Corn Leaf Counting Methods
By Greg Stewart, OMAFRA Corn Specialist

As we rely more and more on post emergent herbicides it has become increasingly important to be able to know and describe the growth stage of the corn crop in order to ensure crop safety. This system in corn is based fairly simply on counting the number of leaves on the corn plant. However, there has been some confusion as to when a corn leaf is included in the count. There are three principal systems in use.

Tips - Many people, including myself have referred to the "critical weed free period" for corn. Much of this work was done by Dr. Clarence Swanton at the University of Guelph and in this research a corn leaf is counted once it's tip has emerged from the whorl of the corn plant. Based on this counting system the critical period, where producers need to keep corn weed free, extends from when the 4th leaf tip has emerged from the whorl through to when the 10th leaf tip has similarly emerged from the whorl.

Leaf Over - In OMAFRA Publication 75 corn leaves are counted on the "leaf over" basis. In this system a leaf is counted once it has emerged far enough out of the whorl to no longer stand vertical but begins to arch and the end of the leaf blade leans over.

Collars - This approach counts a leaf only when it is completely out of the whorl and is used predominantly in the U.S. When a corn leaf is fully expanded, and free of the whorl the leaf "collar" is visible and the leaf is now counted. The leaf collar is where the leaf blade joins to the leaf sheath (The leaf sheath being that part of the leaf that wraps around the stem). Using this system there is perhaps less ambiguity as to whether a leaf "tip" has actually emerged from the whorl or if a leaf is in the "leaf over" position.

Comparisons - If we compare the three leaf numbering systems relative to the previously mentioned critical weed free period for corn we get a table that looks like this.

Numbering System

Leaf stage

Leaf stage

Tips (i.e. C. Swanton work)



Leaf Over (i.e. Publication 75)



Collars (i.e. V2 stage)



Remember that the first leaf to be counted in any system has a rounded end while all the subsequent leaves have a pointed tip. When discussing corn growth stages and leaf numbers be sure you are talking the same system. A herbicide, for example may be safely applied to corn that is up to the 8th leaf stage using the leaf over system but a corn plant that had eight visible collars would be too advanced to safely apply that same herbicide.

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