The radish you pull from the garden is a pause partway through the plant’s life, a food store it builds to fuel flowering later. Left in the ground, a radish keeps going: it flowers, gets pollinated by insects, and ripens seed in papery pods. The part most gardeners see lasts only weeks, while the full trip from seed back to seed takes about five months.1Organic Seed Alliance. Principles and Practices of Organic Radish Seed Production in the Pacific Northwest
Germination and the First Leaves
Fresh radish seed is not dormant, so it can sprout without a rest period.2Weed Research. The roles of light and pericarp on seed dormancy and germination in feral Raphanus sativus (Brassicaceae) Cold soil and light slow it down, though.3Weed Research. The roles of light and pericarp on seed dormancy and germination in feral Raphanus sativus (Brassicaceae) Seed still inside its pod also sprouts less well, because the pod wall slows water uptake.4Weed Research. The roles of light and pericarp on seed dormancy and germination in feral Raphanus sativus (Brassicaceae) Shelled seed sown into moist, warm soil and covered lightly does best. Dry stored seed holds only about 4% moisture, so it must take in a lot of water before it can grow.5PubMed Central. Improvement of Germination and Early Growth of Radish (Raphanus sativus L.) through Modulation of Seed Metabolic Processes
In warm soil, the seed swells and splits within a day or two of watering. A root pushes down first, then a hooked stem lifts the seed leaves into the light. Seedlings usually appear in about 3 to 7 days in mild weather. Thin them early so each root has room to swell.
Those first two leaves are seed leaves, rounded and quite different from the jagged true leaves that come later. They pass stored food and fresh sugars from sunlight to the first true leaves.6Journal of the Japanese Society for Horticultural Science. The Role of Cotyledons in Growth and Photosynthesis of Radish Plants That is why slug or flea beetle damage to them sets the whole plant back.
A few sowing mistakes cause most of the trouble. Sowing too deep delays emergence, because small seeds carry little stored food. Letting the surface dry out stalls a seed partway through soaking.
Pressing the soil gently over the seed keeps it in contact with moisture and supports even water uptake. Sowing in marked rows lets you tell a slow patch from a failed one, so you don’t dig up seed that is still working.
The Root Swelling Stage
The root fattens because a thin ring of dividing cells inside it keeps adding new tissue outward and inward.7PubMed Central. Exogenous gibberellin suppressed taproot secondary thickening by inhibiting the formation and maintenance of vascular cambium in radish (Raphanus sativus L.) The leaves’ sugars are pumped down and stored in that new tissue. Several plant hormones together control how fast those cells divide and expand.8PubMed Central. Transcriptome Profiling of Taproot Reveals Complex Regulatory Networks during Taproot Thickening in Radish Extra amounts of one growth hormone, gibberellin, can slow the thickening.9PubMed Central. Exogenous gibberellin suppressed taproot secondary thickening by inhibiting the formation and maintenance of vascular cambium in radish (Raphanus sativus L.)
Water and timing decide the final size. Under moderate water shortage, drought cut root weight by about a quarter, and under severe shortage by about half.10Annals of Applied Biology. Responses of radish (Raphanus sativus) to drought stress Stressed plants send less of their food down into the root.11Annals of Applied Biology. Responses of radish (Raphanus sativus) to drought stress Even, steady watering matters more during swelling than at any other stage.
Spring radishes go from seed to harvest in four weeks or less.12Garden Betty. The Difference Between Winter Radishes vs. Spring Radishes: What to Plant, When to Plant Winter radishes take twice as long, about eight to ten weeks.13Garden Betty. The Difference Between Winter Radishes vs. Spring Radishes: What to Plant, When to Plant Winter types like daikon are sown in late summer to mature into autumn, and they grow larger and keep better in storage.
To tell when a radish is ready, check the shoulder of the root poking above the soil. Most spring types are ready at about an inch across. Pull one test root rather than waiting by the calendar, and harvest in batches, since roots pass their best quickly.
Sometimes a plant makes leaves and no root. Crowding, too much nitrogen, or heat and long days are the usual causes.
Why Roots Turn Pithy or Woody
A radish with pithiness has spongy patches or empty cavities in its flesh, formed late in root swelling.14PubMed Central. Oxidative stress response and programmed cell death guided by NAC013 modulate pithiness in radish taproots Stress triggers a chain of signals that causes storage cells in the core to die.15PubMed Central. Oxidative stress response and programmed cell death guided by NAC013 modulate pithiness in radish taproots Then the glue between cells breaks down, small gaps open, and they widen into hollows.16Journal of the Japanese Society for Horticultural Science. Studies on the pithiness of radish. IV. On the process of pithy tissue formation in the radish root Pulling roots on time and avoiding stress such as drought reduces the problem.
You can often spot a pithy root before cutting it. It feels light for its size, and a gentle squeeze may give a little. Cut one open to be sure: white streaks or a spongy center show that root is past prime, and its neighbors may be too. Pithy roots are safe to eat, just dry and less pleasant.
Pithiness also ties to the plant’s next stage. As a radish prepares to flower, it begins drawing on stored root food, while roots harvested during growth stay crisp. Bolting plants usually have tough, pithy roots.
A pithy root is a sign of age or stress, not disease. Test a few others in the row rather than discarding the bed, and harvest earlier next time.
Bolting and the Switch to Flowering
Bolting is when the plant shoots up a tall flower stalk from its center. Many radishes need a spell of cold before they bolt, though long days and heat also push spring types to bolt.17PubMed Central. Identification of Flowering-Related Genes Responsible for Differences in Bolting Time between Two Radish Inbred Lines How much cold is needed differs between varieties.18PubMed Central. Identification of Flowering-Related Genes Responsible for Differences in Bolting Time between Two Radish Inbred Lines That same growth hormone can push some varieties into flowering even after too little cold, by switching on flowering genes.19PubMed Central. Gibberellin Promotes Bolting and Flowering via the Floral Integrators RsFT and RsSOC1-1 under Marginal Vernalization in Radish Other varieties do not respond, and lower gibberellin delays flowering in some types.20PubMed Central. Gibberellin Promotes Bolting and Flowering via the Floral Integrators RsFT and RsSOC1-1 under Marginal Vernalization in Radish21PubMed Central. Epistatic interactions among multiple copies of FLC genes with naturally occurring insertions correlate with flowering time variation in radish
When a radish bolts, the center stretches into a tall stem with small leaves. Flower buds appear at the top within days. Root growth stops and the root toughens. Once bolting begins, it cannot be reversed.
To prevent early bolting, sow spring radishes early so they finish before long hot days. Choose slow-bolting varieties for late spring sowing. Sow winter types after midsummer so the cold comes after the root has formed. Keep plants watered, since stress hastens the switch. The same sowing date can bolt one variety and not another, so read variety descriptions for bolting resistance. A single cold night doesn’t always mean bolting, since it depends on a stretch of cool conditions.
Flowers and Pollination
Radish flowers have four petals and come in white, pink, or purple. Their size and shape vary widely between varieties.22Euphytica. Inter-accession variation in floral morphology in radish (Raphanus sativus L.) Most radishes reject their own pollen, so seed needs pollen from another plant.23PubMed. The effectiveness of insect-pollination test to evaluate the level of self-incompatibility and their genetic analysis in radish (Raphanus sativus L.) Bees and other insects carry it between plants. In wild radish, a plant’s own pollen does not block pollen from other plants.24PubMed. Self-pollen interference is absent in wild radish (Raphanus raphanistrum, Brassicaceae), a species with sporophytic self-incompatibility
In the garden, a flowering radish can reach waist height with many branches. Flowers open over several weeks from the bottom up, and bees and hoverflies visit heavily. Young green pods are edible and spicy.
Insects can also carry pollen between different radish varieties, and wild and weedy radishes can cross with garden types. Seed from mixed plantings may not grow true.
A lone flowering radish often sets little seed. If you only have one bolted plant, empty or nearly empty pods are a pollination issue, not a disease.
Pods and Ripening Seed
Radish pods do not split open, and the seeds stay sealed in a rigid pod wall.25PubMed Central. Comparative pericarp biomechanics and germination physiology of Raphanus raphanistrum and Raphanus pugioniformis indehiscent fruits Pods higher on the plant tend to be shorter, thicker-walled, and more often twisted.26Agronomy Research. Matrix quality variability of oilseed radish (Raphanus sativus L. var. oleiformis Pers.) and features of its formation in technologically different construction of its agrophytocenosis Seed is ready when about 60 to 70% of pods turn from green-yellow to brown, thin, and papery.27Organic Seed Alliance. Principles and Practices of Organic Radish Seed Production in the Pacific Northwest Harvest the whole plant at that point rather than waiting for every pod.
Pods ripen unevenly because flowers open over weeks, so green and brown pods appear together. The odd look of higher pods is normal.
To get seed out, make sure pods are fully dry before threshing. Crush them in a bag or on a tarp, then winnow off the chaff in a light breeze. Store cleaned seed cool and dry. Fresh seed can sprout right away, so self-sown seedlings may appear, returning the plant to the first stage.
Saving Your Own Radish Seed
Growers keep a variety true by pulling roots at eating size, choosing the best, and replanting them.28Organic Seed Alliance. Principles and Practices of Organic Radish Seed Production in the Pacific Northwest These replanted roots get wider spacing for the large flowering plant.29Organic Seed Alliance. Principles and Practices of Organic Radish Seed Production in the Pacific Northwest Keep about a mile from other radishes in open land, or half a mile with barriers like woods or buildings.30Organic Seed Alliance. Principles and Practices of Organic Radish Seed Production in the Pacific Northwest Seed crops take about 150 days to mature.31Organic Seed Alliance. Principles and Practices of Organic Radish Seed Production in the Pacific Northwest Cut plants and leave them to dry 10 to 14 days before threshing.32Organic Seed Alliance. Principles and Practices of Organic Radish Seed Production in the Pacific Northwest
At home, pick at least several plants with good shape and color and replant them about a foot apart. In a small garden, flower only one variety per season. Stake tall plants against wind. Skip any root that looks off or that bolted early, since you don’t want to pass along that tendency.
Hang cut plants somewhere airy and dry, and lay a sheet underneath to catch fallen seed. Rain on drying plants can spoil seed.
How Flavor Changes as the Plant Ages
Radish heat comes from compounds that break down into a mustard-like bite. These are higher in roots than leaves in autumn and early spring.33Scientia Horticulturae. Glucosinolate variation among organs, growth stages and seasons suggests its dominant accumulation in sexual over asexual-reproductive organs in white radish Once flowering starts in spring, they build up most in the flowers and stalk.34Scientia Horticulturae. Glucosinolate variation among organs, growth stages and seasons suggests its dominant accumulation in sexual over asexual-reproductive organs in white radish Taste and few side roots were favored as radishes were domesticated.35PubMed. Identification of candidate domestication regions in the radish genome based on high-depth resequencing analysis of 17 genotypes
In the kitchen, young roots are mildest, while heat and dry weather make roots hotter. Flowers and young pods carry a strong peppery bite, and the leaves are edible cooked.