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Growing plants for spice seeds is all about timing and patience. While leaf crops are picked early at their prime, seed crops need to stay in the ground until the plant finishes its lifecycle and produces dry seeds. It takes a bit longer, but it's worth it for extra fresh spice seeds.
How To Grow For Seed Harvest
The real difference between a leaf crop and a seed crop is how long the plant stays in the ground. Cilantro doesn't take long to grow and produce leaves, but coriander seed (the seed of the cilantro plant is coriander) takes most of a season.
The seed plants can also take up a lot of garden space. A cilantro plant grown for seed takes up a foot of the garden bed for roughly three months, and each plant produces about a tablespoon or two of coriander. This means a cook who uses coriander regularly needs a dozen plants, not one, to get a usable amount.
If you're planting to harvest the seed, give the crops a back row or a corner of their own instead of mixing them into the main beds. This also helps keep their long, messy drying stage from being an eyesore.
The Five Worth Growing
The first four listed here are straightforward in most gardens. The fifth, cumin, is finicky, and its success depends entirely on your local climate.
Coriander
Cilantro and coriander are the same plant; the leaves are called cilantro and the seeds coriander. If you want to harvest coriander seeds, don't plant the slow-bolt varieties of cilantro. Those are developed to grow copious amounts of leaves and will take even longer to produce seed.
A standard coriander variety, or one sold specifically for seed, will bolt on schedule. It takes about three to four weeks for the plant to go from flower to seed. The seed changes color as it matures; it starts out green and soft, turns tan, then becomes brown and brittle. It should be harvested for seed when it is tan, and before it becomes brittle.
How To Grow: Sow seeds in early spring and again in late summer. Thin the plants to about 6 inches apart, and stop picking their leaves once the central stalk starts to grow. Cut the whole plants when roughly three-quarters of the seed heads have turned tan.
Dill
The variety of dill plant matters more than most people expect. Compact types like 'Fernleaf' are bred for leaf production, and because of that, they stay short and don't produce many seeds. The taller types, sold as 'Mammoth', 'Long Island Mammoth', or 'Bouquet', produce wide umbels and have many seeds.
Dill is sown in spring, usually flowers by midsummer, and the seeds will be ready about 90 days after planting. The flowers grow in flat clusters called umbels. The first cluster opens at the top of the main stem and ripens first, while the smaller clusters out on the side branches follow over the next few weeks. When the seeds on a cluster have gone tan and flat and rub off under a thumb, that cluster is ready to cut. Just cut the ones that are ready and let the others continue to mature.
What to Do: Grow a tall seed-type variety and direct sow it, since dill doesn't transplant well. Cut individual heads into a paper bag as they tan instead of waiting for the whole plant to finish.
Fennel
Fennel in seed catalogs comes in two forms. The bulbing type, Florence fennel, is bred for its swollen base and is harvested before it flowers. The other is common fennel, sometimes sold as sweet fennel, which doesn't form large bulbs. It grows to four to six feet tall, and puts out large umbels of seed.
This plant is perennial in mild regions, roughly zone 6 and warmer, and one planting can produce for several years. Fennel has escaped cultivation across California and stretches of the Pacific Coast, and it is now an invasive weed. Please check local guidelines before planting.
The seed ripens late, often into early fall, and it goes from bright green to a dull gray-green as it hardens. The seed can be eaten at the green stage, and it tastes sweeter and more aromatic than it will once dried. This is because the oils that have the strongest flavor dissipate as the seed dries.
What to Do: Grow common or sweet fennel rather than Florence. Cut the individual umbels once the seeds turn dull gray-green and come off the head under light thumb pressure. Harvest the umbels individually, as the seeds ripen.
Mustard
The mildest mustard seed is Sinapis alba, the yellow or white mustard that is used to make table mustard. Brown mustards are Brassica juncea, and they are used in Indian cooking, Dijon-style blends, and the hot Chinese mustard served with egg rolls. The hottest mustard is Brassica nigra, black mustard.
The yellow types mature in about 85 to 90 days, while the brown and oriental ones take roughly a week longer. The pods ripen from the bottom of the stalk upward and split on their own once fully dry. When about two-thirds have gone tan, and the seeds rattle when you shake the pods, the plant is ready to be harvested.
What to Do: Cut plants at the base when two-thirds of the pods are tan, then hang them upside down inside a paper bag for two weeks and shake the bag hard.
Cumin
This plant needs roughly 120 frost-free days with sustained heat, low humidity, and dry weather through the final weeks while the seed sprouts. If it doesn't have all these things, the plants will be shriveled or won't appear at all.
The real problem with growing cumin is humidity because it can cause a blight that stops seeds from maturing. Another problem is Fusarium wilt, which lives in both soil and seed, kills plants outright, and has no cure once it takes hold.
For gardeners in the Southwest, inland California, and similar hot, dry regions, cumin is an option. For most of the humid East and the cool North, it is a long shot, and the plant transplants badly enough that starting indoors to stretch the season only helps a little.
What to Do: Skip cumin unless summers are long, hot, and dry. If trying it anyway, direct sow after nights are above 60°F. Space plants 4 to 8 inches apart, and don't expect more than a small yield from each plant.
Seed Harvest Timing
The word for a ripe head releasing its seed on its own is shatter, and it is how every one of these plants reproduces. The seed matures a few days before the plant releases it, so the harvest window is very short. Check the plants every day or two once the seeds start to change color.
You can tell whether a seed head is ready by color, touch, and sound. The seeds change color as they mature, so watch this closely as the plant ages. A gentle shake at this stage usually makes a dry rattle instead of a soft rustle, which means the seeds are ready. Or, if you rub the seed head lightly, it will release some seeds. Once seeds begin dropping without any help at all, the window has already closed.
The weather during this time will affect the harvest date a lot. A stretch of hot, dry days can push a crop from ready to shattering in two or three days, while rain landing on a nearly ripe head can sprout the seed where it sits or start mold inside the flower cluster.
If you're unsure when to cut the seed heads, err on the early side. A seed head that is removed from the plant a little ahead of schedule will keep drying on its own and should be fine. If you leave them too long, though, the seeds all end up on the ground. The seeds won't all be ready at once, so wait until almost all of them have darkened before cutting.
Drying, Threshing, and Winnowing
The cut stalks need a warm, dry spot with moving air and no direct sun. A garage, a covered porch, or a spare bedroom all work well enough. Tie five or six stalks into a bundle, slip a paper bag over the heads, and cinch the mouth of the bag around the stems so anything that falls lands inside. Hang the bundle upside down for about two weeks. In humid weather, it might take longer; just wait until everything is dry.
Once the stalks are dry, the seed has to be knocked loose from the heads or pods, which is called threshing. The dill, fennel, and coriander heads will release their seeds when you rub them between the palms. Mustard pods hold on harder, so a good method is to fill a pillowcase with dry pods and crush them under a rolling pin. Stepping on the bag also cracks them open without crushing the seed inside.
After threshing, the bowl will have seeds mixed with broken bits of stem, pod, and flower, which is called chaff. Separating the two is called winnowing, and it works because the chaff weighs less than the seed. Pour the mixture back and forth between two bowls in front of a fan set on low, or outside in a light breeze, and the seed will drop straight down while the chaff blows off to the side. It takes a few passes to get it clean. You can also run everything through a kitchen sieve first, which pulls out the big stem pieces and speeds the whole thing up. The seed does not need to come out spotless. A little chaff in the jar doesn't hurt anything.
Before anything goes into a storage jar, press the seeds with a fingernail. If they dent, they still have moisture, and moisture in a sealed jar can lead to mold. To combat this, spread the seeds on a tray for a few days and test again.