Why is moss growing in my lawn?

Moss grows in lawns because of poor growing conditions for grass — typically acidic soil, compacted earth, excessive shade, or poor drainage. In New Brunswick's naturally acidic soils and humid Maritime climate, moss problems are extremely common and often indicate multiple underlying issues that need addressing.

The primary culprit in most NB lawns is soil acidity. Our province's soils naturally run between pH 4.5-6.0, which is too acidic for healthy grass growth but perfect for moss. Grass thrives in slightly alkaline conditions (pH 6.0-7.0), so when your soil is acidic, grass struggles while moss flourishes. This is especially common in areas like the Saint John River valley and around Fredericton where clay soils tend to be quite acidic.

Soil compaction is another major factor, particularly in our clay-heavy soils. When soil becomes compacted from foot traffic, heavy equipment, or just our wet spring conditions, water sits on the surface rather than penetrating down to grass roots. Moss loves these constantly moist, oxygen-poor conditions. You'll often see moss in high-traffic areas, under downspouts, or anywhere water tends to pool.

Excessive shade also promotes moss growth. If you have areas under trees or on the north side of buildings that receive less than 4-6 hours of direct sunlight, grass will thin out and moss will move in. Our dense Maritime forests mean many NB properties have significant shade challenges.

Poor drainage compounds all these problems. In areas where water sits after rain or snowmelt, the constantly wet conditions stress grass while providing ideal growing conditions for moss. This is particularly problematic in our spring thaw cycles when frozen ground prevents drainage.

To fix moss problems, you need to address the underlying conditions. Start with a soil test to check pH — if it's below 6.0, apply lime in early spring or fall. Core aerate compacted areas in late August or early September when soil conditions are ideal. For shady areas, consider shade-tolerant grass varieties like fine fescue, or embrace alternative groundcovers in heavily shaded spots.

Timing matters in New Brunswick's climate. Apply lime in April or October when it has time to work into the soil. Overseed thin areas in late August when warm soil helps germination but cooler fall weather reduces stress on new grass. Improve drainage issues before winter to prevent ice formation that worsens compaction.

When to hire a professional: While you can handle soil testing, liming, and overseeding yourself, significant drainage issues, large-scale aeration, or landscape redesign for shade problems often require professional expertise. A landscaper can assess your specific site conditions and develop a comprehensive plan to create better growing conditions for grass while addressing the root causes of moss growth.