Starting Herb Seedlings: Basil, Parsley and More

Herbs divide into two groups for the home gardener. Basil, parsley, dill, cilantro, chervil, chives and summer savory are raised from seed without much trouble. Rosemary, thyme, oregano, sage, tarragon and mint are usually bought as plants, because their seed germinates unevenly, runs off-type, or takes so long that a beginner loses interest. Start with the seed-grown group, give the seedlings steady light and careful watering, and set them out only when the soil and the nights have warmed.

Which herbs to sow and which to buy

Seed is the cheap and simple route for annual herbs and for parsley, which is a biennial grown as an annual. The woody, perennial herbs are a different matter. Rosemary and thyme germinate slowly and unevenly and grow slowly from seed; oregano from seed can vary from the parent plant; French tarragon is grown from cuttings or divisions because seed does not come true. Buy those as transplants or start them from cuttings, and use seed for the rest.

Parsley, dill and cilantro also have long taproots and resent disturbance, so sow them where they are to grow, or start them in a deep cell and move them young. Basil transplants well and is commonly started indoors.

Starting basil from seed

Basil is tender and dislikes cold at every stage. Sow it indoors a few weeks before the last frost date, not months ahead. The general indoor schedule in the Maryland Extension tables runs 5 to 6 weeks for tomato, and basil is a warm-season crop set out at about the same time, though it is not on that list; the seed packet and your frost date set the exact week. The full sequence for flats, cells and light is in how to start seeds indoors, step by step.

Sow the seed shallowly, about twice its diameter, and keep the mix at 65° to 75°F, the range Maryland Extension gives for most vegetable seeds. Basil germinates quickly in warmth. A clear cover holds moisture until the sprouts appear, then comes off at once.

Once the seedlings are up, give them a 55 to 60°F night and a 65 to 70°F day. Keep the light close: Maryland Extension suggests 2 to 6 inches above the plants for 14 to 16 hours a day on a timer, and Clemson suggests 16 to 18 hours with tubes 2 to 4 inches above the seedlings. Weak, pale, stretching basil is usually a light problem, not a feeding problem.

Pinching for a bushy plant

Basil grows straight up if left alone. When a seedling has a few sets of true leaves, pinch out the growing tip just above a pair of leaves. Two shoots grow from that point, and each can be pinched again. Maryland Extension gives the same advice for basil in the garden: pinch stems for bushy growth. Do not strip the plant hard while it is still small; take the tips and leave the lower leaves to feed the roots.

Parsley: slow germination and short seed life

Parsley is the herb that makes gardeners think their seed is dead. It germinates slowly and unevenly, and old seed germinates poorly. Maryland Extension lists parsley among the short-lived seeds, good for 1 to 2 years when stored cool and dry. If the packet is older than that, run a germination test before sowing a whole flat: roll 20 seeds in a moist paper towel, bag it, keep it warm, and count sprouts after 7 to 10 days. Fewer than 50 percent means discard the seed. The method and the other causes of failure are set out in seeds not germinating? common causes and a germination test.

Sow parsley shallowly and keep the mix evenly moist but not saturated. Because the seedlings emerge over a long stretch, do not judge the flat after a few days. Thin or transplant carefully; parsley has a taproot and does not like being moved after it is established.

Light and water for herb seedlings indoors

A windowsill is seldom enough. Stems stretch and lean toward the glass, and the plants that result are soft and slow to establish. Use a fixture on a timer, keep it close to the tops, and raise it as the plants grow.

Water from the top until it drains, or from below by setting the containers in a shallow tray; Clemson suggests 1 to 2 inches of water in the tray for 3 to 10 minutes. Then let the surface of the mix dry slightly before watering again. Saturated mix starves roots of oxygen and invites damping-off, the water-mold disease that collapses seedlings at the soil line. Once seedlings have two true leaves, a small fan nearby makes them sway and begins toughening them, but it also dries the mix faster, so check moisture more often.

Most potting mixes carry enough fertilizer for at least 6 weeks, and some labels say 6 to 8 weeks after germination. Clemson’s practice is to feed at every other watering once the first true leaves appear, with plain water between feedings, and to rinse any liquid fertilizer off the leaves because young seedlings burn easily.

Hardening off and planting out

Basil is not tolerant of chilly temperatures. Set it out after all danger of frost, when the soil has warmed, in full sun with at least 6 hours of direct summer light. Harden the plants first: over 5 to 14 days depending on the crop, Maryland Extension moves plants from a shaded, wind-protected spot into more sun each day, and Clemson starts the process once plants are about 3 inches tall. Bring basil in if the night will drop below 50°F. The step-by-step routine is in how to harden off seedlings.

Parsley and dill are hardier and can go out earlier, but they still benefit from a gradual introduction to sun and wind. Set transplants on a cloudy, calm day, water them a few hours before lifting, and shade them for a few days after planting.

Spacing

Maryland Extension gives basil spacing of 12 inches between plants and 18 inches between rows. Do not crowd basil to fit more plants into a bed. Downy mildew spreads in warm, wet, humid weather, and crowded plants stay wet longer. Good airflow and drip irrigation rather than overhead watering are the practical defenses. Follow the seed packet for parsley, dill and cilantro; squeezing plants closer causes disease and lowers yield.

What goes wrong

Basil set out too early. Basil sulks in cold soil. A plant set out before the nights settle can stay yellow and stunted while a later planting passes it. Wait for warm soil and frost-free nights, and harden off properly.

Crowded basil. Several seedlings left in one cell compete, stretch and damp off. Thin to one strong plant per cell, or pot up shortly after the first true leaves appear. Leaving seedlings crowded in the flat is one of the most common indoor mistakes.

Old parsley seed. Parsley seed loses viability quickly. Buy fresh seed, store it sealed in the refrigerator, and test anything more than a year or two old before sowing a whole flat.

Too little light. Herbs on a windowsill lean and stretch. Keep the fixture close and on a timer, and turn the containers if the light comes from one side.

Overwatering and damping-off. A mix kept constantly wet grows water molds that kill seedlings at the soil line. Infected seedlings cannot be saved: remove them with their mix, wash the containers, soak them for 1 to 2 minutes in 1 part chlorine bleach to 10 parts water, and rinse. Fungus gnats are mainly a sign of overwatering; let the mix dry between waterings and use yellow sticky traps to monitor.

Sowing basil with the tomatoes and leaving it indoors too long. Basil grows fast, and started on the tomato schedule it can outgrow its cell before the weather allows it outside. If the plants outgrow their cells, pot them up rather than holding them in a flat. The tomato timeline is a useful comparison: growing tomato seedlings from seed to garden covers the 5- to 6-week schedule and the 8- to 10-inch stocky transplant that Maryland Extension describes.