This article was first published in 1991 in the journal of the Royal Caledonian Horticultural Society. It is with permission of Bill MacConnachie and RCHS that this article is reproduced to further the interest in and knowledge of chrysanthemums. The 'Caley' as it is affectionately known is particularly keen to share knowledge on all gardening topics to further the interest in horticulture. Not all societies are as open and forward thinking - Well Done Caley!
This article provides a comprehensive account of the cultivation of late flowering sprays. The article is quite lengthy and to help readers locate particular paragraphs a list of bookmarks is shown below. Click on the bookmark to go directly to the paragraph selected.
Since the introduction to Britain at the end of the eighteenth century the chrysanthemum has maintained a high popularity rating and over the years it has fully merited the title of "Queen of the Autumn".
There has been a considerable change in the genus over the years but the advancements that have resulted from the introduction of the modern garden sprays are unparalleled. Hybridizers found the new break to be a golden opportunity to parade their skills and over the last twenty years they have produced a wealth of new cultivars with combinations of colour and type that could not have been anticipated.
Specialist nurserymen quickly realised the commercial possibilities of a type that was both decorative and had long-lasting cut flower qualities. Today in Europe and North America the big organisations talk of millions of plants annually, not simply in terms of total production but of one variety.
With the knowledge of modern growth retardants to restrict the height of plants and the effect of light on the initiation of flower buds they were soon able to provide us with dwarf pot plants in full flower all year round. It didn't take them long to develop the technique of producing long lasting spray-type cut flowers over a like period. The chrysanthemum of the nineties is no longer the "Queen of the Autumn" but the "Queen of all Seasons".
The majority of readers will be conversant with early garden sprays rooted in spring from a cutting taken from the previous year's stool which had successfully overwintered in a cold frame or greenhouse. The young plant is planted out in the garden in late May and at that stage it has a single stem. Having established itself by June, the centre growing tip is removed (stopping or pinching) so that the plant will "break" to produce three or four strong flowering stems.
Additional stems growing from the leaf joints are removed to ensure vigour is directed to the selected flowering stems. Each early spray cultivar has its own temperature response and it appears that for each cultivar a minimum temperature over a specific period of time is needed for flower bud development. Until these temperature requirements are met vegetative growth continues to take place. The buds appear in the leaf axils and if left to their own devices, will develop rather untidily with the late lower buds eventually growing up through the more mature top flowers. You have to anticipate this by selecting the five or six flowers at the top of the stem and disbudding those below. As the apical bud develops faster than the others, this is also removed to give the appearance of even development.
Early sprays are normally grown through five inch mesh nets to avoid time consuming and unsightly alternative methods of staking and given reasonable attention, nutrients, weeding, spraying, etc., early flowering sprays in the garden give immense pleasure for garden display, as cut flowers or for exhibition.
The late flowering sprays are not as widely grown by the amateur, probably because they have to be brought in to the greenhouse or conservatory during early October when the buds begin to show colour. They are however quite spectacular when well grown, standing at least three feet high on a single stem, with a fully developed flower on a short leafless pedicel at each and every leaf axil. The evenness of flower development is a result of blacking-out the plant for a period of fourteen days in the growing cycle.
The flowering of mid-season and late chrysanthemums is largely controlled by the length of day. We can control the day length either by extending it by the use of artificial light or reducing it by blacking-out during normal daylight hours.
To be precise, the plants have to be in darkness for fourteen hours daily for at least fourteen days at the end of August and the beginning of September to initiate flower buds. These light conditions are not naturally achieved at this time in Scotland so we have to shorten the daylight hours by covering the plants at six o'clock in the evening and uncovering at eight o'clock in the morning.
Each cultivar which is suitable for our purpose is put into a Response Group which is a figure denoting the number of weeks it takes from the start of the blacking-out to the full development of the flower. If we require our flowers to be at full development from the beginning of November we have to ensure that any light control commences appropriate to the Response Group.
Varieties suitable for flowering in early November.
For example; if you require flowers of Rynoon or its sports on 1st November, counting back nine weeks (63 days) from 1st November brings you to 31st August. On this date there is less than 14 hours of darkness so blacking-out has to commence on 31st August ensuring that there is a minimum of fourteen hours darkness for the next fourteen days.
Cuttings have to be produced from the old stools that have been overwintered in a cold greenhouse. Because of the late rooting the previous year (June/July), they appear to be smaller and thinner than other late types, but when given a little heat in February they quickly confirm that they are alive and well. By March, cuttings will be available at the base of the stool for rooting but the date of striking need not be precise because these cuttings are simply for producing mother plants which eventually provide cuttings for the final crop. This final rooting will not take place until the end of June and the beginning of July.
Chrysanthemums root fairly easily in early spring when day temperatures are relatively low. A fairly open mixture of equal parts of loam, peat and grit with a bit of bottom heat and they will root within a fortnight. They have to be moved from the cutting tray, and there are three options, each quite successful. The objective is to produce short, healthy cuttings approximately two inches long for rooting during the last days of June and the first days of July.
In the first two methods the plants will grow on a single stem to reach a height of approximately 18 inches by the end of May. In the pots they will
be harder and more spindly. In each case they are cut back to a few inches from soil level to create a mini stool. Keep both growing with adequate water with a light balanced feed and the fresh cuttings will emerge from the soil or the neck of the old stool stalk at the required time.
The third method is to pot up into 3.5 inch plastic pots in John Innes Compost No. 2 and when they have become well established, say the third week of April, the growing tip is removed (stopping or pinching) to have the plant produce laterals. At the end of the month they are potted on into 5 inch plastic pots, again in John Innes Compost No. 2. On the 25th May a further stop is given, this time to the tips of the new laterals. The plants will be large and bushy in comparison with the other two methods and in this case the cuttings will emerge from the tips of the laterals that have been stopped. There will be cuttings at the base of the stool but these should be disregarded. I would be looking for perhaps five good quality cuttings from each mother plant irrespective of the method used and this will give some indication of the number of mother plants to be taken on to meet one's personal need.
During the period of producing cuttings the plants need the protection of a heated greenhouse but they would benefit from being in a cold frame after they were well established in pots or boxes. I am fortunate to have a large, well ventilated greenhouse and opt to keep them inside, right up to flowering time.
It goes without saying that good hygienic practices are important. There is disappointment to get everything right only to find that the foliage becomes distorted with age due to earlier insect damage.
Up to this point there is nothing difficult in what I have outlined but I think there is a question that you may be asking.
Why the importance of producing a SHORT cutting from the soil base, the neck of the stool or a side shoot?
All late flowering spray cultivars produce a number of leaves before the initiation of the flower bud. This leaf number varies between cultivars and is known as the Long Day Leaf Number (L.D.L.N.) and is counted from the point where the cutting originated from the mother plant. Of the cultivars previously mentioned the L.D.L.N. varies from 35 to over 80.
The ideal short two inch cutting will have two mature leaves, two semi-mature leaves, twelve undeveloped leaves in the growing tip and will probably develop a further four leaves during the fortnight it is rooting. A further two leaves will probably have been left on the old plant when the cutting was taken which means that by 10th July the shoot selected for the cutting will have produced 22 leaves. The cultivars with a lower L.D.L.N. will only have to produce a further 13 to 15 leaves before the flower bud is naturally initiated, which can cause problems if this happens before blacking-out. If a cutting is taken leaving six leaves on the shoot at the mother plant the resulting plant will produce six leaves less than its L.D.L.N. at the time of bud initiation.
Remarkable as it may seem, the plant appears to have a memory. In Scotland it takes about two days to produce each leaf; therefore six leaves lost, means that bud initiation will occur twelve days earlier and flowers develop without the benefit of blacking out. Plants allowed to initiate flower buds naturally in mid-August have been found to give variable results.
Rooting cuttings in the middle of summer has its own problems. The sun is high in the sky; day temperatures can be extremely high even in well ventilated greenhouses, and the atmosphere can be very dry. The cuttings to be rooted no longer have a plumbing system to keep them going and because of the rapid growth they are likely to be on the soft side. In these circumstances there is always the risk of loss of moisture in the cutting material due to transpiration and if they are left flagging for too long they will be difficult to root. Even if they recover you may find that they will only have rooted on one side of the stem.
I root in a mixture of equal parts rough Irish peat and grit. Barra shell from a builder's merchant, if I can afford it, is ideal. It has too large a particle size but if it is placed in a hessian sack and given a good going over with a heavy hammer it gets into better shape. This is a very open mixture and consequently it can take regular watering without doing any harm.
The tray of cuttings is placed on an open bench without bottom heat where they get good light but are shaded in such a way that they are never in direct sunlight. The cuttings are sprayed regularly in an endeavour to create some humidity. They normally strike in ten days and are then moved quickly to 3.5 inch plastic pots in John Innes Compost No. 2 to avoid the ever expanding roots becoming entwined.
Growth is very rapid in July and early August but it is nevertheless important that they are kept moving and are not checked. As soon as the pots are filled with roots, which is in early August, they are moved into 7 inch pots with a John Innes Compost No. 3 mixture.
Ail my other late types i.e. Japs, Decoratives and Charms go through a series of clay pots which are porous and assist in ripening the wood. Plastic appears to be more suited to late sprays which require a softer growth.
The single stem has to be supported and a split cane will suffice initially but a thin bamboo cane will be needed finally. They are now in their final pots and throughout August they will appreciate good ventilation and sufficient space to encourage good lateral growth. Turning the pots occasionally to let each area of the plant share the sunny side encourages balanced growth.
In the last few days of August the dreaded "black-out" time arrives and for three weeks you will have the chore of covering the plants morning and evening.
The necessity for blacking-out has been explained. Response Group 10 plants will go into the black-out on 24th August and will be removed on 8th September. Response Group 9 plants will go in on 31st August and come out on 15th September. Could anything be simpler!
The actual blacking-out arrangement need not be elaborate. I have known a grower with only a few pots who lifted them into his windowless garage in the evening and carried them out in the morning. Another, with a few more pots, placed them on an old door to which he fitted four wheels and he moved them in and out of his coal house without ever having touched the pots (the plants must have belonged to the same Response Group!).
Whatever arrangements you have, it must be capable of completely blacking-out the light. Close woven black cloth or black polythene is suitable, draped over a light wooden structure or snag-free net. The cloth is preferable because the morning sun on polythene just before removal can create high temperature and humidity.
My own arrangement is a rectangular frame made of two by two inch timber. Four foot square ends are permanently covered with polythene and are joined together at the corners to make an eight by four foot rectangular frame. It stands temporarily on an ash base path in the centre of the greenhouse and when the plants are inside it is a simple matter to place a wide strip of black polythene over the centre section to totally enclose the unit.
Fourteen hours of black-out has to be provided and the timing arrangement for covering and removing is what suits you best. However an important consideration is that the cover should not still be over the plants at 9.30 on a late August morning when the sun will have been on the greenhouse for one and a half hours. Even with good greenhouse ventilation the temperature under the black-out would be dangerously high. My own preference is to black-out in the evening at six o'clock and remove at eight o'clock in the morning. If you go out one morning and find that you forgot to cover the previous evening don't get too upset as a single night's error will do them no harm.
The high humidity and temperature to which I refer would suggest that the plants could be in an environment that could cause an attack of powdery mildew and it would therefore be prudent to go over the foliage with a drench of Benomyl solution before starting the black-out. Similarly the leaf miner maggot can run riot in these conditions.
It is depressing to say the least to have a good batch of sprays in November only to find that the leaves are riddled with the crazy white tunnelling patterns which only become apparent after they have departed. A spray of Hexyl does no harm in mid-August.
The challenge to the perfectionist is to have a good placement of fresh individual flowers of good colour. It is also important to have an evenness of flower development throughout; but even in the hands of the most experienced grower the top and bottom flowers develop properly, but somewhere in the middle they are slightly later and smaller when they develop. This irregular development is now believed to correspond to uneven night temperature during the blacking-out stage. To overcome this I am installing a thermostatically controlled mini-heater under the covers with the thermostat set at 60°F.
At the conclusion of the blacking-out period the plants are removed to the open bench where they will have more space to continue their development. Close scrutiny of the leaf axils will reveal the first traces of the buds which will develop rapidly. Do what you can to avoid large fluctuations of temperature. Too high a night temperature could lead to flower development which is too rapid and this would be harmful to both colour and flower size. If you have the facility, a night temperature of 54°F from late September onwards would be suitable and with prudent day-time ventilation, temperature fluctuations can be controlled.
The top flowers tend to be crowded and it is common practice to remove the apical bud to improve their spacing. Most cultivars tend to produce their lower flowers on rather long pedicels and it will improve the balance of the spray if they are removed. The floral artists will want to keep them on the plant because they are ideal material for arranging.
Late October and November brings its own reward as those remarkable specimens unfold and provide us with a long lasting display to adorn greenhouse, conservatory and living room.
W. D. "Bill" MAcCONNACHIE, M.B.E., S.H.M., was a Vice-President of the National Chrysanthemum Society. He was founder and for many years Secretary of the Scottish Group of the Society. Sadly Bill MacConnachie died in 2001. A great man, sadly missed.
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Last updated on 22 December, 2001