Narcissus and Daffodil

108 G.R. Hanks
In practice, moulds associated with bulbs may be difficult to control during
incubation of the propagules, perhaps because of the presence of fungicide-resistant
strains (Lyon, 1978) or due to using grossly infected bulb stocks (Hanks, 1989).
Bulbil production on twin-scales is inherently variable because of the different
properties of bulb scales from different parts of the bulb (Hanks, 1985).
Chips or twin-scales are usually incubated after cutting, usually by mixing with
damp vermiculite and holding in trays at 20 °C for 12 weeks (Vreeburg and van
der Weijden, 1987b; Hanks, 1989). These conditions are ideal for fungal growth
as well as bulbil production. Attempts to scale up the treatment of machine-cut
chips by using deep crates, or by omitting the medium and controlling humidity,
gave promising results (G.R. Hanks, unpublished data). In the period immediately
after cutting, it is important to control chip temperatures which increase as a
result of wound respiration and the production of heat by the damped vermiculite
(Hanks, 1989), although the exact temperature and duration of incubation is not
critical (Hanks, 1986). By the end of incubation, the bulb scales should be largely
depleted of reserves: bulblets over 10 mm in length grow well, but smaller ones
often remain dormant (Hanks and Rees, 1979). The propagules are planted in the
field following the gentle removal of the vermiculite, and in some cases after a preplanting
fungicide dip (Vreeburg and van der Weijden, 1987c). In the case of
twin-scales, better growth has been reported following planting in a frost-free
glasshouse than in an unheated gauze house (Mowat and Chambers, 1977). As an
alternative to incubating chips, they may be planted in the field directly after cutting
and fungicide treatment, but chipping should take place in July so that bulbils
are produced before soil temperatures become sub-optimal, or a polythene mulch
may be used to raise soil temperatures from planting to emergence (ADAS, 1987;
Vreeburg and van der Weijden, 1987b).
In the field, plant growth should be maximised through careful husbandry, for
example controlling weeds by using non-damaging herbicides or a straw mulch,
and having a prolonged fungicide spray programme, and by using a low planting
density (Vreeburg and van der Weijden, 1987c; Hanks, 1989). Planting densities
quoted include 0.5 million chips per ha or 1–5 t original bulb weight/ha.
These methods would be useful in bulking supplies of bulbs for extraction,
perhaps following an initial phase of micropropagation. Chipping is likely to be
more practical than twin-scaling, as chips are robust, may be cut by machine, and
are more uniform.
Optimised production in the field
Standard bulb production methods are designed to give the most cost-effective
production of bulbs and (or) flowers, but in the case of valuable bulb stocks it may
be more important to maximize bulb yields. This can be achieved by greatly reducing
planting density and by adopting one-year-down growing. Planting density
should be reduced so that inter-plant competition is minimised, provided there is
no wind damage. In a trial of optimised bulb production, low planting densities
and annual planting and lifting were effective, but there was little additional benefit
of using combinations of foliar feeding, top-dressing, de-heading, irrigation or
fungicidal sprays (ADAS, 1982c). However, all these methods might be considered
beneficial in specific circumstances where it is desired to maximize bulb yields.