Citrus (Indoor/Container)

In USDA zones 4-6 covering New Hampshire and Vermont, citrus trees are exclusively container-grown plants that spend summers outdoors and winters inside. This unique growing situation creates fertilization challenges that differ dramatically from orchard citrus in Florida or California. Container citrus trees are heavy nitrogen feeders — nitrogen is the single most important macronutrient, directly influencing not just vegetative growth but also fruit production and the tree's ability to absorb other elements. The ideal NPK ratio is approximately 2:1:1 or 3:1:1 (twice to three times as much nitrogen as phosphorus and potassium), which runs counter to the typical fruit tree advice of reducing nitrogen during fruiting. Citrus also has uniquely high micronutrient demands, particularly for iron, manganese, zinc, and magnesium — deficiencies of these trace elements are the most common nutritional problems in container citrus. Frequent watering required by container plants constantly flushes nutrients from the limited soil volume, making regular fertilization essential rather than optional. The critical management challenge for northern growers is transitioning between active outdoor growing season (spring through fall) and the indoor overwintering period. Fertilization must ramp up as trees move outdoors into strong sunlight and warm temperatures, then taper off by late summer to early fall so trees can harden off before moving inside. During winter, most citrus enters a semi-dormant state indoors with minimal growth, and fertilization should be reduced to quarter-strength or eliminated entirely depending on how much light and warmth the tree receives.