Foliage: The leaves are dark green, serrated, and provide a lush backdrop for the flowers.

Flowers: Produces large, lacecap flower clusters with small fertile flowers in the center surrounded by showy, sterile florets. The flowers can be blue or pink, depending on soil pH, and bloom from early summer to early fall.

Light Requirements: Prefers partial shade to full sun. For the best growth and flowering, plant "Mariesii" in a location that receives morning sun and afternoon shade, particularly in hotter climates. While it can tolerate full sun in cooler regions, too much direct sunlight in hot areas may cause the leaves to scorch and reduce flower production.

Soil Requirements: Thrives in well-draining, fertile soil. "Mariesii" prefers rich, loamy soil with a slightly acidic to neutral pH (5.5-7.0). The flower color can be influenced by soil pH—acidic soils produce blue flowers, while alkaline soils yield pink flowers. To maintain or alter flower color, you can adjust the soil pH by adding garden lime to increase alkalinity or sulfur to increase acidity. Adding organic matter, such as compost or peat moss, helps retain moisture and improve soil structure.

Watering: Requires regular watering. Keep the soil consistently moist, especially during the growing season and dry periods. Hydrangeas are water-loving plants, and "Mariesii" needs adequate moisture to thrive. Deep watering is recommended to ensure the roots receive enough moisture. Mulching around the base of the plant helps retain soil moisture and regulate temperature.

Temperature and Humidity: Hardy in USDA zones 6-9. "Mariesii" is well-suited to temperate climates and can tolerate cold winters. It prefers moderate to high humidity levels, performing best in areas with consistent moisture. In regions with hot, dry summers, additional watering and protection from intense afternoon sun will help the plant flourish.

Fertilization: Apply a balanced, slow-release fertilizer in early spring to encourage healthy growth and abundant blooms. A second feeding in mid-summer can help sustain the plant during its peak blooming period. Use a fertilizer formulated for hydrangeas or one that is low in phosphorus to avoid excessive foliage growth at the expense of flowers.

Pruning: Prune lightly in late winter or early spring. "Mariesii" blooms on both old and new wood, so avoid heavy pruning, which could reduce flowering. Remove dead or weak stems and shape the plant as needed. Deadheading spent flowers throughout the blooming season can encourage continued flowering and maintain the plant's appearance.

Pests and Problems: Generally pest-resistant, but may occasionally be affected by aphids, spider mites, or powdery mildew. Ensure good air circulation around the plant to prevent fungal diseases. If pests or diseases are detected, treat with insecticidal soap or neem oil. Regular inspection and maintaining garden hygiene can help keep the plant healthy.