The windmill palm is a common name given to palms in the botanical group Trachycarpus, the most widely grown windmill palm species being Trachycarpus fortunei and Trachycarpus wagnerianus. Demonstrating excellent winter cold tolerances, windmill palms are naturally slow-growing, but their pace of growth can be hastened with consistent soil moisture, an even fertilizer regimen and a long, warm growing season. At best, a windmill palm can be coaxed to grow no more than 12 to 18 inches each year, rarely 20 inches.
Provide one to two inches of irrigation water to the root zone of a windmill palm during the warmth of the growing season. Although drought-tolerant, the palm will increase its growth if the soil is always moist but never soggy. Reduce watering in the cooler winter months.
Apply a slow-release granular fertilizer formulated for palms and the soils in your region. Often called "palm special," the fertilizer contains nutrients essential for healthy palm frond growth. Contact your local cooperative extension office for the specific timing of fertilization of palms in your region, but in general, fertilizer should be applied in spring, summer and early autumn.
Increase the warmth received by the palm in the cooler spring and mid-autumn months. Palm root growth increases once soil temperatures are above 70 degrees Fahrenheit. Pulling back mulch in early spring so the sun more quickly warms the soil or positioning the palm in a warm microclimate, such as along a southwest-facing building facade, can lengthen the growing season.