1. Why Does Kelp Have a Tiny Form and a Giant Form?
A kelp called ma-konbu has a life cycle in which two very different bodies take turns. One is a microscopic gametophyte, smaller than 1 mm. The other is the giant sporophyte, up to several meters long, which is the kelp most people picture [1].
Here is how the turns go, according to the Hokkaido report [1]. From around summer, spots form on the surface of the big blade. Tiny swimming spores, which work like seeds, are released from the spots. A spore lands on a rock or a rope, sprouts, and becomes a male or a female gametophyte. Kelp spends summer and autumn in this tiny form. In winter, the female makes eggs and the male makes sperm. After an egg and a sperm join, the egg develops into a new giant sporophyte. The exact months vary by place and year.
So the speck is not a baby kelp in the way a puppy is a baby dog. It is a different-looking stage that has to come first. The next question is how long the giant form lives, and which year's blade ends up on the table.
2. Which Year's Kelp Do People Harvest?
Mostly the second year's. The first-year sporophyte grows until around summer. After that its blade dies back from the tip and slowly gets shorter. Some plants survive, though, and regrow from what is left of the blade to become second-year sporophytes. The kelp caught in the wild is generally these second-year plants [1].
This is not true for every kelp. Rishiri kelp lives two years, and it is biggest and heaviest in May and June of its second year [3]. Kelp in warmer parts of Japan can follow a different schedule, so the "two years" rule belongs to the Hokkaido kelp described here. That gives the big picture: a tiny stage in summer and autumn, fertilization in winter, a giant stage that grows for a year or two, and a harvest. But why does a winter event start the whole thing off?
3. Why Does Winter Matter for Kelp Growth?
Kelp needs nutrients such as nitrogen dissolved in the water. In winter on the southern Japan Sea coast of Hokkaido, the sea surface cools. Cold water is heavier, so it sinks, and richer deep water is pushed up to the surface. This stirring is called vertical mixing. Nutrient levels at the surface start rising in mid to late November and peak in January and February [2].
Researchers in Hokkaido found in lab experiments that gametophytes need about 5 micromoles of nitrate nitrogen per liter of seawater (5 μM) to mature and sprout. At one spot on this coast, Tomari, the winter reading was above that level in 7 of the past 15 years and at or below it in 8. So in about every other year, the winter reading fell at or below the level the experiments pointed to [2].
Two cautions apply. The winter measurements came from only one or two visits per winter, so they are rough. And the 5 μM figure comes from work focused on a kelp called hosome-konbu, so it cannot be copied over to other kinds of kelp or other seas [2]. With the nutrients in place, where on the blade does the new growth actually happen?
4. Which Part of a Kelp Blade Grows?
Many people would guess the tip, as with a fingernail or a blade of grass. For kelp, growth happens at the base, where the blade meets the stem-like stalk. That region is the meristem, a patch of tissue that keeps making new cells [4]. Because new tissue is added at the bottom and pushes older tissue upward, the tip is the oldest part of the blade. This is a general description from an encyclopedia entry, not a finding from the Hokkaido reports.
It fits another observation from Section 2: the blade dies back from the tip. A reasonable guess is that the oldest tissue is the part that fails first while fresh tissue keeps arriving at the base. The reports do not say this directly, so treat it as a guess.
How fast can it grow? The Hokkaido fish guide says Rishiri kelp, during its best period in May and June of the second year, grows about 1.6 cm (0.6 in) a day [3]. If Rishiri kelp grew at that pace for 30 days, its blade would grow about 48 cm (19 in). The pace is the peak, not an average for the whole year, and other kinds of kelp will differ.
One more part of the body is easy to misread. The root-like base is called a holdfast, and it anchors the kelp to the rocky sea floor [4]. It does the job of holding on, which is why a plant that is pulled hard by waves stays put. Whether it also takes in nutrients is a separate question that the sources used here do not answer.
5. How Long Does Farmed Kelp Take Compared With Wild Kelp?
Farmers can skip some of the waiting. In the quick-growing method used in southern Hokkaido, spores are collected in August and September from wild second-year plants with spore spots. They are attached to twisted string and grown in indoor tanks for two to three weeks. In that time they pass through the gametophyte stage and become sporophytes a few millimeters long [1].
The string is then hung in the sea around October to November and the kelp is harvested the following July or August, about 10 months later [1]. Wild kelp is generally caught in its second year. Farming moves the delicate tiny stage into a tank, which shortens the time. Note that this describes one farming method for one kelp. It does not mean that kelp always grows in one year.
6. How Can You Check the Numbers Yourself?
Take a ruler and look at 1.6 cm. That is one day of the best growth for Rishiri kelp. Multiply by 7 and you get 11.2 cm, about the length of a pencil. If you see a piece of dried kelp at home or in a shop, look at its length and width and picture which part was growing and which part was oldest. Dried kelp from a shop cannot tell you its age or kind, so this is only for fun.
If you want to read more, the Hokkaido research reports listed below are written for the general public, and they cover topics left out here, such as how kelp forests are lost along parts of the coast.