The cooler weather in the last half of March and early April has significantly slowed ginseng crop emergence, but it is probably accelerating again in the last few days. As of April 9, 68 growing degree days (GDD)(Base 5) have accumulated at Delhi. As a reminder, GDD is a standard model often used to look at natural processes like plant and insect development. It assumes that no development takes place if the average temperature for a day is below the base temperature chosen. If the average temperature on a day is 1 degree higher than the base, then 1 GDD accumulates.  Looking at the 22-year average from 2002 to 2023, we normally reach 68 GDD on April 18. That roughly means that many perennial plants and insects would be at the same stage now as they normally are on April 18, meaning we reached that stage 9 days earlier this year.

Even if the model is perfect and the rest of April is normal, being 9 days ahead of schedule now does not translate to being 9 days ahead of schedule in late April when ginseng normally emerges. Temperatures in late April are much warmer and more GDD accumulate per day at that time. It is better to compare with previous years. Looking at every year since 2002, this is now tied for the third earliest we have reached 68 GDD after 2012 and 2010 when we reached that level on March 20 and April 7, respectively. We are at the exact same level now as we were on April 9 of 2021 for a more recent comparison.

The weather over the next few weeks is much more important for determining the risk to ginseng. At this point, expect ginseng to emerge at least several days ahead of normal.

Seed Stratification Reminders

At the OGGA Industry Day I gave a presentation on seed storage and handling. Here is a summary of seed stratification and things that can negatively affect germination.

There are two types of dormancy that prevent ginseng seeds from germinating at seed harvest: 1) The embryo in the seed is not mature at harvest and needs a warm period of  15-20°C for around 120 days to fully mature, and 2) Once the embryo is mature (around 4 mm long or longer – Figure 1), a cold period of 0-5°C for 80-90 days is required to break physiological dormancy. The full process of breaking dormancy is called ‘stratification’.

Figure 1. A close-up of a ginseng seed with a fully mature embryo ready to germinate. This embryo is around 3.5 mm long, so the 4 mm requirement is only an average and an approximation.

Normally in nature, the berries fall to the ground and the berry pulp inhibits embryo growth. Then over the first winter it is too cold for any embryo development. As it warms up in the spring the embryo begins to grow reaching around 5 mm in length by the fall. A second winter is then needed to break physiological dormancy and initiate germination.

In the past when most growers were putting seed into buried seed boxes, this natural cycle was mimicked. Now that most seed is stored in coolers, growers still follow the same natural cycle but there are more chances for things to go wrong.

Most of the issues with poor or no germination after planting are due to improper conditions during the embryo growth phase in the summer. As I mentioned, the embryo needs 120 days at 15-20°C to fully develop before physiological dormancy can be broken. Some of this warm period comes before seeding by warming up the seed box around May, and some of it comes in the field after seeding. Embryo growth is much slower when temperatures are even a little higher or lower than this range. Embryo growth stops completely when the seed dries out. As a result, the three most common causes of poor germination (other than pathogen contamination) or delayed germination (seed germinating in the 2^nd year after seeding) are:

1. Temperatures too hot or cold: While some embryo growth occurs above or below the 15-20°C range, it is much slower at those temperatures. This means that the seed may need a longer period of warmth to reach the proper size if temperatures cannot be kept in the proper range. Simply warming up the seed and storing the seed boxes in a barn likely results in temperatures much higher than 20°C for a significant portion of the summer. This could delay embryo growth and may prevent it from having enough time to reach the proper size before winter.
2. Seed too dry: The embryo will not grow when the seed is dried out. Drying out may not kill the seed, but all development stops. If the seed dries out for a month, that is one month less time for the embryo to develop before winter. Monitor the moisture levels in the seed boxes to ensure they have not dried out during this period.
3. Seed warmed up too late: If you keep your seeds in the cooler too long in the spring, the embryos may not have enough time to develop before winter. If you cannot keep your seed at the optimum temperature during the summer, then you can compensate by warming up the seeds a little earlier in the spring. The only risk of having the seed warm for too long is that there is more time for pathogens to grow and infect the seeds, but adding an extra week to a 14 week process will only marginally increase risk.

Understanding the stratification requirements of ginseng also allow for longer term storage. If an immature embryo is kept cold indefinitely it will just not develop until a warm period occurs. This can also be done after air-drying the seed which reduces pathogen contamination and allows for a longer storage period. However, care must be taken not to let seed dry down too much or the embryo could die. It is best to store dry seed at no lower than 35% relative humidity. Dried seeds would have to be soaked for 24 hr to rehydrate them and then given the full 120 days of warmer temperatures for the embryo to develop. Another major issue that occurs is when excess seed with a nearly mature embryo is placed back into cold storage after seeding. In this case the embryos are often large enough to break physiological dormancy and the cooler can provide the proper cold period to do this. If the grower tries to plant that seed the following August, the seeds will germinate soon after planting only to be killed off by frost in October. There is a method for properly storing seeds at that stage that involves drying the seed under very specific conditions before exposure to cold, but it is a riskier method and requires strict adherence to proper temperature and humidity throughout the process.