Arguably the most cold-hardy of trunked fan-palm species, windmill palm (Trachycarpus spp.) grows its roots and new fronds during the warm months of the growing season. Once temperatures drop below 60 degrees Fahrenheit, root growth greatly slows and nutrient uptake is slowed. Thus, any nutrient deficiencies seen in fall will likely deepen across winter. Having a fertile soil with all necessary nutrients year-round ensures a windmill palm is healthfully with no lapses in soil nutrition. Windmill palms readily survive winters that drop no colder than -5 degrees Fahrenheit.
Understand your Soil
All palms need ample amounts of nitrogen, potassium and phosphorus, as well as trace amounts of micronutrients to sustain key metabolic processes or tissues. Trace elements like magnesium, iron, calcium, boron and manganese must be present in ground or container soils. Conduct a nutrient analysis and pH test of the soil near the windmill palm. Knowing exactly what nutrients are lacking helps you create an effective fertilizer regimen. Acidic soils permit palm roots to absorb more molecules of trace nutrients.
The Australian Palm and Cycad Society and Jungle Music Palm and Cycad Nursery of San Diego, California, note that fertile soils don't necessarily warrant the use of fertilizers for the health of windmill palms. Sandy, nutrient-poor soils certainly warrant fertilization to prevent leaf yellowing or weak growth. Gardeners choose either organic or synthetic (man-made) fertilizers. Organic fertilizers include compost, mulch, fish emulsion and bone meal.
Organic fertilizers are slow to decompose and release nutrient molecules for use by windmill palm roots. Such fertilizer is applied anytime of year, particularly compost and top-dressing mulch, but the warming months of spring and summer requires the most attention. Granular synthetic fertilizers degrade over months based on moisture and temperature; applications according to product labels ensure fertilizer is present year-round. New, fresh applications are best in spring and summer months to coincide with the natural growth of new roots and fronds.
A general rule for a good synthetic formulation for palm fertilizers is 3-1-3 or 3-1-2 with micronutrients, according to APCS and Jungle Music Nursery. These "well-balanced" fertilizers, often marketed as "general all-purpose" or "landscape" formulas have nitrogren, phosphorus and potassium levels in the 3-1-3 or 3-1-2 ratios. For example, a 12-4-12 fertilizer with micronutrients is ideal; whereas a 30-0-0 is not, since it is too rich in nitrogen and is lacking other necessary elements. Liquid fertilizers are perhaps not the best, since the salts in the solution can burn palm roots and leach through soil quickly when compared to slower-acting organic and slow-release synthetic granules. Approximate ratios are acceptable, as you may find a "rose" fertilizer has the nutrients in a proportion that is best for your windmill palm.
Never overfertilize the windmill palm, especially with fast-acting liquid fertilizers, since they can damage roots. Be consistent with your fertilizer regime, whatever components you decide to use. Supplementing your natural soil's fertility with slow-release fertilizers guarantees the soil contains nutrients for the palm roots at any time, but especially in spring and summer. Always apply fertilizer to moist soil, and never focus primary applications in late fall and winter periods.