Growing Cluster Mallow From Seed: Soil Temperature, Depth and Thinning

Wait for warm soil, not for a date on a calendar

Cluster mallow, also sold as Chinese mallow and known in Japan as okanori, is Malva verticillata: a mallow-family leafy green whose crinkled leaves turn faintly slippery when cooked, which is where the name land seaweed comes from. Everything about the way it starts is governed by soil temperature rather than by the date.

The seed will technically move at 55F (13C), but it moves badly. Emergence is ragged, half the packet rots, and the seedlings that do appear sit still for a fortnight. Sow once the top two inches (5 cm) of an unmulched bed hold above 60F (15C) overnight, and it comes up evenly in five to eight days. At 70 to 77F (21 to 25C) it is up in four or five.

There is no prize for being early. A sowing made into cold spring ground is routinely overtaken by one made two weeks later into warm ground, because the later sowing never checks. Push a soil thermometer in at mid-morning, four inches (10 cm) down, and read it three mornings running before you commit the packet.

Softening a hard seed coat

The seed is a small, flattened disc with a coat built to survive years in the soil. Left dry, a proportion of any packet simply will not take up water in the first season. Soaking fixes most of that. Put the seed in room-temperature water for twelve to twenty-four hours before sowing, drain it on a paper towel until the seeds separate again, and sow the same day. Soaked seed that dries out hard on a bench loses the advantage.

If you have old seed, or seed you saved yourself, scarify it lightly as well: rub it for a few seconds between two sheets of fine sandpaper before soaking. You are scuffing the coat, not grinding it. Do not soak beyond a day, and never leave it in standing water overnight in warm weather, because the seed ferments and you lose the lot.

One consequence of that tough coat shows up later. Seed shed by a plant you let flower survives the winter in the ground and germinates the following season, often more evenly than the packet did. That volunteer habit is useful if you want it and a nuisance if you do not, and it is dealt with by cutting flower clusters rather than by weeding seedlings.

Soak in plain water only. Bleach and hot-water treatments are used on other crops to kill seed-borne disease, and on this hard, thin seed they mostly kill the embryo.

Depth, spacing and thinning

Sow shallow. Half an inch (1 cm) is the maximum, and a quarter inch (5 mm) into a firmed surface is better. Deep sowing is the second most common cause of a failed row after cold soil, because the seedling exhausts itself before it reaches light. Firm the seed down with the back of a rake so it sits in contact with damp soil, then cover lightly and water with a fine rose.

Sow more thickly than you intend to keep, then thin hard and thin twice. A plant left at two inches (5 cm) from its neighbor makes a single thin stem and gives you three cuts a year. The same plant at eight inches (20 cm) branches from the base after topping and gives you a cut every ten to fourteen days all season.

Thinnings are not waste. At the four-leaf stage they are already a usable salad leaf, mild and slightly mucilaginous, so pull the thinnings with a pinch at soil level and eat them rather than composting them.

1. Prepare a firm, level seedbed Rake to a fine tilth, then firm with the back of the rake. Cluster mallow will not tolerate a fluffy, hollow bed that dries out an inch down. Aim for a soil that holds a footprint without smearing.
2. Sow in drills a quarter inch (5 mm) deep Draw drills 12 inches (30 cm) apart. Space seed roughly an inch (2.5 cm) along the drill, cover, and firm again. Water the drill before sowing rather than after if the surface is drying fast.
3. Thin at the two-true-leaf stage to 4 inches (10 cm) Take out the weakest, the tallest and any that are leaning. Do this on a damp morning so neighboring roots are not torn. Water the row afterwards to close the gaps you have opened.
4. Thin again at 6 inches (15 cm) tall to a final 8 inches (20 cm) This second thinning is what makes the crop. Eat what you pull. If you want very large, long-lived plants for a single long row, go to 12 inches (30 cm) instead.
5. Mulch once plants are 6 inches (15 cm) tall An inch (2.5 cm) of straw or fine compost holds surface moisture and keeps rain from splashing soil onto the low leaves you will be picking. Keep the mulch a finger's width clear of the stems.

Direct sowing, transplants and a second sowing

Direct sowing is the better route. Cluster mallow makes a taproot early and resents having it checked, and a direct-sown plant will overtake a transplanted one within a month. Sow modules only if you are fighting slugs, birds or a short season, and if you do, use deep cells, sow two seeds per cell, and plant out at the three-leaf stage without teasing the roots apart.

A second sowing is worth making. The first sowing carries the early and middle part of the season, but by high summer those plants have been cut many times and are increasingly stemmy. A sowing made six to eight weeks after the first gives you clean, tender young growth for the cooler end of the season, when the first sowing is running to flower.

In a mild-winter garden a third, late sowing will overwinter as small plants and start away very early the following spring. It will not give much before hard weather arrives, but it is the earliest cutting you will get.