Ginseng growers have faced many weather-related challenges this year that have stunted the canopy, increased disease, caused earlier senescence of leaves and collapsed shade structures. When making decisions about how much to invest in protecting the ginseng canopy after damage to the canopy or the shade structure, it is important to keep in mind how much of this year’s photosynthesis has already occurred and how much you may be protecting.
There are many variables that affect the amount of photosynthesis over the growing season, many of which have not been studied for ginseng. These include the timing of canopy development, cold temperatures that affect canopy emergence, cloud cover, air temperatures by month, disease levels, and the efficiency of photosynthesis as the leaf ages. The only relatively well-known variable is the amount of solar radiation. These values are available by location and are affected by sun angle, cloud cover and day length. If we consider the canopy fully emerged in an older garden by May 20 and close to fully senesced by October 10 (or temperatures too low for much photosynthesis beyond that date), then the estimated proportion of solar radiation by month for Tillsonburg is shown in Figure 1 (blue bars). That means that the ginseng crop receives an estimated 73% of its solar radiation by August 20 and 80% by the end of August.
Figure 1. Estimated proportion of solar radiation (Tillsonburg) and photosynthesis by month for ginseng based on a May 20 to October 10 growing season.
But this number does not account for the health of the leaves. Obviously, the leaves are healthier in June than they are in September, even if diseases are under control. Leaves in September are showing much more purple and red colours, an indication that the green chlorophyll levels are much lower and other pigments are showing through. Even now, leaves are showing definite signs of senescence. Temperatures are also less than optimal in September for most of the day. In an average year, a reasonable assumption is that photosynthesis is near peak from canopy closure in late May through July, then drops from August 1 onwards. If we assume that photosynthesis is only about 70% of the peak in August, 50% in September and 20% in the beginning of October, then the amount of photosynthesis that has already occurred by the end of August is even more dramatic. Factoring these numbers in, then the proportion of photosynthesis by month is shown as the orange bars in Figure 1. That means that an estimated 82% of the season’s photosynthesis has occurred by August 20 and 89% by the end of August.
If we assume that the crop has been de-budded, then the energy captured by photosynthesis is used for continued respiration of the leaves and roots and growth of the root. Respiration is probably constant all season long meaning that 82% of photosynthesis occurring by August 20 is probably roughly equivalent to 82% of the season’s root growth. These estimates are based on a healthy canopy unaffected by disease or shade damage. If half the canopy is weakened or defoliated by these factors, then only around 9% of an average year’s root growth is yet to occur.
If a healthy 3- or 4-year-old garden adds 1,500 dried pounds of root in a year, then that leaves only 135 lbs yet to be produced in a canopy with 50% damage. At current prices, that would be around $1,900 worth of root. Even if the canopy is healthy, beyond September 1, only 11% of the root growth is yet to occur in an average year, roughly 165 lbs. By September 15, only 5% of root growth is yet to occur, roughly 75 lbs. This is probably an overestimate for most years based on how much colder it is at that time of year and the normal condition of the canopy at the time.
Growers need to consider how much money it is worth investing in that remaining canopy, especially when the canopy is already damaged and root prices are low. Based on estimates provided by growers for the OMAFRA Cost of Production of Ginseng in Ontario factsheet last updated several years ago, each pesticide application averages $157 per acre including labour. One weekly fungicide spray between now and the beginning of October, roughly 7 weeks, would already cost $1,100 (probably more at today’s prices). That does not include any costs for repairing shade, adding fertilizer or other sprays, or weed control nor the added cost of harvesting, processing and drying the additional root. As each week passes, the value in protecting that canopy goes down. Even if it is profitable to maintain the canopy now, it may not be once we get to early September. At the very least it is worth considering backing off on the spray program and tolerating low levels of foliar disease.
Obviously, the numbers provided here are estimates and haven’t been evaluated. Also root prices are never set in stone and can change, changing the math in the examples above. There is also the possibility that a healthy canopy moves some sugars from the leaves to the roots in the fall, which may increase root growth a small amount. The main point of this article is to show that some inputs may not be worthwhile late in the season like they were in the past. When prices were much higher, it was important to protect every pound of root, but the math becomes much less certain when prices are low. When prices are low, the only way to recover profit is to lower cost of production per pound of root. That means investing in those things that increase yield and quality the most and cutting back on things that only marginally increase yield. Those practices that would only marginally impact yield, or have no impact at all, after this point in the season would include fertilization (i.e. nitrogen, foliar nutrients, biostimulants), controlling insects and slugs, and potentially manual weed control (as long as the weeds can be controlled with herbicides in the off-season and seed production is prevented).
Keep in mind that root rots can still develop and spread even if the canopy is destroyed. It is definitely worthwhile to protect the roots from further damage through fungicide application until soil temperatures become too low for continued spread, especially for Phytophthora.