Quick Answer
Most Sarnia lawns should be cut when the grass reaches about 3.5–4 inches tall, and trimmed down to 2.5–3 inches. Never cut off more than one-third of the blade in a single mowing.
Why Mowing Height Matters
The height you cut your grass directly affects its health. Cut it too short and you stress the plant, making it vulnerable to weeds, heat, and pests. Leave it too long and it can mat down, suffocate itself, and encourage disease. The right balance leads to a thicker, greener lawn.
The One-Third Rule
One of the golden rules of mowing is the one-third rule:
- Never cut more than one-third of the grass blade at once.
- Example: If your grass is 4.5 inches tall, mow it down to 3 inches.
This prevents shock to the plant and keeps your lawn consistently healthy.
Ideal Mowing Heights for Common Ontario Grasses
| Grass Type | Cut When It Reaches | After Mowing Height |
|---|---|---|
| Kentucky Bluegrass | 3.5 – 4 inches | 2.5 – 3 inches |
| Perennial Ryegrass | 3.5 – 4 inches | 2.5 – 3 inches |
| Fine/ Tall Fescue | 4 – 5 inches | 3 – 3.5 inches |
Seasonal Adjustments
Spring: Growth is fast, so you may need to mow weekly. Stick to the 3-inch range to encourage deep roots.
Summer: Heat stress means you should leave grass a bit taller (closer to 3.5 inches). Taller blades shade the soil and reduce evaporation.
Fall: Return to the standard 2.5–3 inch cut. A final late fall mowing should leave grass around 2.5 inches to prevent snow mold.
Signs You’re Cutting Grass Too Short
- Lawn looks yellow or brown right after mowing
- Bare patches or thin grass start to appear
- Weeds spread quickly (short grass exposes soil to weed seeds)
- Soil dries out faster between waterings
Solution: Raise your mower blade and stick to the one-third rule.
Tips for Healthier Mowing
- Sharpen your mower blade at least once a season. A dull blade rips grass instead of cutting it clean.
- Mow when grass is dry to avoid clumping and tearing.
- Alternate mowing patterns each week so the grass doesn’t lean in one direction.
- Leave grass clippings on the lawn—they act as free fertilizer.
Final Word
For homeowners in Sarnia, the sweet spot is cutting your grass when it reaches 3.5–4 inches, bringing it down to 2.5–3 inches. Follow the one-third rule, adjust slightly for the season, and you’ll enjoy a greener, thicker lawn all year long.
Cut too short, and you’ll be battling weeds and brown patches. Cut at the right height, and your lawn will reward you with healthy growth and curb appeal.