Plug plants in tray

How to grow plug plants

You can fill your garden with masses of colour, drama and variety by buying mail-order plug plants. Martyn Cox offers some seasonal advice on how to grow plug plants.

Plug plants are essentially well-rooted seedlings (or sometimes cuttings) that allow anyone to raise perennials, bedding plants and vegetables without having to sow seeds from scratch. They are a great option for those pressed for time or who don’t fancy growing plants from seed, and are a fraction of the cost of fully-grown specimens.

Nurseries create plug plants by sowing seeds in special trays or modules, which contain a number of ‘cells’ – they are sold to gardeners after they’ve germinated and the roots filled the cell. Plugs come in varying sizes, and are usually despatched in protective plastic blister packs.

Different sizes

Suppliers offer a number of different sized plugs, using their own terminology like mini plug, extra value plug, super plug and jumbo plug. Essentially, the larger the plug, the easier plants are to handle and the quicker they will establish. Tiny plugs are fiddly, delicate and require more care. When learning how to grow plug plants, it’s always useful to consider what size of plug plant you’re dealing with.

If you browse the range of plants offered by Richard Jackson you’ll find four different sizes of plugs. Rather than give them an ambiguous description, they are sold by measurement – 4cm, 4.5cm, 5cm and 7cm. This refers to the length of the actual plug (not the plant), measured from top to bottom. 

Some suppliers offer ‘garden ready’ plants later in the season. These are essentially super large plugs that are suitable for planting straight into the ground or final containers, without having to be grown on in smaller pots. 

When they arrive

Open the box, lift out the plug plants and water if the compost seems dry – the best way to do this is to stand them in a saucer of water (room temperature) and let them soak it up from the base. When they are visibly damp, place onto a dry saucer to allow excess water to drain away. 

As a general rule, it’s best to pot up your young plants as soon as possible after delivery. However, don’t worry if you can’t get round to it straight away. Plug plants will keep quite well for several days as long as they are stood in a light, warm place and the compost doesn’t dry out.

Potting up 

All but the tiniest plug plants are perfect for potting into 7.5-9cm (3-3½ in) pots of multi-purpose compost. Fill the pot with compost, tap to settle and swish off excess compost to leave a level surface. Use a pencil or dibber to make a hole in the centre of each pot and pop in a plug, firming in place with your fingers.

It’s important not to set plug plants in larger containers. They tend to be slower to establish and the greater volume of compost holds a lot of moisture, which can result in plugs becoming saturated and put them in danger of rotting. From a practical point of view, larger containers also take up a lot more space. 

Martyn Cox potting on plug plants
Pot up your plug plants into 7.5-9cm pots of multi-purpose compost. Image: Louis Cox

Always use fresh, good quality compost (Richard Jackson’s Flower Power Peat-Free Compost is ideal) and never the stale remnants of a bag that’s been knocking about in the shed for a year or so. Also give homemade garden compost the cold shoulder – it can harbour pests and diseases that can be harmful to plants.  

Once they’ve been potted, water gently with a can fitted with a sprinkler head. As they are sensitive to frost, move plants to a light place indoors, but out of hot, direct sunlight. Stand on individual saucers or place a number of pots on top of a rectangular drip tray.  

On-going care

One of the most important factors in learning how to grow plug plants is to consider how to keep them going once they’re established. Once their roots start to fill containers and top growth develops, plants will require regular watering to keep them in fine fettle. It can be a bit of a chore to move them to a place where you can use a sprinkler with abandon, so I prefer to trickle water gently across the surface from the spout of an indoor can. 

As they grow, you’ll notice plants start to bend towards the light. Don’t worry, this is a perfectly natural phenomenon known as phototropism. All you need to do to encourage upright growth is to turn containers round every few days. 

Some plants aren’t naturally bushy and will attempt to grow tall and leggy. Prevent this by pinching back the growing tips of fuchsias, petunias and geraniums when they are about 10cm (4in) high to encourage the development of side-shoots further down. Repeat after they’ve produced two or three more sets of leaves.

To avoid a check to growth, move plants into slightly larger pots once you notice their roots are starting to poke through the drainage holes at the base. Depending on when the plugs were delivered or the vigour of the plant, you may need to repeat this process several times. 

Moving plants outside

Hardy perennials can be planted outdoors at any time, as long as the ground isn’t frozen or waterlogged. But keep tender bedding plants indoors until there’s no longer any danger from frost, usually late May or early June. Acclimatise plants by moving them outside during the day and back inside by early evening, this is called ‘hardening off’. Do this for a few days, and then leave out all night – cover with fleece if cold temperatures are forecast.

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