Growing basil: the complete guide from seed to pesto

Growing basil at a glance

Contents

• Growing basil at a glance
• Why does supermarket basil die after a week?
• Sowing basil indoors: timing, temperature and depth
• How much light does basil need?
• Soil and pots: size, drainage and self-watering
• Water from below, and why wet leaves cause trouble
• Why can basil not take anything below 10 °C?
• Pinching and harvesting: how to get a bushy plant
• Feeding basil without wrecking the flavour
• Basil varieties and mildew-resistant types
• Basil in a greenhouse and on a windowsill
• Basil diseases and pests
• Overwintering and rooting cuttings in water
• Harvest, storing, freezing and pesto
• Sowing and harvest windows in USDA zones 3–7
• Troubleshooting: symptom, cause, fix
• Common mistakes
• Frequently asked questions

Why does supermarket basil die after a week?

It dies of three things at once, all built into how the plant was produced.

It is not one plant, it is twenty

Knock a supermarket pot out and look at the root ball. Fifteen to thirty seedlings are in there, sown thick so the pot looks full on the shelf, sharing about 200 ml of peat. The roots are one solid mat, the feed is gone, and each seedling has a fraction of the root system it needs. That worked in the greenhouse, with liquid feed several times a day.

The peat is too dry, and it travelled cold

The mix is usually straight peat in a 10–12 cm (4–5 in) pot, and peat that has dried once repels water. You pour a cup on, it runs down the side into the saucer, and the middle stays bone dry. The plant wilts, you water more, and the roots end up sitting in water while still dry in the middle.

Basil is also damaged at 10 °C (50 °F), so a pot that sat on a loading dock or by an open store door has taken a chill that shows days later as dark brown patches. It was forced under 16 hours of strong light too, and an October window gives a tenth of that. With too little sugar for all those leaves, the plant drops them from the bottom up.

How to rescue it, step by step

1. Soak the root ball first. Stand the pot in 3 cm (1 in) of lukewarm water for twenty minutes so the peat wets through.
2. Wash the peat off. Dip the roots and let the mix fall away.
3. Split into three to five clumps, four or five seedlings at a time. Broken roots are fine – basil makes new ones in days.
4. Repot properly. One clump per 15 cm (6 in) pot, or three in a 25 cm (10 in) pot, in fertile mix with drainage holes.
5. Set each clump 1 cm deeper than it was; basil roots from the buried stem.
6. Cut the tops back just above the second pair of leaves. The plant cannot support long stems and new roots at once.
7. Water from below, then keep it bright at 20–25 °C (68–77 °F), with a lamp in winter.
8. Let it rest a week. No feed, no harvest. New shoots in the leaf joints mean it is away.

A divided, repotted plant usually lasts two to four months. The same applies to most herbs grown in pots.

How much light does basil need?

Basil wants 6–8 hours of direct sun a day. Give it four and the stems stretch, the leaves thin out and the flavour goes flat, because light is what builds the aromatic oils.

From April to August a south or west window works anywhere in zones 3–7. Keep the pot close to the glass but not touching it – a leaf against cold glass in April picks up chill damage. From September to March the day length is not enough at any window: the plant survives but barely grows. That is when a lamp earns its keep.

• Duration: 12–16 hours a day on a timer. Basil needs darkness too.
• Distance: 20–30 cm (8–12 in) above the plant for a household LED, raised as it grows.
• Position: directly overhead. Side light gives you a leaning plant.

If you can read a newspaper comfortably in that light, it is too dim for basil. Lamp types and heights are in the guide to grow lights for seedlings.

Soil and pots: size, drainage and self-watering

Pot size decides how long the plant lives

The RHS specifies a container at least 20 cm (8 in) wide and deep, roughly 5 litres (1.3 gal). In 5 litres there is moisture left over a hot day and feed left for six weeks; in the supermarket's 300 ml there is neither. Depth matters as much as width, because basil sends down a taproot and wants 18–20 cm (7–8 in).

Drainage is not negotiable

The pot must have holes, or salts and surplus water collect at the base and the roots suffocate. Do not add gravel either – a coarse layer under the mix raises the saturated zone and makes the soil wetter, not drier.

Mix and self-watering pots

Use fertile potting mix, not seed compost and not straight peat, with a fifth perlite or coir stirred in if it feels dense. Aim for pH 6.0–7.0, which bagged mix almost always sits at.

A self-watering pot with a reservoir and wick is close to purpose-built for basil: even moisture from below, dry leaves, and a week away without harm. Let the reservoir run dry for a day each week so the mix gets air.

Water from below, and why wet leaves cause trouble

Basil wants evenly moist soil and dry leaves. You satisfy both at once by watering into the saucer.

How bottom watering works

Stand the pot in a saucer holding 2–3 cm (1 in) of lukewarm water. The mix draws it up over ten to twenty minutes; tip away what is left. Repeat when the soil feels dry a finger's depth down – daily in a hot July window, every second week in November. The whole ball wets evenly instead of just the surface, roots grow downward, leaves stay dry, and fungus gnats get no damp surface for eggs.

Why wet leaves cause trouble

Almost every basil disease is a fungus or water mould that needs a film of water on the leaf to germinate. Downy mildew, grey mould and leaf spots need a few hours of leaf wetness, and evening overhead watering leaves the foliage damp all night. Water from above only in the morning, at the soil.

Drooping leaves mean too much water more often than too little. Wet soil means the roots have begun to rot and the mix must dry out; dry soil means the plant will stand up within an hour of bottom watering.

Why can basil not take anything below 10 °C?

Basil comes from tropical Asia and has no cold protection. Below about 10 °C (50 °F) the cell membranes start to leak, and the damage is permanent long before anything freezes. University of Illinois Extension is blunt: air or soil below 10 °C (50 °F) causes damage and blackened leaves, and growth stalls. The RHS sets the same floor.

Symptoms arrive late. After a night at 7 °C (45 °F) you see nothing in the morning; two or three days later the leaves develop glassy dark patches that dry to brown, and the lowest ones drop.

• Into an unheated greenhouse: once nights hold above 12 °C (54 °F), usually late May in zone 5–6.
• Outdoors in pots: two to three weeks after the last frost, which is June in most of zones 3–5.
• Back indoors in autumn: when the forecast shows below 10 °C (50 °F) at night.
• Indoors: never on a cold stone sill or under a window you open.

A cold draughty spot is more dangerous than a slightly dim one. Basil at 14 °C (57 °F) neither grows nor drinks, and then gets root rot – not because the soil was wet, but because the plant stopped taking water up.

Pinching and harvesting: how to get a bushy plant

This is the difference between picking three leaves a week and cutting whole bunches.

Why pinching works

The tip bud chemically suppresses the side shoots. Cut the tip out and the two buds in the leaf joints below break. Every pinch turns one stem into two, then four, then eight. A plant pinched four times carries sixteen harvestable tops; an unpinched one carries a single top on half a metre of bare stem.

The first pinch

Pinch when the plant has three leaf pairs, at 10–15 cm (4–6 in) tall, four to six weeks from sowing. Cut the stem 5 mm above the second leaf pair, just above a leaf joint – a stub with no bud only rots.

How to harvest after that

• Cut tops, not single leaves. Picking from the middle leaves the plant sparse and still growing upward.
• Always cut just above a leaf pair, where the new shoots come from.
• Never take more than a third of the foliage at once.
• Harvest every 7–10 days in summer; regular harvesting is regular pinching.
• Cut in the morning with sharp scissors. Blunt blades crush the stem and invite grey mould.

Cut the flower buds off

Once basil flowers it switches from leaf to seed: leaves shrink, the stem goes woody and the flavour turns harsh. University of Illinois Extension recommends pinching flower buds out immediately to keep leaf quality. Look for a small square, tight bud between the two topmost leaves, and cut the whole shoot one leaf pair lower.

Early bolting has a cause: drought stress, a small pot, too little feed, or a cold night. Variety matters too – 'Prospera' and 'Rutgers Obsession DMR' are bred to be slow to bolt, while Greek bush basil runs to flower quickly.

Feeding basil without wrecking the flavour

Basil is a leaf crop and wants nitrogen, but overfeeding makes the flavour bland.

• Weeks 1–4 after repotting: nothing. Fresh mix carries the plant four to six weeks.
• After that: liquid feed at half strength every second week. Utah State University Extension gives liquid fertiliser every 14 days once the plant is harvestable, and weekly for containers.
• Choose a balanced or herb feed. The RHS advises against high-potassium feeds, so tomato food is the wrong bottle – it pushes flowering.
• Under lights in winter: halve the dose again.
• Feed into the saucer, in the water you bottom water with.

Pale lower leaves on a plant that is otherwise growing means nitrogen shortage; dark thick leaves, soft stems and sudden aphids mean too much. Yellowing leaves with grey fuzz underneath are downy mildew, not a feeding problem.

Basil varieties and mildew-resistant types

There are well over a hundred named basils. These six cover what a home gardener needs, and all are sold in North America, the UK or both.

For pesto quantities, a Genovese type or a DMR selection is right. For one narrow windowsill, Greek bush basil is.

Basil in a greenhouse and on a windowsill

Greenhouse and polytunnel

A greenhouse is the best place for basil in a cold climate: the warmth is there, rain never hits the leaves, and you gain two to three weeks at each end of the season. Three things separate a lush greenhouse basil from an infected one:

• Ventilate. Still humid air is perfect for downy mildew. Open vents and the door every day it is above 15 °C (59 °F) outside.
• Give it room. 25–30 cm (10–12 in) between plants in a bed, not 10.
• Water the soil in the morning. Never overhead, never in the evening.

Basil shares a bed happily at the feet of greenhouse tomatoes. The claim that it improves tomato flavour is a myth, but the use of space is real – see companion planting. If the greenhouse is full of tomatoes, stand basil in pots along the path.

Windowsill year round

A potted basil stays productive for eight to twelve months if four conditions hold: above 18 °C (65 °F), 12–16 hours of light, bottom watering and pinching. Let one slip in November and the plant is bare in January.

Basil diseases and pests

Basil downy mildew (Peronospora belbahrii)

The most serious basil disease today, and the usual explanation for a plant that yellows and collapses for no visible reason. It turns up in bought plants and on seed.

Symptoms, per University of Minnesota Extension: pale yellow areas on the upper leaf surface that look like a nutrient problem, and a velvety grey-purple fuzz of spores underneath, starting on the lowest leaves and moving up. The water mould infects from as low as 15 °C (59 °F), needs humidity and leaf wetness, and spreads on airborne spores, infected plants and seed.

• Turn the bottom leaves over and check the undersides every time you water.
• Remove infected plants at once, bagged, and keep them out of the compost.
• Increase spacing and ventilation, and water the soil in the morning.
• Grow resistant varieties: 'Prospera' and 'Prospera Compact DMR', and the Rutgers series 'Devotion', 'Obsession', 'Passion' and 'Thunderstruck'. 'Amazel Basil' is sold as plants, and lemon basils carry some resistance.
• Cut an infected plant hard and use the clean tops – the disease is harmless to people.

Fusarium wilt

Fusarium oxysporum f. sp. basilici causes sudden wilting on one side, brown-black streaks along the stem and a darkening stem base. The plant cannot be saved. It travels on seed and persists in soil for years, so do not reuse the mix or replant basil in the same bed – the same logic as crop rotation.

Grey mould, spider mites, aphids and fungus gnats

• Grey mould (Botrytis) is a furry grey coating on damaged stems and cut surfaces, usually after evening watering. Cut it out and increase airflow.
• Spider mites hit indoor plants in warm dry winter air: fine speckling, dull grey leaves and webbing in the leaf joints. Rinse the foliage in the morning and raise the humidity.
• Aphids cluster in the shoot tips with sticky honeydew. Cut the tops off and wash with weak soft soap; they often follow too much nitrogen.
• Fungus gnats breed in the top damp centimetre of mix, so bottom watering plus coarse sand ends it.
• Slugs go for basil ahead of almost anything, so grow it raised.

Overwintering and rooting cuttings in water

Basil is an annual and dies after flowering. But it is one of the easiest plants to propagate: a cutting taken in September is a young plant by October.

Overwintering the plant

Bring the pot in before nights drop below 10 °C (50 °F) – August in zone 3–4, September in zone 5–6. Cut it back by a third and check the leaf undersides for mites and aphids first. Give it bright light, above 18 °C (65 °F), a lamp for 14 hours and half-strength feed. It will go woody by February, when cuttings or a fresh sowing serve you better.

Cuttings in water, step by step

1. Cut healthy shoot tips with four to six leaves, 8–10 cm (3–4 in) long, just above a leaf pair. Avoid shoots with flower buds.
2. Strip the two lowest leaf pairs so 4–5 cm (11/2–2 in) of stem is bare; leaves below the waterline rot.
3. Stand them in lukewarm water somewhere bright but out of scorching sun, 20–25 °C (68–77 °F).
4. Change the water every other day. Stale water goes cloudy and the stem blackens.
5. Wait 7–14 days for white roots from the stem and the leaf joints.
6. Pot up at 4–5 cm (11/2–2 in) of root into a 10–12 cm (4–5 in) pot of moist mix, three to five per pot, shaded a week.

This is also the quickest way to multiply a bought plant you like. Other herbs differ – lavender roots far more slowly and wants compost rather than water.

Harvest, storing, freezing and pesto

Never put basil in the fridge. Below 10 °C (50 °F) the leaves develop dark glassy patches within a day – the same chilling injury the plant suffers. University of Illinois Extension recommends standing the stems in a glass of water at room temperature instead, which keeps them three to five days.

Basil's aroma sits in volatile oils that leave during drying, which is why dried basil tastes of hay. Freezing keeps far more of it.

• Whole leaves on a tray. Freeze spread out, then bag. They darken but keep the flavour.
• Ice cube trays with oil. Chop, pack, top with olive oil and freeze. One cube per portion of pasta.
• Pesto base. Blend with olive oil and salt, no cheese or nuts, and freeze in portions.

Reckon on 50–60 g (2 oz) of leaves for a batch of pesto serving four – the harvest from two or three well-grown plants. For a winter's worth in the freezer you want six to eight Genovese plants in 5 litre (1.3 gal) pots. Blend briefly; a long run heats the leaves and turns the pesto bitter.

Sowing and harvest windows in USDA zones 3–7

Basil is started indoors in every one of these zones. The table gives sowing dates, last frost, when pots can go out, and how long the harvest lasts. A cold or exposed site behaves like the next zone north.

Sowing dates shift about six weeks between zone 7 and zone 3, but the harvest window shrinks by nearly three months. In zones 3 and 4 a greenhouse roughly doubles the useful season, and a windowsill under a lamp beats an outdoor bed for yield over a year.

Troubleshooting: symptom, cause, fix

Common mistakes

• Leaving supermarket basil in its pot. Twenty seedlings in 300 ml of peat have no chance. Divide and repot.
• Sowing too cold. Seed in 16 °C (61 °F) mix germinates badly or not at all.
• Watering overhead in the evening. Wet leaves overnight are what downy mildew needs.
• Delaying the first pinch. Pinch at three leaf pairs, not when the plant is 30 cm tall.
• Letting it flower. The flavour turns harsh within a fortnight.
• Feeding with tomato food. High-potassium feed pushes flowering.
• Putting it out too early. Below 10 °C (50 °F) the cells are damaged permanently.

Frequently asked questions

How do I keep supermarket basil alive longer than a week?
Divide and repot it the day you buy it. Wash the peat off, pull the root mat into three to five clumps and plant each in a pot at least 15 cm (6 in) across. Cut the tops back to the second leaf pair, water from below, and keep it above 18 °C (65 °F).

When should I start basil seeds indoors?
Six to eight weeks before your last frost: mid March in zone 5, the first week of April in zone 3. The seed wants 20–25 °C (68–77 °F) and germinates in 5–10 days at that temperature. Under grow lights you can sow any time. Direct sowing outdoors rarely works in a short season.

How deep do you plant basil seeds?
Barely at all. Basil seed germinates better with light, so it belongs on the surface or under no more than 6 mm (1/4 in) of compost. Press it down with a fingertip and water from below or mist, so the seed is not washed sideways.

How do you pinch basil?
Cut the stem 5 mm above the second pair of leaves once the plant has six true leaves, at about 10–15 cm (4–6 in) tall. The cut belongs just above a leaf joint, where the buds that become new shoots sit. Each pinch turns one stem into two.

Why is my basil tall and leggy?
Too little light makes the plant stretch toward the window with long stem sections and thin leaves, and never pinching lets the tip bud suppress every side shoot. The fix is 12–16 hours of light a day plus a hard cut back to the second leaf pair.

Should you water basil from the bottom?
Yes. Stand the pot in 2–3 cm (1 in) of lukewarm water for ten to twenty minutes, then tip the rest away. The whole root ball wets evenly, roots grow downward, leaves stay dry and fungus gnats get no damp surface to breed in. Water from above only in the morning.

What temperature is too cold for basil?
Anything below 10 °C (50 °F). Cell membranes begin to fail, and symptoms appear two or three days later as dark glassy patches and dropping leaves. Move plants outdoors only in June, once nights hold above 12 °C (54 °F), and bring pots in before 10 °C nights.

Can you grow basil indoors all year?
Yes, but only with light. Four conditions have to hold: above 18 °C (65 °F), grow lights 12–16 hours a day from September to April, bottom watering and regular pinching. Use a pot at least 20 cm (8 in) wide and deep, with half-strength feed every second week.

For more guides like this on herbs and season-by-season growing, sign up for the GardenLovers newsletter and get the sowing calendar before February.