A Critical Look at Sweetpotato Yield

Yields for most Mississippi sweetpotato growers are anticipated to be on par with expectations.  However, growers not content with their yields should take a critical look at the vines that ride up the front of the digger- each one tells a story.  Here are three non-pest related contributors to insufficient yield:

Planting depth

The first thing I look at when I see an unproductive hill is planting depth.  Sweetpotato roots develop from nodes (the point where leaves attach to the stem of the slip) and from the callus tissue that develops on the cut end of the slip.  Increased planting depth places more nodes below ground and increases the potential for more roots to develop.  Increased planting depth also places more nodes deeper into the soil profile where temperature and moisture levels are often more moderate than nearer the soil surface.  Research shows that sweetpotato yields increase as planting depth increases from 0 to 5”.    Increasing planting depth is one of the simplest ways to increase potential yields.

Root type

In the broadest sense there are three types of roots sweetpotatoes produce (Figure 1).  Storage roots are roots that thicken to greater than 1” in diameter and are ultimately marketed and sold by growers.  Fibrous roots are thin, white roots.  Pencil roots are slightly thickened (less than 1” at harvest), very elongated, and pigmented.  Ultimately the goal of the grower is to produce the greatest number of storage roots possible.  However, when environmental conditions are not favorable for storage root formation or if roots are damaged early in their development, fibrous and pencil roots develop.  A lack of soil moisture, excessive nitrogen, and a lack of soil oxygen (as a result of soil compaction or excessive soil moisture) can all decrease storage root number and contribute to increased pencil root number.  Root type and number are determined by environmental conditions in the first 2 to 3 weeks after transplanting.  If pencil roots are abundant, one should review transplanting conditions and weather events the first 2 to 3 weeks after transplanting to determine the likely culprit.

Fig. 1. Sweetpotato plant with thickened storage roots; slightly thickened, pigmented pencil roots; and thin, white fibrous roots.

Note: As a pencil roots grows, it may form a storage root if it finds a suitable environment (Figure 2).  The length of the pencil root before it becomes a storage root can be a good indicator of where conditions within the soil became ideal for storage root development.  The storage roots located at the end of pencil roots will be delayed and will likely only reach canner or No. 2 size by harvest.

Fig. 2.  Sweetpotato storage roots (left).  Small storage roots developed at the end of pencil roots (right).  Note the shallow planting depth of this plant.  Although this plant did yield 4 No. 1 roots, yields likely would have been increased by deeper slip planting.

Running Out

A third common non-pest reason for reduced yields is the use of slips from older generation seed.  Slips planted from first (G1) and second generation (G2) seed are often superior to later generations.  The further the generations extend from the virus-indexed G0 stock, the greater the variability of plant characteristics that define a sweetpotato clone/variety.  This variability can appear in as little as three generations in some varieties and can be evident in a change of root shape, skin color, flesh color, and even yield.  While a G1 seed purchase represents an additional expense, it is an important part of a successful sweetpotato production program.