Corn: Leaf Stages

Table of Contents

1. Introduction
2. Comparative Growth Stages
3. Why is Early Season Weed Control so Important in Corn?
4. Leaf Over Method of Counting Corn Leaves (Figure 9-1)
5. Corn Herbicide Weed Control Ratings (Table 9-1) (PDF file - 214kb)
   • Annual Grasses
   • Broadleaf Weeds
   • Perennial Weeds
6. Other Corn Weed Control Recommendations for Conventional Corn (Field, Seed and Sweet)
   • Preplant Burndown and Residual Control
   • Soil Applied Grass and Broadleaf Herbicides (Preemergence Timing Only)
   • Soil Applied Tank-Mixes (Preemergence Timing Only)
   • Soil Applied Broadleaf Herbicides (Preemergence Timing Only)
   • Soil Applied and Early Postemergence Grass Herbicides
   • Preemergence and Early Postemergence Grass and Broadleaf Herbicides
   • Soil Applied and Early Postemergence Tank-Mixes (for Control of Grass and Broadleaf Weeds)
   • Postemergence Grass Herbicides
   • Postemergence Broadleaf Herbicides and Tank Mixes (for Broadleaf Weed Control)
   • Postemergence Tank-Mixes (for Control of Grass and Broadleaf Weeds)
7. Related Links

Introduction

Counting leaves on a corn plant may sound like an easy task, but there are complications that can cause miscounting. There are several methods of counting leaves. It is important to know which leaf counting method is being referred to.

Publication 75 uses the leaf over method, (see Figure 9-1) where counting starts with leaves that have emerged from the whorl and the leaf tip is starting to arch over. This normally occurs when leaves are about 50% emerged. Most product labels also use this method of leaf counting, but check the label or with the product representative to be sure. The comparative growth stages table in the next column gives a comparison among the count methods.

Another complication with leaf counting is where on the plant leaf counting begins. In Publication 75, the first leaf is the bottom leaf of the plant. The first leaf is shorter than other leaves and has a round leaf tip. However, as the plant grows the bottom leaves die and drop to the ground. For example, a 10 leaf corn plant may be incorrectly identified as a 7 leaf corn plant because 3 leaves may be "senesed" or fallen off. These leaves may not be immediately apparent and care must be taken to count them.

Hint: Start counting from the bottom leaf and check the first leaf to look for the rounded leaf tip.

It takes about 75-80 Crop Heat Units to produce each corn leaf. Therefore at temperatures of 30°C day, and 20°C at night, there is one new leaf every 2-3 days; and at 20°C day, and 10°C at night, one new leaf every 5-6 days.

Critical Stage: The Critical Weed-Free Period in corn is from the 3-8 leaf over stage.

Some product labels also use plant height to indicate crop growth stages. In general, plant height is more variable depending on plant genetics and on the weather of the season. The following table gives some comparative heights for each leaf stage but individual plants may be slightly more or less than the stage given depending on genetics and weather. The standing height is measured from the ground surface to the top of the plant as it stands. Leaf extended refers to the height of the plant with the leaves pulled up to their full height.

1 Number of leaf tips emerged from the whorl.
2 Number of leaf whorls emerged from the whorl.

Figure 9-1. Leaf Over Method of Counting Corn Leaves

Related Links

• Weeds in Ontario
• Corn in Ontario
• Problem Weed Control in Field Crops
• Integrated Pest Management (IPM)
• Pesticide Application and Safety
• Pesticide Companies and Agents
• OMAFRA Sprayer Calibration Calculator
• Search the PMRA website
• OMAFRA Publication 505: Ontario Weeds
• OMAFRA Publication 75: Guide To Weed Control