How I Grow Onions & Leeks by Ivor Mace

Sitting here preparing to write these notes I think I should state first of all what my association with vegetables is these days, and in fact what involvement I have with horticulture in general, and how this could influence my thinking on growing vegetables and other plants.

I have been involved in horticulture for 45 of my 57 years. Vegetables were my first experience with plants. We only had a small back garden which my mother stuffed full of herbaceous perennials. I got my first allotment at about 11 or 12 years of age and I well remember the school attendance officer (we called him something else which I can’t print here) looking down on me from the rather large coal tip adjacent to the allotment site.

So how appropriate now that I work in the horticultural department at Pencoed College. A land based College in South Wales. I suppose you could say finishing off my education so to speak in lieu of my misspent years early on in life.

I have grown lots of plants in my years as a professional gardener, roses, shrubs, herbaceous perennials, turf, and in my early day’s vegetables. I have never tired of growing them because I like eating them. I like to know what goes into and onto them, and ultimately into me and my family. I am not a completely organic gardener but I am almost there nowadays. I look for the friendliest approach to growing when ever I can. When I run an organic gardening course at Pencoed College it always attracts the most students of which at least two thirds will be younger women. Quite naturally concerned about what they feed their children.

I only grow two vegetables to exhibit these days Onions and Leeks. There are a few reasons for this. One reason is I also like to grow Chrysanthemums (37 years) Roses (15 years) and Carnations (4 years). Onions and Leeks fit in well with these other plants because the vast bulk of the work is done early in the year. Ground preparation, and growing the young plants.

Ground Preparation

This is pretty straightforward, I single dig the beds sometime after the old crop has finished, certainly by Christmas time. I use one of the two 20foot x 14foot polytunnels I have for the onions, and one of the three beds in the 40foot x 10 greenhouse that I use for the chrysanthemums in the autumn to grow the leeks in the summer.

This way I don’t need to sterilise the beds because I am able to practice crop rotation. This has got to be a healthier option as far as I am concerned, and it is a lot easier.

Up until 8 or 9 years ago I used to dig in quite a bit of well rotted manure but I found that it resulted if very high soil conductivity. Base dressings of fertiliser added to the problem. The results of soil testing was alarming with conductivity readings at index 5 or 6 and high phosphate levels before the season even started. This would mean that my plants would find it difficult to pull in enough water in hot weather to replace the water lost through transpiration.

This was the first year (2005) since testing that I have added any phosphate. It took at least 6 years to reduce to an acceptable level. In fact I have used no manure or base fertiliser for at least 6 years. I have relied on soluble feeds with no phosphate.

Young Plants

The leek grass is started off in late Oct, early November with a later lot about the 3 rd week in November. They are rooted on a soil warming cable and as they begin to grow I transfer them under lights. I have four 8ft florescent tubes beneath my propagation bench with another soil warming cable fitted. This enables me to propagate my chrysanthemum cuttings in natural light on the top bench, while my onions and leeks are enjoying 24 hour daylight underneath.

Usually they will get too big to remain there by the end of the first week in February because the tubes are only a foot above the trays/Pots. The onion seed is sown from 20 th to 27 th December whenever I have time.

A week after germination I transplant the seedlings into Plantpak 40s. At 40 per tray they don’t take up too much room under the lights. The leeks are potted on about 20 th December into 9cm pots. A cane and clip is used to keep them straight. I usually have about a dozen spare in case the odd one develops a bend.

As you know leeks with bends are not worth planting out because you will not win prizes with them. I usually have 200 onion seedlings at this stage, and I finish up planting 84 in the tunnel. Maintenance of the young plants is every morning and again at 6pm when I return from work. I look over the plants and with the aid of a small houseplant watering can I water any dry cells and pots. They dry out fairly quickly because it is a constant 16 oc with continual light.

The next move for the onions is into 9cm pots with 2 canes and 4 clips. Keeping them straight is the top priority. I aim for a night temperature of 10 oc and vent at 15 oc. This coincides with a move into a 12’x8’ greenhouse. I have 2 sodium lamps up in the roof apex, I raise the staging up on milk crates, as the plants grow I drop the staging down. I find this works better than having a low staging and raising the lights as the plants grow.

Early in the year I find it is warmer at night if the plants are higher in the greenhouse. Watering at all stages is only carried out when the surface of the compost has dried out, and then I water to get about a third run through. It is important to get them wet right to the bottom of the pot, and equally important to get air to enter the compost before watering again.

Compost

After much experimentation over several years I have arrived at the following compost which I grow most things in including onions and leeks.

• 2 Parts Sterilised Loam.
• 2 Parts Moss Peat.
• 1 Part Perlite.
• To every bushel (8 Gallons) I add 2oz Dolomite Limestone.
• 2oz Calcified Seaweed. (I am informed that this will be discontinued) in which case I would use Calcium Carbonate (Ground Limestone
• 2oz Vitax Q4
• 2oz Antergon organic fertiliser with beneficial fungi.

This is the equivalent of a John Innes number 1. I use this for sowing, transplanting, 9cm and 15cm pots. Once the plants have been in their pots for 4 weeks I introduce a balanced half strength liquid feed to the water.

At about the same time that the onions are moved from their 40 cells to 9cm pots the leeks go into 6inch clay pots, they are now all in the 8 x 12 greenhouse with lights on for 16hours a day. Margaret may switch them off on really sunny afternoons and switch them on again when the sun goes down. The leeks are moved up into 8-9” clay pots in early April. The onions go into 1 1/ 2 litre pots at the end of March. The same level of care of watering and ventilating is used and the grow lights by now are only used in the evenings for a few hours.

I send a soil sample off in early April just to see if I need any fertiliser. I add what is needed and cultivate into the top 6 inches. I plant out in early May. The main reason for this is do not want to heat the tunnel.

Experience has taught me that if I plant out large plants at this time of year and we are unlucky enough to have several successive nights near to a frost the onions will either have double necks or worse still run to seed.

This is often the case with any plant that flowers naturally in its second season of growth, if it gets several cold nights after the plants have passed their adolescent stage they can often run into flower, and a seasons work is wasted.

I realised many years ago if you are after large onions with a good finish you must not get them too early, or too soft. If you get the bulbs too early the skins will split before the shows arrive because they only need 3 to 4 weeks to ripen before your show.

If the plants have had too much nitrogen they can be too soft and will not keep. Since my problems with high conductivity I only manure the beds on the year I intend changing the polythene sheet on the tunnel.

The ground will have 3 months rain over the winter. On the other 4 years no manure is added. I have a soil salinity meter which through trial and error on various plants. I know I will give good growth at a reading of .2, if the soil salinity drops bellow this I liquid feed through the lay flat tubes that are bellow the black/white polythene sheets the plants are grown through.

The method of planting is to plant empty pots the same size as the ones the onion plants are growing in. I allow 18 inches between each plant and each row. I then lay the irrigation hoses followed by the black/white polythene, then hold it in place with planks or bricks. Cut holes around the inside of the pots, remove the pot and replace with an onion plant. Make sure the leaves run along with the rows and not out over the path.

I feed with 2 parts nitrogen to 1 part potassium. This will give the plants about equal parts N to K because nitrogen leaches quicker.

I use.

• 7ozs Potassium Nitrate.
• 6ozs Ammonium Nitrate.
• 12ozs Calcium Nitrate
• Into a gallon of water and then dilute it at 1 in 200 parts water.

If the onion necks are thick and the plants look very vigorous then I cut the Ammonium Nitrate and Calcium Nitrate by half, this will then give a feed of 1 part nitrogen to 1 part potassium.

The leeks are not collared in the pots but the foliage is held up with clips to encourage the centre of the plants to elongate. Once they are planted out they are caned and collared with 12inch black damp course. This is 12 inches by 15 inches. After a while as they grow I turn it around so that it is 15 inches high. Later on this is replaced with an 18 inch collar. The leeks are planted 4 inches deep so you should land up with at least 18 inches to the button. When you re collar you can take off any old leaves but I think it is a good idea to keep one split skin on to protect against marking.

The canes and clips are left on the onions for a while after planting, but as the plants begin to grow quickly then they must be removed to prevent overcrowding. All that is left to attend to now during the next 2 months is to weed around the plants a couple of times and make sure the plants remain upright. I water around the plants after planting for the first 2 weeks then I use the hoses when I am fairly certain the roots are running out into the bed. Usually this is once a week depending on the weather.

Soil Salinity Meter

I Have a Hanna Instruments soil salinity and water conductivity meter which I use while the plants are growing. If I get a soil reading less than .2 I use a soluble feed through the hoses. As an example most potting composts would have a reading of around .4 to .5 so you can see I don’t aim for soil that is over fertile because of the problem with osmosis or lack of it. Plants can absorb water freely when the soil solution is weaker than the plants sap. When the water dissolved minerals enter the plant and when they reach the leaf they play an important part in photosynthesis.

On the other hand when the soil water solution is too strong the plant cannot absorb enough to replace the rate of transpiration, the plant in an effort to prevent dehydration will shut the stomata to avoid water loss, the result of this is no carbon dioxide can enter the leaf and photosynthesis will slow down or stop. Therefore overfeeding should be avoided as it inhibits plant growth.

In Polytunnels and Structures there is no rain to leach away surplus salts so irrigation has to be used as a substitute. I have found that I have to reduce the amount of organic matter I use and be very sensible with fertiliser use. I under fertilise and rely on my conductivity meter. When the soil conductivity drops below .2 I used my injector and layflat tubes to supplementary feed.

This is what I use.

• 7ozs Potasium Nitrate.
• 6ozs Ammonium Nitrate.
• 12ozs Calcium Nitrate.

I dilute this in a gallon of water then use it at 1 in 200 or in my case I adjust my injector to dispense at 1000micro siemens. I give the beds about 20 minutes, then after a few days I check again with the meter and if the soil reading is above .2 I use plain water the next time I irrigate. If on the other hand the nutrient level is bellow .2 then I use the injector again.

Ripening the Bulbs.

I find about 25 days from the show is needed to produce a good ripe finish. If you go longer the bulbs could wrinkle or the skins could crack. Less and they won’t be quite ripe. In reality I find that if I get 3 shows from the same bulbs, that’s about it. One week a little under ripe, the next week they will be spot on, then after this the skin condition will be going down hill. Over the last 3 seasons I have been seeding some of Derek Raw’s strain next to mine in hope that they will cross pollinate.

Some offspring I feel fairly sure have crossed because the bulbs are taller like my strain. I have kept these to produce seed because my aim is to avoid inbreeding. Two observations I have made are Derek Raw’s strain is earlier maturing than mine which is very useful as I then have a potentially longer showing season. The other thing I have noticed with it is it has a stronger flavour. I am very careful to use my strain in salads because Derek’s brings tears to my eyes.

Lastly I think its worth mentioning since I have been careful not to use too much organic matter and have been including calcium nitrate in my feeding the keeping qualities of the bulbs has been 1 st class.

I have had them keep until May of the following year. One word of caution though this feed does not suit all plants. My Carnations like it because they are calcium tolerant, Chrysanthemums get chlorosis but they can cope with it once the top leaver are fully expanded. Celery enjoy it and the calcium seems to prevent heart rot. Tomatoes also display Chlorosis because high calcium locks up iron. So be careful if you use this feed, anything that displays iron deficiency with draw the 12oz of calcium nitrate and replace it with a further 6oz of Ammonium Nitrate.

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