Will My Late Planted Corn Mature Prior to a Frost?


Spring of 2019 was clearly frustrating to be a corn grower. Frequent rains and cool weather delayed planting for large parts of Ontario. A significant amount of corn was planted into June (Fig 1), and many fields tasseled in early to mid-August, later then the middle to late July we normally expect. Naturally, the common question now is what are the risks to this late planted corn?

How Much Season Do We Have Left?

How much time can we expect to have before a killing frost? Because we don’t know when our first killing frost will be in 2019, we can only estimate our risk based on long-term weather normals. Table 1 gives typical growing season end dates used for Crop Heat Unit (CHU) ratings at select locations across the province. Even if frost does not occur before these dates, cooler temperatures typically start to limit CHU accumulation and grain fill a week or two past these dates.

Table 1. Long term average season end dates for 4 locations in Ontario.

Location Average Season End Date
Elora September 26
Ottawa September 29
Exeter October 3
Ridgetown October 10

What If We Get A Late Frost?

If frost does not occur before these dates, cooler temperatures typically start to slow CHU accumulation and grain fill a couple weeks after. At the Weather Station on the Elora Research Station from 1989-2018, the difference in average CHU accumulation between the 10% killing frost risk date (October 2) and a 90% killing frost risk date (October 28) was about 200 CHU. The earliest killing frost in the same time period was September 24 while the latest was November 1.

Figure 1. Frost risks and crop heat unit accumulations at Elora Research Station weather station, 1989-2018.

How Much More Time Will My Late Planted Corn Need to Reach Maturity?

This is a tough question to answer. Naturally one would want to look at their current growth stage, add CHUs required for remaining growth stages until maturity (page 17, Agronomy Guide for Field Crops Publication 811), and calculate which calendar date this would place them at based on average long-term CHU accumulations. The trouble with this approach is that corn plants adapt to later planting by shortening thermal time (the number of CHUs or growing degree days (GDDs)) required to progress through growth stages. Therefore, the CHUs required to progress through a growth stage for corn planted June 1 is less than the number of CHUs required to progress through the same growth stage for the same hybrid planted May 1.

An excellent research paper investigating the impact of plant development and time to maturity from delayed planting was from Nielsen et al (2002) who evaluated the impact of delaying planting of select hybrids from end of April to early June in Indiana and Ohio during the early 1990’s.

Some key observations from their research:

  • Hybrids have some ability to adapt to late planting by reducing growing degree days (GDDs) required to progress through growth stages.
  • Later planted corn emerged in fewer days (consistent with warming soil temperatures) but GDD requirements for emergence was not affected by planting date.
  • Silking (R1) and black layer (R6) occurred in fewer days after emergence for June planted corn than early May planted corn. Vegetative stages required 10 fewer days while reproductive stages required 5 more. June planted corn matured in 5 fewer days after emergence than early May corn.
  • June planting required fewer GDD to reach silking and black layer than early May planting. Overall reductions were 10%, with the majority occurring during reproductive stages. Despite GDD reductions, reproductive stages still take more days to complete for June planting as development occurs later in the year during days with lower GDD accumulation.
  • For a given location, GDD reductions are greatest for later maturing hybrids. Reductions increase as planting is delayed.
  • Increased frost risk and premature black layer formation did not fully explain GDD reductions during grain fill of June planted corn. Reductions still occurred during years where June planted corn black layered prior to frost. 

So Practically, What Does This Mean?

If you planted a hybrid in early June that you would normally plant in early May: You can expect a full season hybrid to mature in fewer CHU and calendar days than normal. If you are estimating days to maturity from silking date, early June planted corn still requires fewer CHU than early May planted corn, but reproductive development is occurring later in the year during days with lower CHU accumulation, so will likely take more calendar days than you would expect from early May planted corn. Nielsen et al (2002) observed average time from silking to black layer increase from 63 days for early May planted corn to 68 days for the same hybrids planted early June, so 5 more days than normal may be a rough guide. General guidelines for time to maturity from various grain fill stages for early May planting dates is presented in Table 2.  

If you reduced maturity of the hybrid you planted in early June: While hybrid maturity differences are more strongly attributed to the length of time spent in vegetative growth stages, some adjustment in length of reproductive stage also occurs (DuPont Pioneer, 2015), so shorter maturity hybrids will likely have a slightly shorter reproductive requirement than a full season hybrid. Later planting will also still likely result in some reduction in CHU requirements for grain fill, though not as much as a full season hybrid planted in early June. These would both reduce CHU requirements from silking to maturity. Late planting, however, is still pushing grain fill into days with lower CHU accumulations. This creates a tradeoff, and thus days from silking to maturity may still fall within the realm of our normal expectations.

A Few Other Caveats

A few other things can impact plant development, grain fill and maturity. Brief, low single digit temperatures without frost damage during grain fill can temporarily impair photosynthesis and delay grain fill. Though plants will resume activity within a couple days once normal temperatures resume, these events will delay grain fill (Brown, 2017). Similarly, extended periods of very cool but non-frost temperatures during grain fill may be associated with premature black-layer formation (Nielsen, 2002).

What is the impact of not maturing prior to a frost?


Figure 2. Mid June planted corn in the milk stage, August 26, 2019

For late planted corn (Figure 2) the impact of not reaching physiological maturity (black layer) before a frost depends on corn stage and frost temperature and duration. A light frost (not less than -2°C) will likely damage leaves and halt any further photosynthesis but not penetrate stem tissues. This will stop dry matter accumulation in the plant, but still allow translocation within the plant to continue some grain fill. A frost of -2°C or lower is generally considered a killing frost, where stem tissue is damaged, stopping any further dry matter accumulation and translocation.

Frost prior to dent stage (approx. 3 weeks until maturity) would have severe impacts on yield and grain quality. Getting a corn crop to at least half-milk stage (approx. 1-2 weeks until maturity) prior to frost removes considerable yield and grain quality risks (Table 2).

Table 2. Expected weeks to maturity, yield and grain quality risks for frost received at various grain fill stages.

1 – general guidelines for a 2800 CHU hybrid, may differ slightly across hybrids, hybrid maturities or planting dates. Weeks to maturity will take longer as pollination/grain fill is pushed later into the season to days with lower CHU accumulations.


Brown, C. Editor. 2017. Pub 811 OMAFRA Agronomy Guide for Field Crops. Ontario Ministry of Agriculture, Food and Rural Affairs.

DuPont Pioneer. 2015. Corn Growth and Development. DuPont Pioneer.

Nielsen, R.L., P.R. Thomison, G.A. Brown, A.L. Halter, J. Wells and K.L. Wuethrich. 2002. Delayed planting effects on flowering and grain maturity of dent corn. Agron J. 94:549-558.