How to Grow Basil: A Practical Guide from Seed to Harvest
Basil is one of our favourite herbs to grow because there are so many different types to try. Most of us know the familiar large-leaved Genovese basil from the supermarket, but once you start growing from seed there's a whole range of flavours, scents, colours and leaf shapes to discover.
We grow quite a lot of basil in pots in the greenhouse, often alongside the tomatoes. It also grows well outside in a warm, sheltered position or in pots on a sunny windowsill.
The main thing basil needs is warmth. It's a tender plant and doesn't appreciate cold weather, particularly when it's young.
I like to grow several pots rather than relying on one plant. We can start picking from one while the others are still developing, and another sowing a few weeks later keeps fresh leaves coming through summer.
Keep it warm, regularly watered and keep pinching out the growing tips. Do that and basil will usually give you plenty of fresh leaves.
When to Sow
Basil can be sown from February or March through to June, although I don't think there's much advantage in starting very early unless you have somewhere warm and bright to grow the seedlings.
March and April are good months for most of our sowings.
Basil needs warmth to germinate, somewhere around 21–24°C is ideal. A propagator works well, but we've also had excellent results from a warm, sunny windowsill.
You can continue sowing small batches through spring and into early summer.
I like succession sowing basil rather than relying on one large sowing at the beginning of the season. A few pots started several weeks apart means you always have younger plants coming along behind the ones you're already harvesting.
If you're growing basil outside, don't rush it. A warm April afternoon doesn't mean summer has arrived. Basil really doesn't like cold nights and there's little to gain from putting plants outside before conditions are suitable.
How to Sow
Basil is easy to germinate from seed.
Fill pots, modules or a seed tray with a good-quality seed compost and sow the seeds thinly over the surface.
It's tempting to scatter lots of seed into one pot — we've done it plenty of times — but sowing a little more thinly makes life considerably easier when the seedlings need moving.
Cover the seeds with a thin layer of fine compost or vermiculite and water gently.
Keep the compost moist and somewhere warm while the seeds germinate.
Basil can start appearing in as little as five days in good conditions, although around 7–14 days is quite normal.
If you're covering the pots or using a propagator lid to maintain humidity, remove the cover once the seedlings have appeared. Leaving young seedlings in very humid, stagnant conditions can encourage damping off.
Once they're growing, give them plenty of light. Warmth gets basil started, but warmth without enough light can quickly give you long, weak seedlings.
Growing Seedlings On
Once the seedlings are large enough to handle, give them more room.
I don't necessarily grow every basil seedling as an individual plant. We often put two or three seedlings together in a pot and gradually increase the pot size as they grow.
How large the final pot needs to be depends on what you're doing with the plant. A basil growing on a windowsill doesn't need to become enormous, while plants growing alongside tomatoes in the greenhouse can happily go into one or two-litre pots and become much larger.
Keep seedlings somewhere warm and bright and don't let the compost completely dry out.
As the plants develop several sets of leaves, start pinching out the growing tips. This encourages shoots to develop lower down and gives you a much bushier plant.
That's one of the most useful jobs you can do with basil. One tall stem isn't much use when you could have a plant covered in fresh shoots to pick.
Planting Out and Growing in Pots
Basil grows brilliantly in pots, and that's how we grow much of ours.
A sunny windowsill, greenhouse or sheltered patio can all work well. We particularly like growing basil in the greenhouse alongside tomatoes — apart from being convenient, they enjoy many of the same warm growing conditions.
You can plant basil outside during summer, but wait until the weather is properly warm and the danger of frost has passed.
Plants raised indoors or in a greenhouse should be hardened off gradually before being left outside. Put them outdoors for increasing periods during the day and bring them back under cover initially.
Suddenly moving a warm-grown basil into cold conditions can stop it in its tracks.
Choose a sunny, sheltered position outside. Basil likes warmth and doesn't appreciate being battered by cold winds.
If you're growing in containers, make sure there are drainage holes. Basil likes regular moisture but doesn't want its roots sitting permanently in water.
Watering and Feeding
Basil grows quickly during warm weather and needs regular water to keep producing soft, fresh leaves.
Try to keep the compost reasonably moist rather than allowing plants to wilt badly between waterings.
Container-grown basil can dry surprisingly quickly on a hot day, particularly once the plants become large. Check pots regularly rather than watering automatically on a fixed schedule.
At the same time, don't leave the compost permanently waterlogged.
Plants growing strongly in containers will benefit from some feeding during summer. A general-purpose liquid feed used according to the instructions is perfectly adequate.
I wouldn't overcomplicate it. Basil wants warmth, water and enough nutrients to keep producing fresh leafy growth.
If the leaves start looking pale and a plant has been sitting in the same pot for months, it may simply be running short of nutrients.
And if a small pot seems to need watering constantly, have a look at the roots. Sometimes the answer is a larger pot rather than a larger watering can.
Harvesting and Pinching Out
You can start harvesting basil once the plants are large enough to have plenty of leaves to spare.
Rather than continually picking individual leaves, I like to pinch or cut the tips of the stems just above a pair of leaves. New shoots will develop from below the cut, making the plant bushier and giving you even more to harvest later.
This is why regular picking actually helps a basil plant rather than hurting it.
Don't strip a young plant bare. If you have several pots growing, take some from one while allowing the others to develop.
We often start several plants for exactly this reason.
Keep an eye out for flower buds too. If your priority is fresh leaves, pinch the flowers out as they appear so the plant keeps putting its energy into leafy growth.
Towards the end of the season, I often let some plants flower. The flowers are attractive and the bees certainly appreciate them.
Growing Basil Outside
Basil will grow outside perfectly well during a warm British summer, but choose the position carefully.
Give it your warmest, sunniest and most sheltered spot.
The difference between a basil growing against a warm wall and one sitting in an exposed, windy corner can be considerable.
Don't plant outside too early. Wait until frost is no longer a risk and nights have warmed up.
During cooler or wetter summers, greenhouse and windowsill plants will often perform better than those outdoors. That's not a failure on your part — basil simply prefers warmer conditions than our weather always provides.
Containers are useful because you can move them. If the weather turns particularly miserable, a pot of basil can be brought somewhere more sheltered.
And don't assume every variety behaves exactly the same way. Compact Greek types, large-leaved Genovese basil and some of the more unusual varieties can all grow rather differently.
Common Problems
Slugs and Snails
Slugs and snails can make short work of young basil plants, particularly outdoors.
Keep an eye on newly planted seedlings during damp weather. A large plant can survive some damage, but a small basil can disappear remarkably quickly.
Aphids
Aphids can gather around tender growing tips and underneath young leaves.
Check plants regularly and deal with small colonies early. They can often be rubbed off or washed away with water.
Damping Off
Young seedlings can suddenly collapse when conditions are too wet and stagnant.
Use clean pots and fresh compost, don't oversaturate the seedlings and remove propagator covers once germination is underway to improve airflow.
Yellowing Leaves
Yellow leaves can have several causes, including overwatering, poor drainage, cold conditions or a plant that's exhausted the nutrients in its pot.
Look at the whole plant and its growing conditions before automatically adding feed.
Plants Becoming Tall and Thin
Basil can become leggy when it doesn't have enough light or when the growing tips aren't pinched out.
Give plants plenty of light and regularly remove the tips above a pair of leaves to encourage branching.
Plants Flowering
Flowering is perfectly natural, but if you're growing basil mainly for leaves, pinch flower buds out as they appear.
Later in the season, I like to let some flowers develop. By then we've usually had plenty of leaves, and the bees make very good use of what's left.
Cold Damage
Basil really doesn't like cold weather. Leaves can become damaged and growth can stop when temperatures fall.
Don't rush plants outside in spring and, if you're growing in pots, take advantage of being able to move them somewhere protected when the weather turns cold.
Basil isn't difficult to grow once you remember that it likes warmth. Sow a few pots, don't put them outside too early and keep pinching the plants as you harvest. And don't feel you have to grow the same basil every year. Genovese is brilliant, but trying a lemon, cinnamon, Thai or purple type alongside it is half the fun of growing basil from seed.
