”default” max_width=”85%” hover_enabled=”0″ box_shadow_style=”preset1″ global_colors_info=”{}” sticky_enabled=”0″][\/et_pb_image][et_pb_text _builder_version=”4.21.0″ _module_preset=”default” text_text_color=”#5a5952″ text_font_size=”15px” max_width=”85%” module_alignment=”center” custom_margin=”-12px||||false|false” hover_enabled=”0″ global_colors_info=”{}” sticky_enabled=”0″]<\/p>\n
Figure 2.<\/strong> Putting-green clipping yield after rainfall of approximately 0.5 inch or more. Rain following a hot, dry week did not produce a larger growth response than rain following other conditions. Each course was compared with itself on dry dates at the same time of year.<\/em><\/p>\n[\/et_pb_text][et_pb_text _builder_version=”4.21.0″ _module_preset=”default” hover_enabled=”0″ sticky_enabled=”0″]<\/p>\n
For putting greens, there is a reasonable explanation. We don’t allow putting greens to experience substantial drought stress. During hot, dry weather, irrigation keeps the root zone above the wilt point. The surface may become moderately dry and evaporative demand may be high, but the plant still has sufficient access to water. While a soaking rain can re-wet the soil profile, it isn’t necessarily rescuing a severely drought-stressed plant.<\/p>\n
In that context, a temporary 15 to 25% increase in growth following a significant rain is probably more reasonable than the dramatic \u201cgrowth explosion\u201d we sometimes perceive following a strong summer storm.<\/p>\n
We were also cautious about clipping volume the morning after heavy rain. Surface water and dew can make clippings wetter, heavier and bulkier. That can be part of the reason we see an increase in clipping volume the morning after rainfall. If those clippings were dried, it’s possible that the amount of leaf tissue – including the sugars and nutrients within that tissue – would not be much different than any other morning. For that reason, our modeling conclusions focused primarily on changes in clipping volume over the several days following heavy rain.<\/p>\n
[\/et_pb_text][et_pb_text _builder_version=”4.21.0″ _module_preset=”default” global_colors_info=”{}”]<\/p>\n
The first cold snap produced a much larger response<\/h5>\n
The clipping volume response to fall temperature changes produced a much more dramatic story.<\/p>\n
On cool-season putting greens, growth dropped rapidly around the first meaningful cold snap of autumn. The clearest example was the first night the air temperature fell below 45\u00b0F<\/em><\/strong>. After accounting for differences that were already present before the cold front (from fertilizer or PGR applications, for example), clipping volume fell by roughly 30 to 40% for a period of three days compared with the same golf course on the same dates in years when the cold came later. Growth began recovering over the following week, although clipping yield remained below the comparison years through much of the ten-day window.<\/p>\n[\/et_pb_text][et_pb_image src=”https:\/\/www.greenkeeperapp.com\/marketing\/wp-content\/uploads\/2026\/10\/fig_cold_snap_45F.png” alt=”Chart of cool-season putting-green clipping yield around the first fall night below 45 degrees F.” title_text=”fig_cold_snap_45F” show_in_lightbox=”on” align=”center” _builder_version=”4.21.0″ _module_preset=”default” max_width=”85%” hover_enabled=”0″ box_shadow_style=”preset1″ global_colors_info=”{}” sticky_enabled=”0″][\/et_pb_image][et_pb_text _builder_version=”4.21.0″ _module_preset=”default” text_text_color=”#5a5952″ text_font_size=”15px” max_width=”85%” module_alignment=”center” custom_margin=”-12px||||false|false” global_colors_info=”{}”]<\/p>\n
Figure 3.<\/strong> Cool-season putting-green growth around the first fall night below 45\u00b0F. Clipping yield fell sharply after the cold night and began recovering as temperatures moderated. Each course was compared with itself on the same dates in other years, mostly years when the cold arrived later, using dates when growth had been similar in the five days before.<\/em><\/p>\n[\/et_pb_text][et_pb_text _builder_version=”4.21.0″ _module_preset=”default” global_colors_info=”{}”]<\/p>\n
I would not interpret 45\u00b0F as a biological switch, but as a marker of a broader shift in the weather pattern.<\/p>\n
In the weather data, the first night below 45\u00b0F typically marked the arrival of a larger cold air mass. Daytime highs were also 5 to 12\u00b0F cooler than in the comparison years. When that first cold night was followed by two or more additional nights below 45\u00b0F, the decline in growth was about twice as large as when the cold night stood alone. Days that warmed back up pulled clipping volume back toward normal levels for that course and time of year. That uneven recovery from course to course is part of why the blue shaded band in Figure 3 is so wide after the cold night.\u00a0<\/p>\n
In other words, the plant did not cross a 45\u00b0F threshold and permanently step down to some new fall growth rate. Growth followed the weather down and then followed it back up over the next 10 days.<\/p>\n
That is why the thermal-time concept in the previous article<\/a> matters. September 25th is still September 25th, but the turf can be in very different places biologically depending on the weather that came before it. A warm stretch can keep growth moving, while an early cold front can slow it quickly. The date is the same; the plant\u2019s recent temperature history is not.<\/p>\n[\/et_pb_text][et_pb_divider _builder_version=”4.21.0″ _module_preset=”default” hover_enabled=”0″ sticky_enabled=”0″ custom_margin=”||1px|||”][\/et_pb_divider][et_pb_text _builder_version=”4.21.0″ _module_preset=”default” hover_enabled=”0″ global_colors_info=”{}” sticky_enabled=”0″]<\/p>\n
Weather changes growth, but not always the way we expect<\/span><\/h5>\nA large rain increased clipping yield modestly for a few days, even after hot, dry weather. A strong fall cold front had a much larger effect, reducing growth quickly before it recovered as temperatures warmed.<\/span><\/p>\nWeather is still only part of the story. Nitrogen, PGRs, irrigation, cultivation, mowing and the normal growth characteristics of each golf course all influence how much turf is actually growing. That is why clipping volume matters in GreenKeeper: weather tells us what the turf experienced, while ClipVol shows how it responded.<\/span><\/p>\nGreenKeeper is using those clipping records, weather data and management histories to build <\/span>site-specific growth models for individual golf courses<\/span><\/strong>, with plans to make them available to users in 2027. The more consistently you collect clipping volume now, the better those future models can learn what normal growth looks like on your greens and how it changes with weather and management.<\/span><\/p>\nThe goal is to turn those measurements into better decisions around PGRs, nitrogen, water and cultivation. Over time, GreenKeeper can move from general growth estimates toward guidance based on <\/span>how your turf actually grows<\/span><\/strong>.<\/span><\/p>\nNext in the Growth Series, we\u2019ll look at how much growth patterns differ from one golf course to another\u2014and why that matters for building site-specific growth models.<\/strong><\/em><\/p>\n[\/et_pb_text][et_pb_button button_url=”https:\/\/www.greenkeeperapp.com\/register” button_text=”Join GreenKeeper” button_alignment=”center” _builder_version=”4.21.0″ _module_preset=”default” global_colors_info=”{}”][\/et_pb_button][\/et_pb_column][\/et_pb_row][\/et_pb_section]<\/p>\n","protected":false},"excerpt":{"rendered":"
In the last article in this series, I argued that a thermometer is often a better biological clock than a calendar. Growing degree days help explain why 14 calendar days in July can represent a very different amount of biological time than 14 days in November. That discussion naturally led to another question. If temperature […]<\/p>\n","protected":false},"author":3,"featured_media":102505,"comment_status":"closed","ping_status":"open","sticky":false,"template":"","format":"standard","meta":{"_et_pb_use_builder":"on","_et_pb_old_content":"","_et_gb_content_width":"","_jetpack_memberships_contains_paid_content":false,"footnotes":""},"categories":[5,64],"tags":[31,55,52],"class_list":["post-102490","post","type-post","status-publish","format-standard","has-post-thumbnail","hentry","category-blog","category-greenkeeper-growth-series","tag-clipvol","tag-modeling","tag-ptm"],"jetpack_featured_media_url":"https:\/\/www.greenkeeperapp.com\/marketing\/wp-content\/uploads\/2026\/10\/Stormy-Mountain-Golf-Course-Panorama.png","jetpack_sharing_enabled":true,"_links":{"self":[{"href":"https:\/\/www.greenkeeperapp.com\/marketing\/index.php\/wp-json\/wp\/v2\/posts\/102490"}],"collection":[{"href":"https:\/\/www.greenkeeperapp.com\/marketing\/index.php\/wp-json\/wp\/v2\/posts"}],"about":[{"href":"https:\/\/www.greenkeeperapp.com\/marketing\/index.php\/wp-json\/wp\/v2\/types\/post"}],"author":[{"embeddable":true,"href":"https:\/\/www.greenkeeperapp.com\/marketing\/index.php\/wp-json\/wp\/v2\/users\/3"}],"replies":[{"embeddable":true,"href":"https:\/\/www.greenkeeperapp.com\/marketing\/index.php\/wp-json\/wp\/v2\/comments?post=102490"}],"version-history":[{"count":7,"href":"https:\/\/www.greenkeeperapp.com\/marketing\/index.php\/wp-json\/wp\/v2\/posts\/102490\/revisions"}],"predecessor-version":[{"id":102507,"href":"https:\/\/www.greenkeeperapp.com\/marketing\/index.php\/wp-json\/wp\/v2\/posts\/102490\/revisions\/102507"}],"wp:featuredmedia":[{"embeddable":true,"href":"https:\/\/www.greenkeeperapp.com\/marketing\/index.php\/wp-json\/wp\/v2\/media\/102505"}],"wp:attachment":[{"href":"https:\/\/www.greenkeeperapp.com\/marketing\/index.php\/wp-json\/wp\/v2\/media?parent=102490"}],"wp:term":[{"taxonomy":"category","embeddable":true,"href":"https:\/\/www.greenkeeperapp.com\/marketing\/index.php\/wp-json\/wp\/v2\/categories?post=102490"},{"taxonomy":"post_tag","embeddable":true,"href":"https:\/\/www.greenkeeperapp.com\/marketing\/index.php\/wp-json\/wp\/v2\/tags?post=102490"}],"curies":[{"name":"wp","href":"https:\/\/api.w.org\/{rel}","templated":true}]}}