Dark Opal Basil Plant Profile

A Striking and Intensely Aromatic Basil Variety

Close up of the leaves of Dark Opal Basil

Quinn Dombrowski / Wikimedia Commons / Creative Commons Attribution-Share Alike 2.0 Generic

 

Dark Opal Basil (Ocimum basilicum 'Dark Opal') is a pretty-looking, aromatic basil cultivar. This upright, bushy perennial is more frequently grown as an annual and has edible deep purple leaves.

Although this variety might be a little slower to grow than other basil plants, it's dark leaves, intense sweet-spicy flavor and pretty pinkish flowers, which appear mid to late summer, make it worth the effort.

The foliage has a much stronger smell than more common green basil varieties, and this adds to their garden appeal as well. The buds, as well as the leaves, are edible. But, even if you don't plan to harvest this basil, it looks lovely to look at when grown in containers outdoors or in a well-lit spot indoors.

Botanical Name Ocimum basilicum 'Dark Opal'
Common Name Dark Opal Basil
Plant Type Annual herb
Mature Size Up to 20 inches
Sun Exposure Full sun
Soil Type Loamy, sandy, well-drained
Soil pH Tolerates a wide range
Bloom Time Late summer
Flower Color Purple foliage
Hardiness Zones 9 to 11
Native Area Tropical Central Africa to Southeast Asia 
Flowering Dark Opal Basil
Dark Opal Basil produces purple/pink flowers in the summer months anne heathen / Flckr / Attribution 2.0 Generic (CC BY 2.0)

How to Grow Dark Opal Basil

As with other basil varieties, dark opal likes plenty of light, warmth, a sheltered position and well-drained, moist and fertile soil.

Light

Unless you live in particularly hot regions, your dark opal basil will appreciate a full sun position. Ideally, they'll want at least six hours of full sun to thrive.

Soil

Not terribly fussy about soil type, this basil variety just needs one that is well-drained and not overly dry.

Mulching can help dry soils retain moisture better, particularly in hot regions, and the addition of some organic matter will be appreciated if your basil is planted in a less rich medium.

Water

During the hotter summer months, make sure that the soil is kept moist. It's best to do this in the morning to prevent overly wet conditions through the night.

Temperature and Humidity

The key to success is to locate dark opal basil in a sheltered spot. Without this, it'll be unlikely to thrive.

Although this species can cope with hot, humid or dry conditions, it prefers cooler, more temperate regions. Too much heat will mean the leaves will be more green rather than purple, and it can impact on the intensity of their flavor.

This species is also very frost sensitive and would be better housed indoors if temperatures drop considerably overnight.

Fertilizer

Fertilizing your dark opal basil regularly can help to ensure healthy growth. However, you do need to get the balance right. Over-fertilization can impact on the intensity of the flavor, and it won't be as aromatic. This is because the amount of oil the plant produces will be reduced.

Make sure the fertilizer isn't too strong and that you don't use too much.

Pruning

If you want to maximize the growth and flavor of the basil foliage, you should pinch off the flower heads as they begin to appear. Leaving the flowers on will result in the leaves taking on a bitter taste and not growing as prolifically.

Even if you don't plan to harvest the leaves, cutting back the flowers after they have bloomed is recommended.

Harvesting

It's best to harvest when there are at least several sets of leaves that can be left to grow. This allows for healthier regrowth and better harvest yield.

Always select leaves from the top down and continue to pinch back every month to six weeks, even if you aren't harvesting. This'll encourage bushy rather than lanky and weak growth.

Growing in Containers

Your dark opal basil will grow well in containers kept in a warm and sunny spot in your home. Make sure whatever container you use allows good drainage - these plants don't like to be too wet.

If you have multiple plants in one container, make sure they're spaced far enough apart. Plants that are squashed together can increase the chance of them developing fungus.

Growing From Seeds

Dark opal basil needs warm condition for successful germination of seeds. You should start them off indoors or wait until outdoor temperatures are above 65 to 70 degrees F.

Germination will usually take two to three weeks. The soil should be kept consistently moist during this period and, as the seedlings mature, they can be moved into a more sunny position.