Dark Opal
Grown for the colour more than the harvest. Slower and fussier than green basil, so give it the warmest brightest spot you have.
Dark Opal is a purple leaved sweet basil, dark enough to look almost black in some lights, with a slightly more clove like flavour than a Genovese. It was bred in the United States in the mid twentieth century and remains the standard purple basil in seed catalogues. Be clear about what you are buying: purple basils are generally less vigorous and slower than green ones, which in an Irish summer is a real cost. Grow it because a purple basil vinegar or a two colour salad is worth having, not because you want volume.
Quick facts
- Botanical name
- Ocimum basilicum
- Type
- Open pollinated
- Category
- Purple leaved sweet basil
- Difficulty
- Challenging
- Hardiness
- Tender, no frost tolerance
- In Ireland
- Needs a greenhouse or tunnel
- Germination
- 5 to 14 days at 18 to 25C
- Sowing depth
- 5mm
- Spacing
- 20cm apart, 25cm between rows
- Height
- 40cm
- Position
- Full sun
- Containers
- Grows well in pots
- Sowing to harvest
- 8 to 12 weeks
- Harvest
- June to September
Sowing and harvest calendar for Dark Opal
Months shaded below are the reliable windows for Irish conditions.
Where to buy Dark Opal basil seeds in Ireland
Irish suppliers that stock this, so postage stays domestic and seed arrives in time to sow. These are plain links, not paid placements.
Garden centres and mixed retailers
- Mr Middleton Irish Six basil varieties, the widest choice here Mail order arm of a long established Mary Street shop, carries the familiar named varieties the organic specialists skip. Visit Mr Middleton , Co. Dublin
- Johnstown Garden Centre Irish Four basil lines including organic Third generation family business near Naas with the widest branded range, though popular varieties sell out by midsummer. Visit Johnstown Garden Centre , Co. Kildare
Why it suits Ireland
It is a greenhouse or windowsill plant here, like every basil, and it has no particular Irish advantage. What it does have is presence. Under Irish light, which is diffuse and often flat, a dark purple leaf reads far better in a pot or a salad than another green one, and a row of it in a polytunnel is genuinely handsome.
The honest caveat is that in a cool summer it will grow more slowly than a green basil sown at the same time, so allow it a few extra weeks and put it in the best position you have rather than the leftover corner.
Growing notes
Sow from mid-March to May, at the warmer end of the range, 20 to 25C if you can manage it. It is slightly slower to germinate than green types.
Once the true leaves appear you can see which seedlings have the colour. Keep the darkest and discard the plainly green ones, or grow them on separately as ordinary sweet basil.
Grow in a 20cm pot or a greenhouse border at 20cm spacing. Bright light deepens the purple; a plant in marginal light goes a dull bronze green rather than near black.
Harvest by cutting whole stems above a leaf pair, as with any basil, and pinch out flower spikes on sight. The flowers are pale pink and pretty, which tempts people to leave them, but leaf production stops when they open.
Water in the morning, at the compost, only when the surface is dry. Slower growing plants use less water, so this variety is easier to overwater than a vigorous Genovese sitting beside it on the same watering schedule.
Common questions
Why are some of my Dark Opal plants green?
This is normal and not a sign of poor seed. Dark Opal is known for throwing a proportion of green or green blotched plants in any batch, because the purple colouring does not come completely true from seed. Growers usually sow more than they need and discard the greenest seedlings once the true leaves show the colour. The green ones are perfectly good basil, just not purple.
Does purple basil taste different?
A little. Dark Opal is generally described as having a stronger clove and slightly more medicinal note than a Genovese, and it is less sweet. In cooking it can tint a pale sauce a muddy grey, so it is used more often raw, as a garnish, or steeped in white wine vinegar, which it turns a striking pink.
Is it harder to grow than green basil?
Somewhat. Purple basils tend to be a bit slower and less vigorous, and in an Irish summer that gap is more noticeable than it would be in a warm climate. It needs the same conditions as any basil, warmth, bright light and compost that dries between waterings, but it is less tolerant of anything short of that. It is not a first basil to attempt.
More basil to consider
Sweet Genovese
Open pollinated / Sweet basil, large leaved
The pesto basil, and the one worth the extra fuss, but only if you have a greenhouse or a genuinely sunny south facing sill.
Growing notes and stockistsGreek Basil
Open pollinated / Bush basil, small leaved
The basil to start with if you have failed with basil before. Small leaves, dense growth, and far more tolerant of cool damp air.
Growing notes and stockistsThai Basil
Open pollinated / Thai or anise basil
Worth growing for the aniseed flavour that holds up in a hot wok, but it is greenhouse only in Ireland and slow to start.
Growing notes and stockists