Why Is My Broccoli Head So Small?

Growing a broccoli plant that produces a head the size of a golf ball when you were expecting something much larger is disappointing, but it is one of the most common brassica growing problems. In almost every case the cause lies in one of four areas: stress during the growing season, insufficient nutrients, being planted too close together, or the plant initiating head formation before it has built enough leaf area to support a large head.

Insufficient leaf area before heading

Broccoli and cauliflower need to build a substantial amount of leaf before they initiate a head — those leaves are the solar panels that power head development. Plants that are stressed, undersized, or damaged by caterpillars early in growth may initiate heading before they have enough leaf area to support a large one. The result is a small but structurally complete head. There is nothing to be done with the current plant, but the lesson for next year is to protect the vegetative growth phase carefully.

Overcrowding

Plants competing for light, water and nutrients each get less of everything. Broccoli planted at 30cm spacing produces markedly smaller heads than the same variety at 60cm. Cauliflower needs 75cm or more between plants. If your spacing was too tight this season, note it for next year. The larger spacing can feel wasteful on a small plot, but the yield in terms of head weight per plant is significantly better.

Nutrient limitations

Phosphorus is particularly important for brassica head development. In cold soil (below 10°C), phosphorus uptake is very limited even if the nutrient is present — transplanting into cold soil in early spring can produce small-headed plants even in fertile ground. Add bone meal or superphosphate at planting and wait for soil to warm above 10°C before transplanting in spring.

Temperature stress during heading

A sudden cold snap or heatwave during the few weeks when the head is forming can trigger the plant to rush to a small but complete head rather than continuing to develop. This is a survival mechanism. Growing varieties matched to your climate and sowing at the right time to keep heading within a moderate temperature window is the main prevention.