Boron is one of the most important micronutrients for oilseed crops, especially rapeseed and sunflower. Although required in very small quantities compared to nitrogen or phosphorus, its impact on crop performance is significant. When boron is deficient, symptoms such as hollow stems, poor flowering, weak pod or head formation and reduced oil content can be observed. In many cases, farmers only notice the problem when the damage is already visible and irreversible, which is why early diagnosis and prevention are so important. In rapeseed, boron plays a key role in root development, cell wall strength and the formation of inflorescences and pods. Deficient soils, light sandy textures, low organic matter and years with heavy rainfall can all lead to boron shortages in the root zone. Plants affected by boron deficiency may show stunted growth, discoloured leaf tips, reddish or yellowish hues and weakened stems. When stems are cut lengthwise, brown necrotic tissues or hollow sections are often visible. During flowering and pod setting, boron deficiency can result in flower abortion, fewer pods per plant and poorly filled seeds. In sunflower, boron is closely linked to root development, head formation and successful pollination and seed set. Fields with boron deficiency may exhibit misshapen heads, empty patches in the centre, poorly filled seeds and brittle stems. At first glance, the crop may appear dense and healthy, but closer inspection of the heads reveals a high level of hidden yield loss caused by poor seed formation. Severe boron deficiency is often associated with the so-called “hollow stem” symptom. This problem is not only structural: weakened stems are more prone to lodging, disease infection and breakage under wind or heavy rain. In addition, transport of water and nutrients to the upper parts of the plant is restricted, which directly reduces yield and oil content. Boron deficiency is most common on light, sandy soils with low organic matter, but it can also occur on heavier soils in wet years, when boron is leached beyond the active root zone. Unbalanced fertilization practices that focus solely on NPK while ignoring micronutrients further increase the risk, especially in high-yielding hybrids with intensive growth. Correcting boron deficiency starts with prevention. For oilseed crops, it is advisable to include boron in the fertilization plan from the very beginning, either through compound fertilizers containing boron or through specific boron products applied to the soil. However, because boron is relatively immobile in both soil and plant tissues, foliar applications are often essential at critical growth stages. In rapeseed, one or two foliar sprays with boron in autumn (for example, at the 4–6 leaf stage) can improve root development and winter hardiness. In spring, additional foliar treatments before stem elongation and around the flowering period support inflorescence development and pod formation. Rates must be carefully controlled to avoid toxicity, and applications should be timed for cooler parts of the day to minimize the risk of leaf burn. In sunflower, foliar boron applications are usually scheduled at the 6–8 leaf stage and again before bud formation. This supports proper head development and improves pollination and seed set. As with rapeseed, overdose is a real risk: too much boron can cause toxicity symptoms, particularly in hot, dry conditions, so label recommendations must be strictly followed. Visual symptoms provide useful clues but should not be the only diagnostic tool. Soil analysis and tissue testing (leaf analysis) give a more accurate picture of boron availability and help farmers decide whether additional boron is needed. Regular monitoring makes it possible to avoid both deficiency and excess. In summary, boron may be a “small” nutrient in terms of quantity, but it is a “big” one in terms of importance for oilseed crops. Deficiency can seriously reduce yield, oil content and crop uniformity, while also increasing plant sensitivity to stress. By combining proper soil fertilization with well-timed foliar applications and regular monitoring, farmers can prevent boron deficiency and fully exploit the yield potential of modern rapeseed and sunflower hybrids.