There is no mystery to the art of propagation as long as it is remembered that the propagator provides the total life support for a cutting until it is rooted. Two efficient means of propagating lavender are to use a low tunnel in a greenhouse or to use a mist bench. The effect is much the same. Bottom heat at about 23°C can be supplied by either a soil warming cable in moist sand or a foil panel with a heating element, covered with polythene and capillary matting. The latter provides a more even heat, but rapidly cools if there is a power failure.
The humidity developed in a low tunnel provides similar conditions to a mist unit operated using an electronic wet leaf to control length of misting. With a mist system rooting is aided by keeping the base of the cutting warm and the top cool.
Cuttings are best taken in late August and early September for overwintering as a liner (9 cm pot). The plant material needs to be soft and about 5 cm in length preferably from a non-flowering shoot. The type of cutting whether nodal, internodal or heal, makes little difference at this level. Cutting material should be taken from perfect specimens of the lavender required. Substandard plants either through pest, disease or with a particularly poor habit must be avoided. The need to maintain good quality stock plants is necessary for the quality and uniformity of the next generation of plants.
Method of taking cuttings
All cuttings first require their lower leaves to be removed to provide about 2 cm of clear stem, then dipped in a softwood hormone rooting powder and struck into a proprietary compost. A soilless rooting medium with about 20 per cent vermiculite has proven best. The use of plug trays greatly assists with the ease and speed of handling rooted cutting. The use of preformed plugs either using a gluing agent or an outer ‘jacket’ for the plug have advantages for handling, but tend to require greater vigilance in watering once removed from the propagation bench. It is also not possible to hold the cuttings in plug form for as long.
The speed of rooting varies according to the hardiness of the species. The tender and half-hardy lavenders root fastest and are usually ready to be removed from the propagation bench within two weeks. The frost hardy, hardy and very hardy lavenders take on average about two weeks. Successful rooting of between 95—100 per cent is typical.
Raising lavender from seed
Lavenders can be raised from seed, but cultivars are notorious for not coming true to type and are best raised from cuttings. Some species can be raised from seed with minimal isolation of the stock plants. It is easy to tell whether seed is set by merely tapping the seed head. If it rattles the seed can be collected, sown immediately or kept in a dark, dry place. When sown they can be placed on the mist bench and lightly covered with compost. Germination is usually rapid, approximately four days. Remove them to the weaning bench as soon as the majority have germinated to ensure the seedlings do not become leggy in the heat. Prick out to plugs when the first true leaves are visible. From then on they can be treated as for cuttings.
Once on the weaning bench the occasional mist will ensure that the plants are not unduly stressed. Light shading using fleece suspended 1—2 m above the plants also helps protect the plants from harsh sunlight. The air temperature at this stage should be maintained at about 15—20°C. The rooted cuttings should be kept to 5 cm so any top growth needs to be removed. Trimming will encourage side shooting and ensure plants branch immediately, giving good plant structure.
Nutrient availability is soon exhausted in any plug plant, so the addition of miniature pellets of slow release fertiliser with a 5–6-month release rate enables the life of a plug to be extended. An alternative is to use liquid feed. A balanced seaweed feed works well. Lavender species Lavandula (L.) stoechas and L. viridis and hybrids of the two, form very fibrous roots that require these to be potted on relatively quickly.
Two weeks in weaning should be sufficient to enable the rooted cuttings to survive a harsher environment of approximately 10ºC and some may be ready for potting to liners.
Clearly labelling lavenders from propagation onwards is imperative, as many look very similar for much of the year. An error at this stage, or mislabelling stock plants from which cuttings are taken, can be extremely costly.
To avoid interruption to growth, lavenders should be potted as soon as a good rootball has formed. For lavenders a compost using young peat with a pH of 6–6.2 provides a good medium for healthy growth. The addition of slow release fertiliser at the rate of 2.5–3 kg per cubic metre of compost is sufficient nutrition. The required rate of release is determined by the potting season. Autumn potting for liners kept inside requires a 12–14-month release rate with reasonably balanced N:P:K ratios. This allows the plant to develop gradually and not become too lush and vulnerable to cold weather nor too susceptible to pest and disease before the spring.
Spring potting requires a different approach, particularly if liners are to be sold as soon as possible. By April it is perfectly feasible to use a release rate of 5–6 months with a balanced N : P : K. With this release rate a liner can be ready within 3–4 weeks of potting allowing for one trim on potting, or approximately 4–5 weeks allowing for two trims.
A liner is certainly the most cost effective plant size to sell. Once planted in the ground a liner will be a year ahead of a plug, although a 3 l plant will be a year ahead of the liner. However, it costs more in time, materials and space to pot and maintain a 3 l plant than a liner. Most customers prefer to purchase lavenders as liners because they establish better than a plug and cost about one-third the price of a 3 l lavender. This enables the customer to buy a selection, or sufficient for a lavender hedge, at reasonable cost.
There is however, a growing market for specimen plants of 5, 10 and 15 l. These are best prepared in late autumn with a proprietary coarse potting compost, using 12–14 month slow release fertiliser. The plants should be set out to allow for substantial growth during the following spring. If the existing rootball is well watered when the plants are potted to a specimen pot, there is often no need to water the plants until the following February. The dryer fresh compost surrounding the rootball will provide excellent insulation through winter and enable these plants to be overwintered with the minimum of protection. A polythene tunnel netted along the sides is sufficient. Fleece can always be used in the coldest periods, on the frost hardy and half hardy lavenders, which will keep the temperature under the fleece 5ºC above the surrounding air temperature. The tender lavenders will always need to be protected to 5ºC. Pruning specimen lavenders to 10–15cm on potting will encourage rapid spring growth. Similar treatment can be given to stock plants for spring cutting material. At all stages of the production process there is a need to be vigilant for pest and disease.
Pest and disease control
Lavender is not prone to many pests and diseases. Good hygiene and regular stock inspections can save vast resources and reduce the need to apply expensive chemicals or biological controls on a regular basis. The first pest that may be encountered, in the life of a lavender on the nursery, is sciarid fly whose larvae attack the base of cuttings. This pest can be a problem for all lavenders on the mist propagation bench due to the moist atmosphere and can be summarily dealt with using nematodes. At the next stage vine weevil can be a particularly pernicious pest where prevention is definitely better than cure. Vine weevil larvae attack the roots of plants at the base of the stem. The addition of chemical additives when potting to a liner and larger sizes, will provide almost complete protection. Aphids can affect all lavenders, especially in the spring, but are easily controlled biologically with a soft soap spray, or chemically, if persistent.
Not all lavenders are affected by the same pests and diseases, nor to the same extent. Lavandula angustifolia and cultivars can suffer with red, and two-spotted, spider mite if kept undercover in a dry atmosphere, in the summer months. Lavandula stoechas and cultivars are particularly prone to leafhopper, another sap sucking pest and while this is often more cosmetic than harmful to the plant it will clearly affect the saleability of the plant. New spray applications on the market, specifically designed to kill sap sucking pests, are effective against aphids and leafhopper. The greatest pest problem of tender and half hardy lavenders, kept undercover in the summer, is whitefly and it is certainly the most difficult to eradicate so prompt action and regular treatment is necessary.
Good hygiene should avoid problems with the water borne disease phytophthora, then, the only fungal disease really affecting lavenders is botrytis. During winter months and warm wet summers botrytis can thrive. The first, preventative action, is to ensure that lavenders are not watered overhead in winter and that there is good air circulation. If botrytis does occur then alternating between two sprays with different active ingredients should be effective. This alternation is required because botrytis is very good at becoming resistant to the active ingredients.
Weeds, although not strictly a pest or disease, can harbour both. It is therefore, advisable and good nursery practice, to remove them from the growing area and from the surrounding area, particularly before they set seed.