Aromatic plants contain both volatile and non-volatile fractions and the chemical composition of these two fractions may be influenced by seasonal changes. The essential oil and solvent extracts of S. africana-caerulea, S. africana-lutea and S. lanceolata, collected at the same locality throughout the 2005 growing season, were compared in terms of essential oil composition, yields and biological activities. Mostly quantitative, rather than qualitative variationwas observed in the oil composition of each species.Major fluctuations in the composition of S. africana-caerulea oil included limonene (2–33%) and viridiflorol (2–24%). Levels of α-pinene (1–12%), myrcene (2–12%) and α-eusdesmol (trace-13%) fluctuated seasonally in the S. africana-lutea oil. In S. lanceolata, considerable changeswere noted for β-caryophyllene (1–19%), β-caryophyllene oxide (1–21%) and ledol (3–12%). The extract prepared from S. lanceolata harvested inwinterwas more active against Gram-positive bacteria. The S. africana-caerulea extract exhibited the most favourable antiplasmodial activity when harvested in winter (IC50 value: 12 μgml−1), which contrasts with the lowest anti-plasmodial activity of S. lanceolata obtained at the same period (IC50 value: 43 μgml−1). The anti-oxidant activity of the solvent extracts also displayed variation over seasons with thewinter collection of S. africana-lutea yielding the most favourable anti-oxidant activity (IC50 value: 10 μg ml−1). All the solvent extracts prepared from the winter collection exhibited the lowest toxicity (20bIC50 valuesb60 μg ml−1), while the three essential oils obtained from autumn collection were more toxic (0.03bIC50 valuesb0.4 μg ml−1).

Peppermint dry weight biomass was higher in Verona (8119 kg/ha) than in Stoneville (6115 kg/ha). Overall both, oil content and yield were higher in bud formation stage than flowering stage. The levels of major essential oil constitutes were (-)-menthol 26 – 30 %, (-)-menthone 14 – 21 %, (+)-menthofuran 5 – 11 %, and eucalyptol 3 – 4 % of total essential oil content at flowering stage. Menthone content and its yield were higher at first cut; however, (+)-menthofuran content and its yield were higher at the second cut at bud formation. Although N fertilizers at rate 80 kg/ha did not affect essential oil content and yield at cut 1, N rate at 80 + 80 kg/ha increased oil yield at cut 2.