Plant Modules & Plugs: Advantages & Disadvantages

Choosing a Plant Module System Offers Many Advantages When You Are Sowing & Growing Garden Plants. Sometimes Referred to As Plug Plants or Plant Module Grown Plants, They Save Gardeners Time, Space, Compost & Money.

Plant module systems are plant trays where you can grow plants in individual cells; they’re usually made of plastic, polystyrene or recycled plastic and come in various sizes and quality. The very best modular plant trays are made from recycled plastic, are robust and last in excess of 25 years.        

The worse are made of virgin plastic and are single use and are often very fragile. Surprisingly, there isn’t a great deal of difference in the price between long lasting recycled and flimsy virgin plastic types. Most garden centres stock the single use trays but not the long lasting ones. Cynics might suggest this is because they can make repeat sales of the cheap, single use ones!      

Polystyrene modular trays are a mid option. They are stronger than single use. Last many years and are lightweight.  

Preparing Plant Module Trays For Sowing

Trays should be filled with open textured compost that isn’t going to set like concrete in the tray. I prefer the compost to be slightly damp and free running so that when it is sprinkled on the tray it flows into the modules with little problems. Clearly the smaller the module the harder it is for a coarse compost to fill them. And any debris such as twigs in the compost will prevent the modules from filling easily. Once filled to the brim I either firm the compost with my finger tips or use another tray of the same size to push down onto the tray and lightly compress the compost. Clearly flat bottomed trays can’t compress the modules as well as the ones that are individual (a bit like an egg tray as in the first image on this page) and can press down on each module cell. The individual module type has the benefit that it leaves a light depression in the compost into which the seeds can be dropped.  

Sowing Seeds In Plug & Module Trays

Single seeds can be placed in each cell or you can multi-seed the cells with a number of seeds. For example if sowing beetroot I tend to put four to a module. Once planted they will push one another apart as they grow. With lettuce I tend to put in one seed per cell as the germination rates are close to 100% and I don’t want to thin them out.

Shallots and onions are another seed that can benefit from multi-seeding. I find that spring onions for example do much better in small groups than in thin drills.  I tend to harvest the best one(s) in each group and this then leaves the smaller ones a bit more space in which to grow. though often there is little difference between the plants. Once the seed is placed in each cell, in most cases you can cover with a small amount of compost and gently firm the compost surface. Some seeds do best if left uncovered .. so check the seed packet before following covering them!  

Seed & Plant Module Cell Sizes

Like pots, module cell sizes can vary. In fact they can be almost any size you want. Indeed, plug plant retailers offer various size plugs, depending on the use of the plant, time of year and other criteria. But often they use three sizes. For example Thompson and Morgan offer Jumbo plugs (surprisingly, with a name like this, it’s the smallest one(, then there’s Postiplugs which fits letter box and has a bigger plant than Jumbo pigs. Finally there are the Garden Ready Plan plugs, which they offer as living up to their name with no need to re-pot or grow them on. Like T&M many producers and retailers talk about plant size in terms of height of plug and plant. When I grew plants for my own use I however recognised another way of nearing them. It was by the size of the plug or module measuring them by length and breadth. We used square plugs and had built our own machinery to produce them. The standard size was 3.8cm square and produced a good sized block/plug for lettuce growing. It was our standard. the plant would have 5-6 true leaves when planted and be a good strong plant with a good root system. When we manufactured plugs ready for sowing we would normally make them as deep as square. So a 3.2 cm plug was 3.2 cm inn all three dimensions. Where we knew we had plenty of time to get the plant in and perhaps already had a en empty greenhouse ready for planting, we used a mini plug, just 3.2 cm square. These plants had minimal compost to grow in and would soon starve if not planted. But we planted quite soon after germination, when the plants had just 2 or 2.5 true leaves. Our last lettuce size was the 4.2 cm plug. This contained a lot more compost and could “hold” a plant for much longer. It was ideal where we wanted to grow the plants on with a gentle heat in the propagation house and hence gain few extra days at harvest time. Of course this only worked if we still planted in time as a plant that is left “hungry” and outgrowing its space and plug will not grow away fast when planted. Theres more on growing lettuce if you follow the link.

Planting-out Module Grown Plants

Plants need to be removed from the modules without damaging the plant. I find the best method is to ensure the compost is gently firmed at sowing time, then allowed to slightly dry out at planting time. This gives a dense block of compost that holds together without being over dense. And, because it has shrunk slightly due to being slightly drier, is easy to remove from the module tray. Being slightly dry the module is capable of being pushed into soft soil. Our where No Dig is the chosen method it can be dropped into a hole made with a dibber. In both of the above cases I advise a good watering after planting. It doesn’t have to be immediate, unless the weather is very hot, but within a few hours. This settles them in and gets the roots working. Where module grown plants are being planted in hard soil a trowel will be needed to make a hole for the module to be placed in. Commercially I did it slightly differently. We didn’t use a tray for module plants. The machinery I used, a blockmaker, “stamped” a bed of compost into a what resembled a chocolate bar. Each module within the bar had a small depression stamped n it in to which we placed a seed. The bar was then placed in the probation house, the seed germinated and was gown until planting time. As we planted we broke off each square of the stamped compost and planted each, containing just one plant, into the soil. Because we were well mechanised I also had a roller type device (a block marker) that went behind my tractor and made individual depressions in the soil into which to plant each plug. This meant we could sown in accurately spaced beds. In my case the outside beds were 100 yards long by 48 inches wide and contained four rows of lettuce. A field would contain many beds and, at peak, I’d often have several hundred thousand lettuce growing, all at various stages of growth. In total I grew half a million lettuce a year using this technique. For greenhouse use I also had a block marker that fitted my rotovator.  

Potting On Plant Module Grown Plug Plants

When potting on module grown plants into flower pots the process if very easy. Fill the pot with the required compost. Make a depression in the compost and drop in the module. Firm gently and water to establish the plant. Variations might include adding a layer of grit to the top of the pot once planted or growing in specific conditions, but hat is dependant on th espy e of plant in the plug and their specific requirements.  

Plant Types That Shouldn’t Be Module Grown

Though module growing suits many species it isn’t ideal for some. It’s ideal for lettuce, beet, most alliums, celery, tomatoes, peppers, various cucurbitaceae including cues, melons, courgettes etc, And though I’ve said its OK for beetroot, it isn’t OK for many root crops. Root crops come in two types. Those where the root is above ground, eg beetroot, turnips, swedes etc. and those where the root is grown underground. Eg. Carrots and parsnips. The reason is that modules distort the tap root, which needs to be long and straight. A twisted, contorted taproot at planting time is going to stay like that. It’s never going to grow deep, strong and straight. And no one loves a twisted contorted carrot! Likewise I wouldn’t grow radish in modules or plugs. They grow so fast there is no advantage. If you’ve questions on plant module growing pot it in the comments section below.  

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