How to plant plug plants

From Thompson & Morgan. This guide draws on the popular video "How to plant and grow plug plants" from Thompson & Morgan, the long-established UK seed and plant company. They supply plug plants by post to UK gardeners every year and know well what goes wrong when they arrive, so the advice is grounded in practical experience rather than theory.

1. Open the parcel as soon as it arrives

Plug plants travel in the dark and arrive slightly stressed. Get them out of the packaging as soon as possible. Inspect each one: healthy plugs have firm green growth and a good root system visible at the base of the module. Any that are yellowed or limp may still recover if you treat them carefully.

If any plugs have dried out, stand the tray in shallow water for twenty minutes to rehydrate the compost before doing anything else. Do not water from above at this stage, as soaking cold compost can cause damping off.

2. Pot on into individual small pots

Most mail-order plug plants are too small to go straight outside. Pot each one into a 7 to 9 centimetre pot using a good peat-free multipurpose compost. Push the plug out of the module from below rather than pulling it from above, which damages the roots.

Plant at the same depth the plug was growing at. Firm gently around the roots but do not compact the compost. Water lightly from the base, or use a fine watering can rose to settle the compost in without disturbing the plant.

3. Keep them in a frost-free place while they establish

Early-season plug plants, particularly bedding such as pelargoniums and petunias, arrive before the last frost date. They need to stay somewhere frost-free: a greenhouse, a coldframe, or a bright windowsill indoors. An unheated shed is not warm enough for tender plants in March or April.

Keep the compost just moist, not soggy. A cold and wet combination is the main reason plug plants fail at this stage. Water in the morning if you can, so any surface moisture dries off through the day.

4. Feed once they start growing away

After a week or two you should see new leaf growth. At this point start feeding weekly with a balanced liquid fertiliser diluted to the recommended rate. Young plants in small pots run out of nutrients quickly, and a light feed keeps them growing steadily without the soft, leggy growth that comes from overfeeding.

If the roots start to fill the pot and circle at the bottom, pot on again to the next size up rather than leaving them pot-bound. A plant left too long in a small pot under stress will sulk when it eventually goes out.

5. Begin hardening off two to three weeks before planting out

Hardening off means gradually acclimatising the plants to outdoor conditions before they go in the ground. Start by putting them outside in a sheltered spot during the day for a few hours, then bringing them back in at night. Increase the time outside over a fortnight.

The aim is to toughen the leaves and stems so they can cope with wind, cooler temperatures, and direct sun. Plants that skip this step often go into shock when planted out, with scorched or wilted leaves. It is worth the patience, especially for tender plants.

6. Prepare the bed or container before planting out

Dig over the planting area and remove any weeds. Work in a little well-rotted compost or a balanced granular fertiliser if the soil is poor. For containers, use a fresh peat-free compost mixed with some slow-release fertiliser granules at the recommended rate.

Check the final frost date for your area. In East Kent the last frost is typically late April, but a cold May night can still catch tender plants out. If in doubt, wait an extra week.

7. Plant at the right spacing and water in well

Spacing matters more than people think. Crowded plants compete for light and air, and dense planting encourages fungal problems. Follow the spacing on the plant label. For most bedding annuals that is 20 to 30 centimetres, though this varies by variety.

Dig a hole slightly larger than the rootball, set the plant in so the compost surface is level with the surrounding soil, firm in gently, and water thoroughly. A good soaking at planting time helps settle the roots and reduces the risk of wilting in the first few days.

8. Pinch out the growing tips on bedding plants

This sounds drastic but it works. Removing the top growing tip on a young bedding plant forces it to branch from lower down, which produces a bushier plant with far more flowers than one left to grow straight up. Do it once the plant is settled after planting, removing just the topmost pair of leaves.

Not every plant needs this. Tomatoes, for instance, have a specific side-shoot management routine of their own. But for petunias, fuchsias, pelargoniums, and most summer bedding, pinching out early pays off by mid-summer.

When to call a handyman

Planting a batch of plug plants is pleasant work on a warm day. If you are filling a large border or setting up multiple containers, though, it can take longer than expected. Richard can help with planting up, border preparation, and general garden work across Sandwich and nearby East Kent villages.