a 
SUPPRESSION OF WEEDS. 9 
impure seed, and to a less extent by the transference of fodder, &c., from 
one district to another. This applies more especially to St. John’s Wort, 
Dodder, Wild Mustard, Chamomile, Digger’s Weed, Thistles, Docks, and 
to a less extent to Ragwort, Stinkwort, and Guildford grass, the seeds of 
the latter plant being sometimes found among early flowering grass seed. 
Thistle seed usually travels a very short distance before it falls to the 
ground ; it is the disarticulated pappus which is often carried by the wind 
for miles, While the seed is attached the fruit is too heavy to go far, except 
- during high winds, and these usually cause the pappus to break off soon, 
and leave the seed behind. Even the fruits of the Dandelion and Sow 
Thistle fall for the most part to the ground within a few feet or yards of 
the plant producing them, and it is usually only stray seeds which are 
carried to a distance of more than 30 or 40 yards. In addition, a 
tall paling fence or a close hedge is usually sufficient to arrest them, and 
the latter especially is of great importance in arresting the spread of the 
air-borne seeds of weeds from one field to another. Hedges and fences act 
in a similar, though indirect, fashion in checking the carriage of adherent, 
hooked or burred fruits by stock. 
Suppression of Weeds. 
There are certain general methods applicable to all weeds, others applic- 
able to annuals, but not to perennials, and others, again, which are of use 
only under special conditions, or for a particular plant or group of plants. 
ANNUALS.—The chief factors are the length of time required to produce 
seed, the amount of seed produced, and the length of time it lasts in the 
soil. The old saying that ‘‘ one year’s seeding is seven years’ weeding,’’ 
though applicable to some weeds, is an exaggerated statement for most. The 
majority of annual weeds can be practically suppressed by two or three 
years’ careful and cleanly cultivation. When the surface is very foul with 
seed, one deep ploughing, by turning the seeds into the deeper layers of 
the soil where they, do not germinate, is a great aid to cleaning the soil. 
If, however, this means bringing up much subsoil, very heavy dressings 
of farmyard manure will be needed to keep the soil open and well aerated. 
The so-called ‘‘ poisonous ’’ action of subsoil when brought to the surface 
is, in nearly all cases, simply due to its deficiency in humus causing it to 
have a strong tendency to puddle, cake or pan, and so form an impervious 
layer shutting out air from the fermenting layers richer in humus which have 
been turned below. As a result, the roots are unable to strike deeply or 
develop freely, and the plants are not only starved but liable to suffer from 
drought. All cultivated plants of any size, and especially cereals, are 
rooted not so much in the surface soil as in the unploughed subsoil below 
the upper 6 to ro inches, and it is this layer which is the medium from 
which the water afd salts are mainly derived ; although they may be washed 
down from the surface layers. Most annual weeds root to a greater extent 
in the surface soil than in the subsoil, as compared with the majority of 
the larger cultivated plants, and hence surface harrowing does much to 
keep such weeds under while the crop is young, without appreciably injuring 
the latter, especially if sown in drills. 
When annual seeds are turned in by deep ploughing, they should remain 
undisturbed, if possible, for at least three years or longer, if the seeds are 
to die. Only the seeds of relatively few weeds last in the soil for a longer 
time than this.* 
* Full details of the duration of vitality of seeds are given in the work on “ Longevity of seeds ’” 
{Proc. Roy. Soc. of Vict. 1908, pp. 1-210). 
