SOIL MANAGEMENT. 59 



Factors Affecting Weed Control. — (1) Seed produc- 

 tion. The following table shows the extremely prolific seed 

 production by certain weeds, the figures representing the 

 number of seeds per single weed plant : — 



Highest recorded. 



Common Groundsel {Senecio vulgaris) 20,000 



Sow Thistle (S. arvensis) .' 19,000 



Ox-Eye Daisy (C. leucanihemum) 26,000 



Scentless Mayweed (M. inodora) 310,000 



From this the necessity for preventing the production of 

 seeds is obvious. 



(2) Germination. The longevity of weed seeds depends 

 largely on the condition of the soil in regard to moisture, tem- 

 perature, and oxygen supply. Mustard seed is supposed to 

 remain viable in the soil for two or more years, while under 

 the same conditions the seed of other weeds may lose their 

 vitality in two years or less. Some will not germinate until a 

 certain period has elapsed after apparent maturity of the seed, 

 e.g., Wild Oats (Avena fatua). Xanthium spinosum produces 

 a plant one year, and often another from the same fruit the 

 following year. Witchweed seed germinates only when in con- 

 tact, or nearly so, with the roots of certain grasses, e.g., maize 

 and sorghum. These characteristics will naturally govern 

 the methods of control. 



(3) Life histories. It is important to know the life his- 

 tory of a weed in order to ascertain the stage at which it may 

 be most easily destroyed. For example, Johnson's Grass can 

 be controlled quite easily if ploughed down before it sends out 

 rhizomes, which occurs relatively late in its growth, but if left 

 until beyond this stage its eradication is both difficult and 

 expensive. 



Annuals and biennials can be effectively dealt with by 

 judicious ploughing and frequent cultivation. They are most 

 easly controlled when in the seedling stage, e.g., Khaki Bos 

 (Tagetes Minuta) can be eradicated with ease by repeated 

 harrowing at this stage. 



Perennials, especially those having rhizomes and adventi- 

 tious roots, are the most difficult to eradicate. In most parts 

 of South Africa these can be easily controlled by ploughing, 

 and thus exposing the rhizomes to adverse conditions, such as 

 dry atmosphere and frost. In the summer rainfall area weeds 

 like Kweek (Cynodon dactylon) are destroyed by ploughing in 



