1. Why Ginger "Seeds" Are Broken Pieces of Last Year's Ginger
When growing most vegetables, you usually plant small seeds from a packet. Ginger is different. You break apart the rootstock—the thick underground stem harvested the previous year—and plant those pieces. These planting pieces are called "seed ginger" in Japanese [2].
According to the Saitama Prefecture cultivation manual, you should break the seed ginger by hand so that each piece has 2 to 3 buds and weighs between 100 and 150 g (about 3.5 to 5.3 ounces). If the piece does not make a good "snap" sound when broken, it is said to be at risk of disease or rot [2]. The size of these pieces is not uniform; it varies depending on the variety and the region.
Ginger is a perennial plant said to be native to tropical Asia. In Japan, the temperature is often insufficient for it to flower, so it is generally grown by dividing the rootstock rather than by seeds [3]. There is a theory that ginger was brought to Japan from China around the 3rd century [1]. Once you have the pieces, the next question is when to plant them. Planting too early or too late can cause problems.
2. Why Ginger Takes Over a Month to Sprout and Must Be Kept Safe from Cold
Ginger likes heat and dislikes cold. The lower limit for temperature for sprouting is about 15°C (59°F), and the ideal temperature for growth is 25 to 30°C (77 to 86°F). If the temperature drops below 10°C (50°F), the rootstock becomes prone to rotting [4].
Moreover, it takes more than one month from planting until the sprouts emerge. If they are hit by late frost during this time, it causes trouble. Therefore, in Saitama Prefecture’s open-field cultivation, it is considered desirable to plant in early to mid-April, working backward from the timing of late frost [2]. This is an example from Saitama; the timing changes depending on the region.
When planting, cover the pieces with about 5 cm (2 inches) of soil so they do not stick out of the surface. The manual also notes that during storage before planting, you should be careful not to let the seed ginger drop below 13°C (55°F) [2]. You want to avoid spring frost, but you also need some warmth for the sprouts to emerge. The planting date is decided in the balance between these two needs.
After the sprouts appear, what happens above the soil?
3. How Ginger Leaves Grow Above Ground While the Edible Part Grows Below, and Why Summer Drought Matters
When sprouts emerge, leaves grow lushly above the ground. However, the part you eat is the rootstock that thickens underground. Even in shade, the leaf color grows vividly, but to make the rootstock thick, high temperatures and plenty of sunlight are said to be necessary. In fields with poor drainage, even if the above-ground parts grow well, the thickening of the rootstock is inferior [4].
Ginger prefers high temperature and humidity, but it is a crop weak against dryness [4]. In the Saitama manual, it is written that during July to September, when the rootstock thickens, you should cover the ground about 5 cm (2 inches) deep with rice straw to prevent soil drying and water it. Keeping the soil moist has a large effect on the harvest amount [2].
Before that, from late June onward, when the first stem is over 20 cm (8 inches) tall, you add fertilizer and mound about 15 cm (6 inches) of soil around the base of the stems. These are the main tasks for summer care: "I like it hot, but I cannot have dry soil" [2].
So, when should the ginger grown this way be dug up?
4. How Young Ginger, Leaf Ginger, and Root Ginger Differ by Harvest Time
Shops display "young ginger," "leaf ginger," and "root ginger." These three names are divided by the harvest time and how it is handled afterward.
Young ginger is harvested from the newly grown part of the seed ginger and shipped immediately. It is said to be suitable for eating raw because the skin is thin and there is little fiber. Leaf ginger is a small rootstock harvested even earlier, while it still has green leaves attached. Root ginger is harvested in autumn and stored for more than two months; it is said to have strong spiciness and more fiber [6].
By the way, young ginger is also sold in summer. According to one source of regional information, greenhouse cultivation plants in January to March and harvests in June to August, while open-field cultivation plants in April to June and harvests in October to November. Since open-field harvest is in autumn, much of the young ginger circulating in summer is said to be harvested early via greenhouse cultivation (the months are examples from this material) [6].
Even for ginger grown in the same field, its role in the kitchen changes depending on which stage it is dug up and how it is stored. Note that the material in this chapter is a site introducing regional information, and the timing differs by region.
So, is it okay to grow it in the same field every year?
5. Why Ginger Should Not Be Planted in the Same Field Every Year
If you grow it in the same field every year, diseases tend to increase. The Saitama manual recommends choosing a field with good water retention where ginger has not been cultivated for 2 to 3 years. Because planting diseased rootstock brings root rot disease into the field, it is also important to use healthy seed ginger [2]. A document from Aichi Prefecture also lists choosing disease-free seed ginger and avoiding continuous cropping (growing the same crop in the same place) [5]. Harvesting is done before frost falls.
Looking at producing regions, the conditions for good growth are clearly shown. Kochi Prefecture has the largest ginger harvest in Japan, said to account for 40 to 50% of the national total (this changes by year) [7]. A paper from Kochi University of Technology lists the natural conditions of the producing region as: a warm climate, sufficient rainfall around September when the rootstock thickens, soil with excellent water retention, and warmth that prevents frost until autumn [7].
This overlaps with the previous discussion. Warmth, moisture, and frost timing. The path of planting in spring, thickening in summer, and digging before frost means these are among the conditions found in producing regions.
6. How a Ginger Label in a Shop Helps You Imagine a Year in the Field
What you can do today: Look at the ginger label in the shop and find the prefecture of origin. Find that prefecture on a map and imagine the weather from spring to autumn. If there is Kochi Prefecture ginger, you can think about which of the producing conditions from Section 5 might apply.
If reading the material: The ginger cultivation manual from the Saitama Prefecture Agricultural Technology Research Center is a public document readable in Japanese, with tasks written by period from planting to harvest [2]. You can grasp the flow even if you skip parts with technical terms.
Things to be careful about: The dates and numbers in this article are examples from Saitama Prefecture and some others. For the cultivation timing in your residential area, ask your local agricultural office for advice.