1. Which part of a turnip is the round white ball?
Most people, and most seed packets, call it a root. A 2019 research team from Wageningen University in the Netherlands looked at it more closely. They write that the storage organ of a turnip, which grows from a true seed, is a swollen hypocotyl with varying amounts of root and stem tissue mixed in. The hypocotyl is the short section of a young plant between the seed leaves and the root. The team suggests calling the turnip ball a "hypocotyl-tuber" to show where it comes from [1].
Why does this matter? A potato is a different kind of storage organ: it grows from an underground stem and has "eyes" that can sprout into new plants. A turnip has a smooth skin without eyes, which fits the idea that it is not a potato-style tuber [1]. The paper also says the top of a turnip can show leaf scars and side shoots, and the bottom can sprout extra roots, so the ball is a mix of tissues rather than one clean part [1].
What is inside the ball? The same paper reports that turnips mainly store the simple sugars glucose and fructose, rather than starch as potatoes do [1]. In everyday gardening talk, "root" is still fine. The scientific point is that the part you eat is not only root.
2. How soon after sowing does the ball start to swell?
In the Wageningen experiments, turnip seedlings and seedlings of pak choi, a close relative that does not form a ball, looked alike for the first days. By 16 days after sowing, the cells in the water-carrying tissue (xylem) of the turnip hypocotyl had already started to differ from pak choi's, and the researchers took this as the early stage of ball formation [1]. The authors also report that a plant hormone called auxin encouraged the growth of the ball in lab dishes. They only tested this with tissue cultures, so it shows that auxin can encourage growth there, not how it works in a whole plant [1].
That timeline fits what gardeners see. A garden guide from the Royal Horticultural Society in the UK says turnips are fast-growing and can be ready in as little as six weeks [2]. So the swelling starts early, in the first weeks of a plant's life. That is one reason gardeners thin seedlings early.
3. Why do gardeners pull out some seedlings after they sprout?
Turnip seeds are sown close together, and a fact sheet from Montana State University Extension tells gardeners to thin the plants to 4 to 6 inches (10 to 15 cm) apart [3]. Other advice differs a little. The RHS gives larger spacings for some UK varieties, so check the seed packet for the kind you grow [2].
The sources we read give the instruction but do not run a crowding experiment, so the reason below is our reading of it. A swelling turnip probably needs room to widen. If seedlings are packed together, each plant has to share the same soil, water and light with its neighbors, and each ball has less space to grow into. Thinning gives the remaining plants more space and may reduce that competition.
The pulled-out seedlings do not have to be wasted. The RHS notes that turnip leaves are edible and can be eaten like spinach [2].
4. When should you pull a turnip, and what goes wrong if you wait?
The Montana State fact sheet says to begin harvesting when turnips are about one inch (2.5 cm) across or larger, and to choose young, small ones because they are the most tender. Overgrown turnips are sharp in flavor and tough and woody in texture [3]. The RHS agrees that turnips are best harvested young [2].
So the answer to "bigger is better?" is no. Unlike a pumpkin, a turnip does not get better as it grows. We did not verify an exact best size, so treat one inch as a starting point and pick them while they are still young and small.
Turnips also like the cold. Both sources say they grow best in cool conditions, and the Montana State sheet says they can survive several light freezes, so harvest can run late into the season [2] [3].
5. How can you check where a turnip's ball comes from?
Buy a turnip with its leaves still attached, or ask an adult at a farmers' market. Ask the adult to cut it in half from top to bottom with a sharp knife while you watch from a safe distance. Look at the cut face. Near the top, where the leaves joined, the skin is rougher, and near the bottom there is a thin tail where the true root is. Does the fat middle look more like the top or the tail? Compare it to what the research paper says about a mix of stem, hypocotyl and root.
If you have a garden or a big pot, you can also sow turnip seeds with an adult and thin half of them to a hand's width apart while leaving the other half crowded. Wash and eat the thinned leaves, and compare the sizes of the turnips after a few weeks.