Evergreen trees and shrubs generally require less feeding than the deciduous plants in your garden. However, a small amount of the right type of fertiliser throughout the year can help your evergreens to perk up and improve their growth. Although evergreens remain leafy year-round, remember when fertilising that they still experience yearly cycles. Never fertilise immediately before winter. Instead, fertilise moderately in early spring. Overfeeding can stress evergreens.
Cottonseed meal is an organic fertiliser that's well-suited to evergreens' needs for acidic soil. Add the cottonseed meal to the evergreen's soil by the first week in June. Adding the fertiliser later may spur late-developing growth that cannot harden by winter. Cottonseed oil generally contains about 7 per cent nitrogen, 3 per cent phosphorus and 2 per cent potash. The nitrogen from cottonseed meal becomes available to plants relatively slowly, over multiple seasons. If you are unable to find cottonseed meal, consider soybean meal or other organic soluble fertilisers formulated for acid-loving plants. However, do not use organic solutions such as lime, wood ashes, or bone meal, as they can raise the soil's pH level.
For broadleaved evergreens, such as holly, you can select from many general purpose fertilisers that provide a high dosage of phosphorus. The phosphorus is important for the formation of flowers and berries among angiosperm evergreens. To determine your plants' particular need for phosphorus, conduct a test on the soil and assess its phosphorus level. For best results, add phosphorus in the form of a superphosphate or triple superphosphate.
Besides phosphorus, many evergreens require additional nitrogen in the form of ammonium or urea. Avoid using nitrogen fertilisers in the form of nitrate, which tends to be less helpful to evergreens that rely on acidic soil. When looking at general-purpose fertilisers, identify their nitrogen content by referring to the first number of the "N-P-K" (Nitrogen-Phosphorus-Potash) formula. You can also select nitrogen-rich fertilisers specifically designed for evergreens. These may either be organic and slowly release the nitrogen, or synthetic and release it more quickly.
In addition to organic or synthetic fertilisers, you can improve evergreens' health by incorporating organic mulches into and over the soil around them. Peat moss or rotted oak-leaf mould are excellent options for conserving moisture, reducing weed growth and protecting the roots from extreme heat during the summer. If applying cottonseed meal or another organic fertiliser, you can mix it directly in with the mulch. The mulch will serve primarily as protection for the trees' roots and, as it breaks down, provide further nutrients.