Basil

Basil is the most commonly grown culinary herb in hydroponics and also one which will grow and produce in many different growing environment and situations. Supplies of your own fresh basil in the dead of winter from an indoor grow room, at a time when prices of the herb at the supermarket have sky rocketed, is one of the many pleasures of hydroponic growing. While basil is not an overly challenging crop to produce hydroponically, there also exists great potential and possibilities in growing this crop commercially, particularly where the out of season markets exist. Basil is also a very versatile hydroponic plant having over 40 recognized types to chose from with a range of flavours and aromas which allows smaller hydroponic growers to `specialize' in the producing of gourmet types. Basil is also a major component of many processed and value added products which growers may consider making themselves or supplying those who manufacture these with their surplus or lower grade herb product.

Basil is the herb which is produced in the greatest volumes for the fresh cut herb market; it is also the herb which is used in the largest quantities in terms of how much is added to various recipes. Basil (Ocimum basilicum), is a fast growing annual herb with is a member of the aromatic Lamiaceae family which includes many other herbs such as mint, oregano, marjoram, thyme, sage, rosemary and lemon balm. Unlike many of the other Labiate herbs, basil differs in that it is thought to originate in India and has been cultivated since ancient times.

Basil is not only cultivated for use as a culinary fresh cut herb, but it is also grown for oil production. Basil oil is used in many food productions, but also in cosmetics, soaps and perfumes. Hydroponic production methods however are mostly used to produce this herb for use in the kitchen or for the fresh cut market, although some growers do process their basil into pestos, sauces, oils, vinegars and salsas. .

TYPES OF BASIL AND CULTIVARS

Most people are familiar with the common `sweet basil' (Ocimum basilicum L.) which is the most widely grown and used type of basil produced, however there are many types of basil, each with its own potential for culinary experimenting and also for fresh cut sales. There are also many different leaf and colour types within the basil range, making this a rather diverse herb to grow. Some varieties such as `Green ruffles' and `purple ruffles' have frilly leaves, much like certain cultivars of fancy lettuce - these types are ideal to grow and incorporate into the salad mixes or sold in bulk as a garnish for catering/restaurants. The dark purple types of basil such as `Purpurascens' and `dark opal' are also used for appearance in salads or in vinegars and oils, although their taste is less favoured than the common sweet basil cultivars.

Of the different flavours that can be obtained within the basil family, those with citrus type overtones are becoming popular with many growers, as are the `Thai basil' and the `cinnamon basil' types. There is also a camphor type of basil, which is generally not used in culinary dishes, but is dried for use in potpourri for its scent. Lemon basil (Ocimum basilicum `Citriodorum) is produced hydroponically alongside the common sweet basil, it is used in with fish, poultry, fruit salads and other dishes which prefer the citrus-basil favour. Cinnamon basil, is a lesser used type, but it is an excellent flavouring for marinades, vinegars, and deserts.

Along with the many different flavours and appearances of the different types of basil, there are also differences in growth habit. While the common sweet basil can grow to one meter or larger, there are varieties of smaller leafed `bush basil' such as `Spicy globe' often referred to as Greek basil (basilicum minimum). These bush types are not often used as fresh cut herbs for commercial sale, but can be grown where indoor growing space is restricted. The short bushy plants can be continually trimmed for use in the kitchen and take up very little space.

Of the sweet basil varieties, the cultivars most suited to continual harvest are Genovese, Italian large leaf and Mammoth. Where the foliage is to be processed into pestos, varieties of large leafed basil such as `lettuce leaf' (O.basilicum Crispum) can be used as they provide the highest leaf yield.

BASIL PROPAGATION

Basil can be propagated from seeds either direct sown or as transplants as well as from cuttings, although this is a lesser used method of propagation. It is important that high quality seeds are obtained, as seedlings must be true to type for uniform production and a high germination percentage. Basil seed germinates easily and rapidly under warm conditions with 24 C being the ideal temperature for germination of most basil types. Once temperatures drop below 15 C, germination is inhibited and growth of seedlings slows. Most hydroponic growers either buy in cell transplants or raise their own rather than direct seed into hydroponic media beds. This ensures the correct plant spacing is obtained and prevents overcrowding and having to thin seedlings or gaps where seeds have not germinated. It also allows a more rapid crop turn around if seedlings are ready to transplant into a growing bed or gully immediately after an old crop has been removed.

Seed germination takes five to seven days under ideal conditions with a higher percentage germination of over 90% in most cases. The media must be sterile, free draining, but still hold reasonable amounts of moisture. Rockwool propagation blocks are often used, as are many different types of hydroponic substrates such as perlite, vermiculite, sterilized coarse sand, coconut fibre, peat, oasis cubes and pumice sand.

Overhead damping of germinating seedlings with cold water can slow growth and stress seedlings, so it is recommended that irrigation water be warmed to greenhouse ambient temperatures before being applied to seedling trays or beds. Basil seedlings are extremely prone to pythium and other damping off pathogens, so care needs to be taken to ensure the surface of the media is not overly wet. Fungus gnats can also become a problem in damp media beds with the young larvae causing root damage and seedling death. As soon as the seedling leaves have expanded, a dilute nutrient solution can be given at an EC of 0.3 -0.5 mScm-1 - this should be a standard herb or basil nutrient formula with a pH of between 5.8 - 6.2. Once the seedlings are larger the EC can be increased gradually to 1.0 so that the young transplants harden off a little before transplanting into the hydroponic system.

Basil seedlings are ready for transplanting when they have developed the first pair of true leaves. Seedlings are fragile at this stage and care must be taken not to handle the young plants by the stem which can easily be crushed.

HYDROPONIC BASIL PRODUCTION SYSTEMS

Basil produces well in a number of different types of hydroponic systems, however the most widely used systems still appear to be shallow media beds usually filled with perlite or vermiculite although many of these do recirculate the nutrient solution. NFT is also utilized for these herbs, just as it is for many others and this method produces excellent yields. Basil has a tendency to develop an extensive root system, particularly were the plants are cut and left to regenerate on a regular basis, so smaller gullies are not always suitable where large plants are intended to be grown for cutting. Like many herbs, aeroponic production holds potential for higher value crops where maintaining high oxygen around the root system is vital.

When the growth, yields and oil production of basil crops grown in a number of different hydroponic systems has been examined it was found that the best type of system used actually depended on the plant part which was required. For the highest oil content per plant, NFT systems appear to be the best system to use, however for the highest oil concentration (g/kg) then DFT (Deep flow technique) was the method which maximized this factor. For the greatest plant fresh and dry weights, where the leaf is to be sold fresh, aeroponic systems produced the best results, while NFT systems produced the greatest root fresh weight.

EC LEVELS FOR HYDROPONIC PRODUCTION

Once the seedlings are in the hydroponic system the EC can be run at between 1.0 and 1.6 Ms cm-1, for sweet basil plants based on environmental and crop conditions. Other basil varieties such as opal basil and bush basil require a slightly higher EC level of 1.8 - 2.0 for optimum growth and quality.

Higher EC levels are usually maintained in winter to ensure high quality stems are produced which are not excessively soft and prone to post harvest problems. Lower EC levels in summer under high light and temperature conditions ensure the plants are able to take up sufficient moisture for rapid growth and development. As with the other aromatic herbs such as cilantro, running EC levels that are to low not only lowers the quality of the foliage, it results in flavours which are less developed and makes the plants more prone to yellowing and or pale foliage and disease attack.

TEMPERATURES

Growing temperatures need to be maintained at all times for optimal basil production. Basil is extremely frost sensitive and even a night of cool temperatures above freezing level will cause severe damaging and blackening of much of the foliage. Temperatures of 24-30 C day and 16-20 night combined with a 16 hour day length produce the highest growth rates and yields per harvest of most types of basil, while higher temperatures can stress the plants and cause wilting during the warmest part of the day. Basil crops also require good light levels, however light levels in summer can be excessive in some areas and the use of shading to reduce both greenhouse light and temperature levels is often recommended. Use of light shading (30 - 40%) has been shown to increase not only yields but also the oil content of basil plants in areas of high summer radiation. In indoor grow rooms, basil performs well under artificial lighting and can tolerate lower levels of light than fruiting plants such as peppers and tomatoes. If light conditions are too low, the plants will continue to grow, provided temperatures are warm enough, but the growth habit will be tall and lanky and the plants’ leaves may become thin and pale and even yellowed. Plant stress such as high light and temperatures, cool conditions, root damage, insect infestations and water or osmotic stress can predispose basil plants to bolting even while fairly small in size, so maintaining good growing conditions is essential for good yields and continual growth.

NUTRITION OF BASIL CROPS

In the edible portion of the basil plant, the amounts of potassium and calcium are relatively high. For this reason the ratio of K to Ca in the nutrient solution needs to kept at a reasonably high level. A ratio of close to 1:1 of K to Ca will ensure a well balanced nutrient formula for systems where basil only is being grown. While potassium and calcium are directly related to the oil and flavour qualities of the foliage, nitrogen is essential for leaf yields and needs to be maintained at all times. Magnesium is another element which needs to be carefully controlled for maximum production of high quality product. Basil which is deficient in magnesium will appear pale with interveinal yellowing and small irregular greyish spots on the leaf margins and this is common in some crops which aren't using a suitable basil formula. Levels of 50ppm Mg in the nutrient solution need to be maintained as there is a direct relationship between magnesium content on the yield and composition of the essential oils which make up much of the characteristic basil flavour and aroma.

NUTRIENT PROBLEMS

Low levels of N in the nutrient solution will rapidly become apparent on basil crops as a discrete and uniform yellowing of the leaves with older leaves becoming completely yellow within 14 days. Basil crops which are deficient in P will demonstrate a light green coloration of the foliage. Mg deficiency in basil causes interveinal yellowing and small irregular greyish spots initially on the leaf margins. Iron deficiency will cause the younger leaves to show a pale yellow colour followed by irregular necrosis and in severe cases, leaf abscission. S deficiency in basil plants has fewer symptoms, usually just a light yellowing of the foliage. Potassium (K) deficiency results in necrotic or blackened patches on the older leaves. Sodium accumulation can also cause problems in hydroponically grown basil plants, just as it can with most other crops.

While basil is considered to be a moderately salt tolerant crop, sodium accumulation from the water supply in recirculating system can result in imbalances occurring in the nutrient solution and a reduction in growth. Once sodium levels reach in excess of 60ppm Sodium, growth of young seedlings is affected, although larger plants can tolerate levels up to 400ppm.

POTENTIAL BASIL YIELDS

Under good growing conditions a yield of between 1 and 1.1/2 pounds per basil per square foot of growing area per month can be obtained under hydroponic production. Some growers are known to obtain higher yields than this under summer conditions when light levels are high, but generally production drops significantly in winter.

PRE-MATURE BOLTING (`GOING TO SEED’)

Basil can also become prone to pre-mature bolting and going to seed before acceptable yields of foliage have been harvested. Production of the seed heads tends to increase many of the volatile oil components in the plant and foliage and for this reason basil crops designated for essential oil extraction are harvested just before or at the time of flowering. However for fresh culinary basil, the foliage needs to be harvested before the flower stalk is produced if fresh flavour and shelf life is to be maintained. Selection of cultivars which are `bolt resistant', particularly under high temperature conditions is important, so too is preventing undue stress on the plant which can also induce bolting. Often, the stress induced by harvesting large amounts of foliage, forcing the plant to regenerate causes sufficient plant stress to promote bolting on even tiny new shoots. For this reason care must be taken to not remove too much foliage at any one harvest, leaving the plant with sufficient reserves to regenerate without inducing premature bolting. Current recommendation for basil harvest is that only 1/3 to 2/3 of the upper portion of the plant be harvested at any one time under conditions such as high temperatures which might induce bolting on the new growth

PESTS AND DISEASES OF BASIL CROPS

Basil is particularly prone to developing severe Botrytis infection where grow room or greenhouse humidity levels are high and this disease is common in winter. The fact that succulent herbs such as basil are regularly cut creating an open wound speeds up the botrytis infection process as air borne spores can land, germinate and rapidly infect the plant. Botrytis spores can also be spread through pruning and cutting tools, on workers’ hands and cut foliage as well as becoming air borne in very high numbers when the crop is disturbed. While there are now effective products which help suppress Botrytis infection, such as those containing the beneficial Trichoderma strains, the best method of control is still prevention by reducing humidity levels, heating the crop and preventing condensation.

Basil can also develop Fusarium wilt, and pythium infection of not only seedlings but older plants and growers are advised to keep a close eye on any plants which appear to be overly soft or wilting with brown, decayed roots. Removal of infected plants is always advised as well as determining where the source of infection is occurring. Often pre-used media and water supplies need treatment before use in the hydroponic system to exclude any disease causing pathogens.

As with most other crops, a healthy, rapidly growing plant is the best line of defense against many pathogens. This is particularly true of herb crops such as basil as there are very few effective synthetic pesticide products which are registered for pest and disease control on greenhouse herbs. However, there are products containing a naturally occurring soil borne bacterium - Streptomyces griseoviridis, which can be incorporated into the growing media to assist with the control of diseases such as Fusarium, Pythium, Phytophthora and others.

Pests which cause the most damage on hydroponic basil plants are those which also affect many other herbs. Whitefly, aphids, mites and caterpillar larvae can be potentially very damaging as they affect the harvestable portion of the crop - the foliage. Honey dew from whitefly and aphids can usually not be successfully washed off delicate herbs such as basil, so early control is vital. Mites can cause a speckling or bronzing of the basil foliage, often without the grower even realizing a pest is involved, they are also difficult to control. Some growers have much success with soap and oil sprays for smothering many pests, provided they don't case a residue on the plants just before harvest. Insect growth regulator products such as those containing Neem a botanical insect can also give good control of many insect pests on basil, provided they are used on a regular basis to break the insects’ life cycle and prevent re-infestation. Caterpillar larvae can be controlled with the use of a natural product containing Bacillus thuringiensis a toxic made from a soil bacterium. Some insects can be controlled with Pyrethrums, although they now seem to have limited effect on whitefly populations which have become resistant.

MAINTAINING BASIL FLAVOUR

Like many herbs, basil flavour and the essential oils contained within the leaf tissue are prone to oxidation and its pungency is easily lost. If the herb is grown in conditions that are too cool, or the harvested stems are stored for too long, basil will lose much of its flavour and pungency. Damage to the foliage will also affect the flavour and aroma of the fresh product as will excessive moisture loss after harvest. Correct growing conditions will ensure the basil type being grown produces its characteristic fresh flavour without becoming overly strong. In hydroponic systems, plant management and nutrient control are ways in which the flavour, appearance and aroma of basil crops can be maximized.

SUMMARY

Basil is a rewarding herb to grow hydroponically, with many culinary uses and the potential for year round cropping in protected or indoor systems. With the right temperature, nutrient and a little care and attention, fresh succulent basil can be ready for its first harvest with 8 weeks from sowing, providing many continual harvests over a number of months. Nothing beats the flavour and aroma of freshly harvested basil, so its a hydroponic plant well worth experimenting with.