Advice
RHS Help & Advice
Growing on seedlings and plug plants
Obtaining young plants that are still in the small ‘plug’ stage can be an economical way of raising large numbers of plants; these can then be potted on, once large enough, and used to maximum effect, displayed in the garden or in windowboxes and planters
Growing plug plants
Whether you raise your own plants from seeds (right) or cuttings, or buy plug plants sold in garden centres or by mail order, young plants are easily grown on for use outdoors when the weather improves.
Restricted root space
Plug plants have little room for their roots to gather nutrients and moisture. To keep plants healthy they must not be allowed to dry out or run short of fertiliser. Drying out is worst around the edges of cell trays.
Keeping well watered
Use a watering can with a coarse rose that floods the cell tray so that each cell fills with water. To soak a dry plug, several floodings may be needed. A well-rooted plug (left) can be extracted and checked for water penetration then returned to its cell, none the worse for the outing.
Plug plants that have yet to fill their cell with roots have to be treated more gently - but without much root, they are slower to dry out.
Keeping well fed
To feed plug plants, apply a balanced liquid fertiliser every fortnight in most cases. Well-developed plants may need weekly or more frequent feeding, but less developed plants might only require an application once a month. Frequent feeding and early potting is advisable when using organic fertilisers and potting media.
Signals of stress
Dull foliage suggests drought stress, and pale lower leaves indicate insufficient feeding. By the time these signs are obvious, the damage has been done, so do not delay in remedying these symptoms.
Placing pellets of controlled-release fertiliser or slow-release organic material, such as chicken manure pellets, in the bottom of plug trays before seeding or planting can help avoid plants running out of nutrients and is especially helpful when growing organically.
Preventing pests and diseases
Young plants, especially those stressed by inadvertent starvation or drought, are vulnerable to pests and diseases. Be ready to treat any problems as soon as they become apparent, using approved pesticides including organic ones when seen.
Encouraging new shoots
When ornamental plants become lanky, pinch out their shoot tips to encourage branching. If necessary, several ‘pinchings’ can be done throughout the growing period of the plant. Avoid excessive elongation (etiolation) by growing plants in bright conditions and avoiding excessive warmth. Pinched-out fragments can be used as cuttings.
Choosing containers
Once the plugs are full of roots
and the root ‘wedge’ can be easily extracted it is time to move the plants into larger pots (left). Although vigorous plants such as petunias and pelargoniums can be put straight into hanging baskets and large containers (right) it is usually best to pot plugs into smaller pots of about 7-9cm diameter. This is to prevent the roots being suffocated by damp, airless potting media.
Easy potting on
The technique is straightforward: fill a pot to the brim with potting compost, tap it on a hard surface, make a plug-sized hole in the compost, insert the roots so the top of the plug is close to the surface, firm lightly and water in.
Once the plug has filled its pot the plant can be hardened off and set out in the garden whenever the weather permits.
Although it is best to grow plug plants without any setback to their growth, it is sometimes helpful to hold plants back; lack of greenhouse space or frosty weather outdoors may necessitate this. Withholding fertiliser for a week or two will slow growth without long-term harm to the plants, when you resume feeding.
Guy Barter