Do potato plants in a field compete against weeds? First needed is to know the growth pattern of potato plants or specifically the vine. The vine sprouts underground and grows toward the surface and, if the tuber piece is healthy and conditions are good, the sprout emerges about three weeks after planting. The rate of vine growth follows an S-curve reaching a flattening area after six weeks. See Figures "Stem Length and Vine Fresh Weight of Superior..." and "Field Stem Length of Three Varieties..."
As the vine grows and canopy increases, the potato plant is able to shade more and more of the ground, depriving light from weed seedlings. In Wisconsin, using several potato varieties were grown and light penetration through the potato canopy during growth was measured. Incident light was measured outside and under the canopy, and the percent of light penetration through the canopy was calculated. This method also gave growth curves for the canopy. See Figure "Shading by Three Varieties in Wisconsin." The growth rate for these three potato varieties is nearly the same for the first five weeks after emergence and all three reach maximum shading at seven to eight weeks after emergence. After reaching maximum shading, there begins a gradual decline in shading as foliage begins to age (senesce). Maximum light interception, however, is dependent on variety. Whereas Russet Burbank, an indeterminate, intercepted as much as 98% of the light and maintained 95% interception for a month, Red Norland's maximum interception was 90% which was maintained for a week. Weed competition by Russet Burbank is much greater than by Red Norland or Atlantic. The goal may be to achieve and maintain about 95% light interception by the canopy to keep weeds from growing below it. Potatoes compete well throughout the remainder of the season once the canopy closed ("row closure"). When a canopy is damaged such as by hail, insects, diseases, or nitrogen shortage, then weed control is drastically reduced because of canopy loss unless a good herbicide program was implemented.