1. Why Tulip Bulbs Planted in Autumn Grow Roots First
When a tulip bulb is planted in autumn, the roots are the first part to start growing. The tulip belongs to the lily family and is said to be native to a region stretching from Central Asia to the Mediterranean coast. It is a typical "autumn-planted bulb," which grows in winter, blooms in spring, and then the above-ground parts die back. The plant spends summer dormant, storing nutrients in a new bulb [1].
According to a cultivation manual from Toyama Prefecture, roots begin to grow all at once after planting. These roots do not branch out, do not develop root hairs, and are said not to regenerate if damaged [1]. Because of this, planting should wait until the soil temperature drops to 15°C (about 59°F) or lower, which is suitable for rooting. After October, the base of the bulb swells, and young roots become visible.
The bulb itself is a structure where leaves that have changed into nutrient-storing scales overlap around a shortened stem. The outside is wrapped in a hard, wood-like skin [1]. These scales play an important role later on. Even though roots are growing, nothing is visible above the ground yet. What is the bulb waiting for during the winter?
2. How Winter Cold Prepares a Tulip to Bloom
Cultivation materials divide the tulip’s growth into four periods. From planting until the snow melts the following spring is the "preparation for growth" period. During this time, the bulb must experience low temperatures, roughly between 5°C and 15°C (41°F to 59°F), to meet the cold requirement for blooming. This is a guideline for production areas and is not a fixed rule for every outdoor garden [1].
The importance of cold is also seen in studies of forcing tulips to bloom early. In a Saitama Prefecture study, bulbs were kept in a refrigerator at 5°C (41°F) for 8 weeks before being potted. In the groups where this cold treatment was shortened, the stems became shorter [2]. However, shorter is not always better. Depending on the variety, 3 weeks was not enough for the sprout to emerge or for the flower to open normally [2]. There is no single correct length; the ideal duration of cold depends on the variety.
After passing through the winter cold, the bulb sends a sprout above ground in spring. Where does the nutrition for blooming come from?
3. How a Tulip Uses Its Nutrients in the Weeks Before Blooming
At the Shimane University Sand Dune Farm, observations of the 'Apeldoorn' variety showed that sprouts appeared above ground in early March. Growth sped up in early April, and blooming occurred in mid-April [3]. These dates apply to that specific location, year, and variety, and are not universal for the whole country.
Where do the materials for this growth come from? A study in Yamagata Prefecture found that when cold-treated bulbs were planted, nutrients stored in the bulb moved to the growing point, allowing stems and leaves to grow [5]. Other explanations note that a small flower is already present in the center of the bulb [7].
As the leaves spread out, photosynthesis using sunlight begins. The Shimane University study organized the findings to show that nutrients made by photosynthesis before blooming were used for stem elongation, leaf expansion, and the opening of the flower [3]. However, this same study showed that the use of these nutrients changes after blooming. Does the work of the leaves end when the flower opens?
4. Why Tulip Leaves Keep Working After the Petals Fall
When the petals fall, the remaining leaves might look like they have finished their job. But the Shimane University study showed that after the blooming period, nutrients were focused on enlarging the new bulb [3].
In May, even as the leaves aged and began to turn yellow, photosynthesis continued at about 50% of its peak rate until the stems and leaves withered. The author states that this result shows the importance of the stems and leaves after blooming [3]. This 50% figure is a comparison to the peak apparent photosynthesis rate at that time; it does not mean half of the total nutrients after flowering.
The size of the leaves matters, which is reflected in farming practices. The Toyama Prefecture manual suggests managing watering from spring until mid-May to maximize leaf area [1]. The Shimane University study also notes that leaf area is a factor that influences bulb yield [3]. This method is for farmers’ fields and may not apply directly to home pots.
There is a limit to how long the leaves can work. The guideline for the period when bulbs grow is around 20°C (68°F). If exposed to high temperatures, such as those from a foehn wind, the leaves wither faster, and bulb enlargement is suppressed [1]. So, where does the new bulb that receives these nutrients form? And what happens to the parent bulb in the meantime?
5. How the Parent Bulb Shrinks as a New Bulb Grows
While the flower blooms and leaves continue photosynthesis, a generation change happens underground. A study at Yamagata University, using bulbs dug up and treated with specific temperatures, found that as the new bulb formed, the scales of the parent bulb gradually shrank, dried out, and died. It was determined that nutrients moved from the scales to grow the new bulb [4].
In the experimental groups where growth was good, the total weight of the new bulbs reached about 80% of the bulb weight at the time of the survey, and about 50% of the initial bulb weight [4]. However, these values come from bulbs stored in constant-temperature incubators, not typical field ratios.
The relationship with temperature was also investigated. Bulbs treated at 0.5–5°C (33–41°F) for more than 60 days formed new bulbs inside the parent. At 30 days, they did not form new bulbs, and at 15°C (59°F), even after 140 days, they formed almost none. It is thought that new bulbs begin to form at 0–5°C (32–41°F) and grow best at 10–15°C (50–59°F) [4]. Another study showed that when cold-treated bulbs were planted, new bulbs grew somewhat in parallel with the stems and leaves. The new bulbs then grew rapidly once the stems and leaves stopped growing and turned yellow [5].
In production areas, digging up bulbs generally happens from late May to late June. They are dug when about 50% of the outer skin has changed color. If dug too early, the skin is pale, cracks easily when dry, and is prone to mold during storage [1]. Are the dug-up new bulbs just resting during the summer?
6. How the Next Flower Prepares Inside the Bulb During Summer
Dug-up bulbs are not merely sleeping during the summer. According to Niigata Prefecture’s flower growth information, one bulb (variety 'Ile de France') dug up on June 10 had reached the stage of pistil formation (from the start of female organ differentiation to the completion of stigma differentiation) by July 31. The progress varies from year to year, so these dates should not be read as fixed [6].
This preparation requires the right temperature. The Toyama Prefecture manual states that if the storage temperature after digging is below 20°C (68°F), flower bud differentiation does not proceed. If it is above 30°C (86°F), deformed flowers increase. The recommended storage temperature is 23–25°C (73–77°F) [1].
Thus, the cycle completes one year. Roots grow in autumn, pass through winter cold, bloom in spring, and after the flowers fall, the nutrients made by photosynthesis in the leaves are used to fatten the new bulb. The parent bulb shrinks, and in summer, the preparation for the next flower proceeds inside the bulb. The nutrients for this year’s flower continue from the leaves and bulbs of the previous year.
7. How to Observe Tulip Leaves After the Flowers Are Gone
You can try this at home: ask an adult to cut an onion in half and look at the cross-section. You will see many thin layers overlapping. Since tulip bulbs are said to be made of overlapping leaves that store nutrients, comparing them helps you imagine the structure. Please ask an adult to handle the knife.
If you want to read more, the Toyama Prefecture "Tulip Bulb Cultivation Manual" [1] and Yoshino Shigeto’s "About Tulip Photosynthesis" from Shimane University [3] are available in Japanese.