Ocimum basilicum var. purpureum or dark opal basil was developed in the 1950s from sweet basil (Ocimum basilicum) by Joseph Lent and John Scarchuk. Wild basil seeds were harvested in Turkey and sent to the US for breeding and research. Scarchuk and Lent, working at the University of Connecticut selected a dark-purple plant from the seedlings for further breeding and research. It took several years of breeding and experimenting to develop a variety with a uniform purple appearance and flavor, finally resulting in the aromatic Dark Opal basil. In 1962, this variety was released by Ferry-Morse, a large seed company, and gradually became commonly cultivated in home gardens. Plants are currently sold fresh by farmer’s markets and specialty grocers and are also available as seeds from online seed companies and retailers. It is cultivated for the decorative appearance of its deep purple and sometimes mottled leaves as well as for culinary purposes.
This perennial is upright and bushy, more commonly cultivated as annuals in temperate climates, and can reach heights of 24 inches and 12 inches wide. The leaves are small to medium, 1 – 3 inches in size. It produces small pink/purple flowers that grow on inflorescences that appear in summer. This plant can grow for quite a few years in warmer climates, with indoor plants also lasting longer under suitable growing conditions.
Although this variety grows a little slower than other basil varieties, the dark leaves, strong sweet-spicy flavor, and attractive flowers make it worth taking the effort to grow it.
The foliage has a stronger aroma than common green basil variations and adds to their appeal in gardens as well. The buds and leaves are edible but, even if you don’t want to harvest and use it in your cooking, it is a lovely plant to grow outdoors or in containers.
This plant prefers full sun in temperate areas to thrive but needs shade, particularly when growing in hot regions.
If you see the leaves wilting in the peak hours of the day in summer, you might have to move the plant into a shaded location.
They need regular watering, about 1 inch each week as they love moisture. Water deeply to get the roots to go down deep. Watering the plants in the mornings will prevent overly wet conditions during the night. These plants like moisture so if they are growing in hot locations, mulching will help the soil hold moisture and prevent weeds. Container plants need more water in summer.
While these plants can tolerate hot, humid, and/or dry conditions, they prefer cooler temperate climates.
The preferred temperature range for these plants is 60 – 90°F. Success growing these plants in warm or hot weather is to grow these plants in sheltered areas or they won’t thrive. Too much heat will make the leaves more green than purple. They are also very averse to frost and should be grown indoors in areas where temperatures tend to drop.
They are not usually fussy about the quality of the soil – they just need soil that drains out water well and doesn’t get too dry. Mulching helps dry soil retain moisture, particularly in hotter regions, and adding some organic compost will benefit the plant if it grows in poor soil.
Plants were grown as annuals generally don’t need repotting, although plants growing as perennials in warmer climates might need repotting, depending on their growth and spread. Choose a larger container with many drainage holes since these plants don’t like being water-logged. Prepare the new container by filling 1/3 of it with soil, moving the plant from the old container into the new one, and watering it well. Generally, repotting isn’t necessary.
Warm conditions are necessary to successfully germinate seeds. They can be started indoors or when outdoor temperatures reach above 65 – 70°F. Use a container filled with potting mix. Germination usually takes 2 – 3 weeks, the soil has to be constantly moist during this time and when the seedlings mature, they can be transferred to a sunnier location.
As an alternative, propagation from cuttings is also possible. This method is not ordinarily successful with cuttings taken from newly purchased potted plants. Let the plants get established and used to environmental conditions before taking cuttings or cuttings from plants you’re presently growing can be taken.
Take 5 – 10 4-inch long cuttings around April and remove the lower leaves. Insert all the cuttings in a vessel filled with clean water and keep it under indirect bright light, with a temperature between 65 – 75°F. Refresh the water every 2 or 3 days. Once roots develop within a week or so, they can all be planted together in a large container with many drainage holes filled with potting mix. Keep the soil always moist, but don’t over-water.
Fertilizing these plants regularly will help ensure healthy growth. Finding the right balance in feeding is necessary as over-fertilization will impact the oil the plant produces, thus affecting their flavor and fragrance.
Pruning helps promote healthy growth and flavor. Pinch off flower heads when they appear, about 1 inch under the flowers, to prevent seed production and stop the flavor from turning bitter.
Cutting flowers back after they bloom helps encourage new growth – you can remove 1/3 of the leaves every month.
If you harvest the leaves for cooking, leave several groups of leaves that will give the plant healthier new growth with a better yield of leaves. Always harvest leaves starting at the top downwards and continue pinching back tips every month or two, even if you’re not harvesting. This encourages bushy growth and prevents the plants from becoming lanky and weak.
Slugs could be a problem, particularly in spring or summer. Use organic or natural slug remedies like beer traps and scatter crushed eggshells near the plants to control them instead of slug pellets. Aphids also can be an issue, so you should be vigilant. A strong water spray will get them off the plants.
If you’re growing multiple plants in one container, make sure they have enough space between them. Plants that are crowded together increase the risk of fungus infection.