When to Plant Herbs Indoors for Balcony Containers

Start tender herbs such as basil indoors about 6 to 8 weeks before your average last spring frost, and slow starters such as parsley about 8 to 10 weeks ahead. Move seedlings to the balcony only after the risk of frost has passed and nights stay above about 10°C, and harden them off for 7 to 10 days first. Perennial herbs such as rosemary, thyme and mint are usually planted outdoors in spring as established plants rather than grown from seed indoors.

Choose herbs that fit your balcony

Before buying seeds or plants, measure how many hours of direct sun your balcony gets, how exposed it is to wind, how much container space you have, and which herbs you actually cook with. Most culinary herbs need at least 6 hours of direct sun per day, but some tolerate partial shade well.

Wind and exposure

Wind dries soil and leaves faster than still air, so pots on high, open balconies can need watering daily in summer, while sheltered pots need it less often. Wind also bruises soft leaves. Put tender herbs such as basil on the sheltered side behind a railing or wall, and place low, bushy or woody herbs such as thyme and rosemary in the most exposed positions. Heavy pots resist tipping; a light plastic pot can blow over.

Containers, drainage and potting mix

Roots need oxygen as much as water. A container without drainage holes lets water collect at the bottom, and roots can begin to rot within days of staying saturated. Choose a pot with at least one drainage hole, and empty any saucer after watering so the pot does not sit in water.

Size matters because small pots dry out quickly. A 15 to 20 cm pot holds enough soil to buffer moisture for small herbs, while rosemary and oregano need more root volume and usually do better in 30 cm pots. Terracotta dries faster than glazed or plastic pots, which suits Mediterranean herbs but can stress basil and mint.

Use a potting mix formulated for containers, not garden soil, which compacts and drains poorly in a pot. For rosemary, thyme, sage and oregano, mix in 20 to 30 percent perlite or coarse sand to improve drainage. Refresh the mix each season, because repeated growing depletes nutrients and the structure breaks down.

Watering by soil moisture, not the calendar

A fixed schedule ignores what the plant actually needs. Water when the top 2 to 3 cm of soil feels dry, and check more often on hot, windy days. In cool, cloudy or rainy weather a pot may need water only once or twice a week, while the same pot in midsummer can need water every day. Pots under a balcony overhang receive no rain and need checking even during wet weather.

How to water correctly

1. Push a finger into the soil up to the first knuckle, about 2 to 3 cm.
2. If it feels dry, water slowly until liquid runs out of the drainage holes.
3. Pour off the water collected in the saucer after 15 to 30 minutes.
4. Wait until the top layer dries again before the next watering.

Why herbs need separate pots

Herbs fall into two moisture groups. Mint, parsley, chives and basil prefer evenly moist soil and decline if they dry out completely. Rosemary, thyme, oregano and sage come from dry, well-drained habitats and suffer when the root zone stays wet. If you combine them in one pot, you must either water for the thirstiest plant and risk rotting the others, or underwater the thirsty ones. Mint also spreads by underground runners and crowds neighboring plants, so it belongs in its own container.

Feeding, pruning and harvesting

Feeding

Watering washes nutrients out of containers, so potted herbs need feeding during active growth. From spring to late summer, apply a balanced liquid fertilizer at half the label strength every 2 to 4 weeks, and stop in autumn. Mediterranean herbs need less. Too much nitrogen produces lush growth with weaker flavor and softer stems that are more vulnerable to frost.

Pruning

Pinching the growing tip of basil just above a pair of leaves makes the plant branch, so each cut yields more leaves. Remove flower buds on basil and cilantro as they form, because flowering shifts the plant's energy into seed and the leaves turn bitter. Trim woody herbs lightly by taking off soft new growth, and avoid cutting into bare old wood on rosemary and thyme, since it often does not regrow. Do not remove more than about one third of a plant at a time.

Harvesting for continuous yield

1. Harvest in the morning after dew has dried, when leaves are dry and cool.
2. Cut each stem just above a leaf node, the point where a pair of leaves joins the stem.
3. Take a few stems from different parts of the plant rather than stripping one side.
4. Harvest regularly during the growing season; frequent cutting keeps plants bushy.

Cilantro and dill bolt quickly in warm weather. Sow a small batch every 3 to 4 weeks so that fresh leaves are always coming along while earlier pots go to seed.

Sowing, planting and transplant timing

The first decision is whether to start a herb from seed indoors or buy a plant. Use your local average last frost date as the reference point and count back from it. Many herbs are simply easier to buy as starts, while others are better sown directly.

Steps for starting herbs indoors

1. Find your average last spring frost date from a local weather service or agricultural extension office.
2. Sow basil 6 to 8 weeks before that date and parsley 8 to 10 weeks before.
3. Use a light seed-starting mix. Barely cover basil seeds, and keep the tray at about 21°C until they emerge.
4. Give seedlings 12 to 16 hours of light a day. A bright windowsill often is not enough and produces leggy seedlings, so a grow light held 5 to 10 cm above the seedlings works better.
5. Harden off for 7 to 10 days, starting with 2 to 3 hours in a sheltered outdoor spot and gradually increasing exposure to sun and wind.
6. Transplant when nights stay above about 10°C. Basil is damaged by cold nights, so bring pots in on cold evenings.

Transplant when seedlings have two to four pairs of true leaves and roots fill their cells, but before they become root-bound. Parsley and dill have taproots that resent disturbance, so start them in biodegradable pots that can be planted with the root intact.

Overwintering herbs on the balcony

Whether a herb survives winter depends on its hardiness and on its container. Roots in a pot are more exposed to freezing than roots in open ground, so a hardy plant in a small pot can still die. Basil, cilantro and dill are annuals and will not return. Let them finish and collect seed if you want more next spring.

Hardy perennials outdoors

Thyme, oregano, sage, chives and mint tolerate cold well in many climates. Keep them outside but protect the pot. Group containers against a wall or railing, lift them off cold floors with pot feet or wooden blocks, and wrap the outside of the pot with bubble wrap or burlap. Water only when the soil is dry and the weather is above freezing, because wet soil that freezes can damage roots. Hardiness varies by species and variety, so check the plant label and your local climate zone.

Tender perennials that need a cool, bright indoor spot

Rosemary is the common example. In colder climates, bring it indoors before temperatures drop toward freezing. A cool room of about 5 to 10°C near a bright window suits it better than a warm living room, where it becomes leggy and more prone to pests. Water sparingly, letting the soil dry further between waterings than in summer, and keep the plant away from radiators and heating vents.

Common problems and how to fix them

A realistic start for a small balcony

If you have room for only two or three pots, start with basil in a 20 to 25 cm pot and one thyme or oregano plant in a 15 to 20 cm pot. Together they cover salads, tomato dishes, roasts and stews, and their watering needs differ enough that they should stay in separate containers. If your balcony gets only partial sun, add chives in a small pot on the shadier side rather than forcing basil or thyme into low light.