To propagate a Leyland cypress, the best method is to take semi-softwood cuttings from a young tree (less than 10 years old) during the early months of the year (like January, February, or March), dip the cut end in rooting hormone, and plant them in a well-draining potting mix, keeping them in a warm, humid environment until they root; due to often sterile seeds, propagation from cuttings is the most reliable way to grow new Leyland cypress trees. 

Key steps:

Select cuttings: Choose healthy, semi-ripe stems from a young tree, around 6-8 inches long, with some brown wood at the base and fresh green growth at the top. 

Take cuttings: Use sharp pruners to make clean cuts, removing the lower foliage from the bottom 2-3 inches. 

Apply rooting hormone: Dip the cut end of the stem into a rooting hormone powder. 

Planting medium: Prepare a pot with a porous mix like equal parts peat moss and perlite. 

Plant cuttings: Make a small hole in the potting mix, insert the cutting, and gently firm the soil around it. 

Care: Keep the cuttings in a warm, humid environment with filtered sunlight, and water regularly to maintain moist soil. 

Important points to remember:

Timing is key: Taking cuttings in early spring (January-March) generally leads to the best rooting success. 

Choose the right tree: Select a healthy Leyland cypress that exhibits the desired characteristics. 

Sterilize tools: To prevent disease spread, sanitize your cutting tools before taking cuttings. 

Provide proper light: Avoid direct sunlight, but ensure adequate light for root development. 

Monitor moisture levels: Keep the potting mix consistently moist, but not soggy.