8 BULLETIN 50, HAWAII EXPERIMENT STATION. 
maturity is lengthened when the crop is grown at the higher eleva- 
tions. Sweet potatoes are classified in three groups, those maturing 
in three months, or early varieties; those maturing between four 
and five months, or medium early varieties; and those maturing 
between six and seven months, or late-maturing varieties. Some 
suppose that the crop is mature and ready for harvesting in about 
four or five months after planting if the leaves turn yellow. Yellow- 
ing of the leaves is not always an indication of ripening and may be 
caused b}^ drought or the disease known as stem rot. Excessive 
rainfall, on the other hand, stimulates the growth of the vines and 
causes the foliage to remain green for months after the roots have 
matured. Again, it is supposed that the crop has matured if the 
roots remain white upon being broken, and that, vice versa, immatur- 
ity is shown by discoloration of the broken surface. In experiments 
with sweet potatoes which were known to be immature, the experi- 
ment station found no discoloration to occur after breaking except 
that due to the milky substance, latex, which is also present in the 
mature root. 
On account of its excellent keeping qualities the sweet potato in 
Hawaii may be left unharvested in the field for months without like- 
lihood of its decaying, even when the period of maturity has passed. 
If the roots are kept in the ground after maturity, not only will the 
quality be improved materially, but the yield will be considerably 
increased. The question of when to harvest, then, is not an impor- 
tant one, so far as maturity is concerned. Under no circumstances, 
however, should the crop be harvested after a rain when the ground 
is very moist, if the roots are intended for market and a large area 
is to be harvested. When the soil is wet, sweet potatoes are difficult 
to harvest and the earth sticks to them. 
A gardener whose sweet-potato area is small may harvest his 
crop before it matures fully. To harvest the required quantity of 
roots without disturbing every hill, the vine should be carefully 
lifted and the ground examined for large cracks or crevices around 
the plant. These cracks are found when the soil surface is dry and 
occur where extra-sized roots are growing. 
In small areas, and where the crop is grown primarily for home 
consumption, harvesting may be facilitated by means of a four- 
pronged spading fork. After the vines are removed the fork should 
be inserted in the soil at a distance of 8 or 10 inches from the plant 
and the roots lifted and brought to the surface. 
When the crop is grown on a large scale in Hawaii, a turnplow is 
used for harvesting after the vines have been removed by hand. 
The sweet potatoes are brought to the surface by plowing, thrown 
to one side by men, and later are graded for market. The roots 
should be left in the field and exposed to the sun for a few hours to dry 
YIELD. 
The yield of sweet potatoes depends largely upon the locality and 
soil in which the crop is grown, the kind and amount of fertilizer 
used, the culture given, and finally upon the variety itself. A yield 
of 10 tons per acre can be secured from a crop that is grown under 
favorable conditions. As high a yield as 17£ tons of roots and 
21£ tons of vines per acre has been produced at the Haiku sub- 
