Fertilization is necessary for maintaining the health and structure of a bonsai tree. Unlike trees planted in the ground, bonsai are restricted to small containers with a limited volume of soil, quickly exhausting the natural reservoir of nutrients. The highly porous, inorganic soil mixes commonly used for bonsai offer little native nutrition, despite being excellent for root health and drainage. This environment requires a consistent external supply of macronutrients and micronutrients to support growth, maintain foliage density, and develop the desired compact form.
Choosing Fertilizer Format
Hobbyists generally choose between two physical forms: liquid and granular. Liquid fertilizers are dissolved in water, offering immediate nutrient availability and a fast boost to the tree. However, these nutrients are rapidly leached out of the fast-draining soil, necessitating frequent applications, often weekly or bi-weekly. Granular or solid fertilizers are placed on the soil surface and release nutrients slowly as the tree is watered. This slow-release mechanism is safer from the risk of root burn and allows for less frequent application, typically monthly.
Another consideration is the nutrient source, differentiating between synthetic (mineral) and organic fertilizers. Synthetic fertilizers are pure, highly concentrated nutrient salts that provide precise, fast-acting doses but carry a higher risk of damaging the root system if over-applied. Organic fertilizers, derived from plant or animal products, require soil microbes to break them down before the nutrients become available. This microbial process ensures a gentle, sustained release of nutrition that is less likely to cause root burn, though some organic types may produce an odor.
Matching Nutrient Ratios to Growth Stage
All fertilizers display an NPK ratio, indicating the percentage by weight of Nitrogen (N), Phosphorus (P), and Potassium (K), the three primary macronutrients. Nitrogen supports lush, green foliage and stem extension, driving above-ground growth. Phosphorus promotes strong root development, flower production, and fruit setting by playing a role in energy transfer. Potassium is responsible for overall plant hardiness, disease resistance, and regulating water uptake.
The ideal NPK ratio changes significantly to align with the tree’s seasonal biological cycle. In early spring, a high-nitrogen formulation (e.g., 10-6-6 or 12-4-8) encourages a vigorous flush of new growth after dormancy. In mid-summer, the focus shifts to maintenance, requiring a balanced fertilizer (e.g., 6-6-6 or 10-10-10) to sustain health without excessive elongation. For late summer and early autumn, a low-nitrogen, high-phosphorus and potassium blend (e.g., 3-10-10) is necessary to cease soft growth and allow new wood to harden. This hardening process prepares the tree for winter by storing carbohydrates and boosting cold tolerance.
Proper Application Methods and Timing
The growing season dictates the timing for fertilization, typically starting after the first burst of spring growth has matured and continuing until late autumn. For trees requiring heavy feeding to develop thick trunks, application may begin earlier as buds swell. Application must cease entirely when the tree enters winter dormancy, as it cannot utilize the nutrients effectively.
When using liquid fertilizer, dilute the product significantly more than suggested for standard potted plants, often to half-strength or less, to avoid chemical burn. This diluted mixture can be applied weekly or bi-weekly, depending on the tree’s needs and soil drainage. Solid organic pellets are placed directly on the soil surface, sometimes held in small baskets, and are reapplied monthly or when the previous pellets have broken down. A primary rule for all methods is to apply fertilizer only to soil that is already moist from a prior watering. Applying concentrated fertilizer to dry soil can draw water out of the roots, causing severe chemical burn.
Handling Specific Tree Conditions
Fertilization must be temporarily suspended when a tree is undergoing a stressful event, such as repotting or illness. Following repotting, the freshly cut roots need time to heal and re-establish themselves in the new soil. Applying fertilizer too soon, especially high-salt synthetic types, can damage the compromised root system; a waiting period of four to six weeks, or until new shoot growth appears, is recommended. A tree showing signs of weakness, stress, or disease should not be fed, as its energy is diverted toward recovery rather than growth.
Certain species require adjustments to the general feeding schedule. Coniferous trees, such as pines and junipers, benefit from a consistent, slightly higher nitrogen ratio throughout the growing season to maintain dense foliage. Deciduous species, like maples and elms, thrive on a balanced formula. Flowering or fruiting bonsai, such as azaleas, require a formula higher in phosphorus to support bloom production.