When to plant bedding plants in the North West

Part 1

When to plant bedding plants

When to plant bedding plants depends on the kind. Summer bedding is mostly tender, so it goes out once the risk of frost has passed. The RHS gives the end of May as the usual first safe time in north-west England, which takes in Warrington and the villages round Rixton. Winter and spring bedding is hardier and goes in during September and October, to flower through mild spells and into spring.

That gives two planting seasons a year. The RHS notes that most bedding displays are changed twice: in late spring for summer, and in early autumn for winter and spring.

The end of May describes an average year. A cold spring, an exposed garden or a frost pocket can push it later, and a sheltered yard in a town can bring it a little earlier. Parts 3 and 4 explain how to judge it for your own garden.

Searches for when to plant bedding climb in March and April, often weeks before it is safe. Buying early is fine, as long as the plants have somewhere light and frost free to wait (part 6).

Part 2

Hardy, half-hardy or tender?

Bedding is a mixed group, and the RHS sorts it into several kinds:

• Half-hardy annuals, such as cosmos, nemesia and tobacco plants, grow, flower and die in one season and are tender to frost.
• Hardy annuals, such as cornflowers, calendula and candytuft, stand frost without protection and can be sown straight into the ground in spring.
• Hardy biennials, such as wallflowers, forget-me-nots and sweet William, take two seasons and flower in spring.
• Half-hardy perennials, such as begonias, pelargoniums (bedding geraniums), salvias and lobelia, would live for years if kept from frost, but are usually treated as one season's display.

The RHS hardiness ratings put numbers to it. A tender plant (H2) copes with temperatures down to about 1 to 5°C but will not survive freezing. A half-hardy plant (H3) copes down to about minus 5 to 1°C, and may come through in a mild or sheltered spot, but is at risk from frost.

In practice the RHS treats both the same way. Tender and half-hardy plants go out after the frost risk has passed, and once nights are reliably mild.

Most of what people think of as summer bedding, the trays of begonias, pelargoniums, lobelia and the like, falls in these two groups. Winter bedding, such as pansies, primulas and bellis, is planted in autumn to flower through mild spells.

Part 3

The North West and your own garden

The RHS gives a rule of thumb for the first safe planting time for tender plants in an average year, moving from the south west to the north:

• the south west tip and the south coast of England: first to second half of May;
• southern England and south Wales: second half of May;
• the Midlands, north-west England, Yorkshire and north Wales: end of May;
• north-east England, southern Scotland and Northern Ireland: early June;
• the Highlands and northern Scotland: mid June.

North-west England is on the third line, so the end of May is the timing to start from round Warrington.

The RHS then adjusts it for the garden itself:

• a garden sheltered by walls, fences or hedges, or in a town or by the coast, may be one to two weeks earlier;
• an exposed, rural or high garden may need to wait one to three weeks longer, and the RHS names the Pennines as an example;
• in a frost pocket or a valley bottom, plant later, or avoid the lowest, coldest corner of the garden.

Cold air sinks and gathers in hollows, so the lowest part of a garden can see frost when the rest does not.

Finally, check the forecast. The RHS advises confirming the timing with the five to ten day forecast before you plant, and part 4 explains what to look for in it.

Part 4

Air frost, ground frost and the 7°C rule

Frost forecasts talk about two kinds of frost, and the difference matters for bedding.

The Met Office defines an air frost as the air temperature falling below freezing, measured at least 1m above the ground. A ground frost is when the ground or a surface falls below freezing. The ground cools faster than the air on a clear night, so a ground frost can form when the air stays above 0°C. The Met Office notes that ground frosts are common even when the air does not freeze.

The RHS explains how this fits planting out. The last air frost of spring usually comes two to four weeks before it is safe to plant out. Ground frosts carry on after it, on nights of about 1 to 4°C, and they can kill tender bedding or check it for weeks.

So the RHS advises planting out once nights are consistently forecast to be above at least 7°C.

The Met Office's own rule of thumb helps with the forecast. If the air temperature is forecast to fall between 0°C and 4°C on a night with little or no cloud and light winds, expect a frost by morning. Cloudy, windy nights are usually milder.

Late frosts still happen. The RHS notes that frost days have fallen in recent decades, but that April 2021 had the most April air frosts in at least 60 years, and the safe planting dates have not moved earlier.

Part 5

Hardening off

Bedding raised under cover needs to get used to cooler air, lower humidity and more wind before it goes out. This is called hardening off. The RHS says it takes two to three weeks, and longer if the plants were raised somewhere warm.

Hardening off toughens the leaves and slows the growth so new shoots are sturdy. The RHS is clear that it does not make a frost tender plant hardy.

With a cold frame, the RHS method is to open the lid a little by day and close it at night for the first week, then raise it gradually over the next two weeks until it comes off before planting.

Without a cold frame, it suggests:

1. Stand the plants in a sheltered spot, in front of a south-facing wall or hedge, under two layers of garden fleece.
2. For the first week, leave them out by day and bring them in at night.
3. In the second week, take off one layer of fleece.
4. Towards the end of the second week, take the fleece off during the day. If the weather is right, leave them out at night, covered.
5. Towards the end of the third week, leave them uncovered, then plant out.

The RHS suggests making each move to cooler conditions on a dull, overcast day, which helps reduce wilting. If a sharp frost is forecast at any point, cover the plants with extra fleece or an old curtain, or bring them back in. If frost does catch them, cut out the damaged growth and carry on hardening them off.

Part 6

Buying bedding before it can go out

Bedding is on sale in garden centres from early spring, long before it is safe to plant out. The RHS warns against being swayed by those displays: follow the timings in part 3, harden the plants off, and keep fleece to hand.

If you buy early, keep the plants somewhere light and frost free until they can be hardened off. The RHS describes a frost free place as one where the temperature stays above about 4°C at night, such as a cool greenhouse or a conservatory.

Small plug plants need potting on as soon as they arrive. The RHS suggests cell trays or 9cm pots, grown on somewhere well lit until the roots fill them.

All of our bedding is grown here from the start, under cover on the yard. Plants raised under cover need a week or two to get used to the cooler air outside, so ask us whether a tray has been hardened off and is ready to go straight out. What is on the tables changes through the season, so ask what is ready this week.

Part 7

Planting out, step by step

When the nights are mild and the plants are hardened off, the RHS sets out how to plant:

1. Make sure the soil or compost is moist at planting time.
2. Rake the soil level and take out large clods and stones.
3. Ease the plants out of their trays by pushing them up from the base, and handle them by the leaves or the root ball, never the stem.
4. Lay the plants out on the soil first, spaced evenly, using the length of a trowel as a guide, and only start planting when you are happy with the layout.
5. Work off a board so you do not tread the soil down.
6. Plant so the top of the root ball sits just below the soil surface.
7. Water in with a watering can without a rose.

For a formal design, the RHS suggests marking the pattern on the soil with fine sand before you lay the plants out. For a bed made of squares or blocks, the spread on the label gives the spacing; multiply the plants along the length by the plants across the width for the number you need.

Bedding has shallow roots and dries out quickly, so water regularly while it grows. Deadhead the flowers as they fade to keep more coming.

Part 8

Winter and spring bedding, and keeping it going

Winter and spring bedding goes in during September and October. The RHS suggests pansies, primulas and bellis for flowers in mild spells through the winter, and wallflowers and forget-me-nots for spring. Bulbs such as tulips, crocus and daffodils can go in among them for colour in early spring.

Through the summer, the RHS lists slugs and snails as the main threat to young bedding, and aphids now and then.

If you would rather plant less often, the RHS suggests drought-resistant bedding, such as osteospermum, gazania, verbena and lambs' ears, which needs much less water, and perennials that give a display for several years.

For baskets, read How long do hanging baskets last? Keeping one going all summer. See what we have in perennials and bedding, or read more in the Gardener's Secrets guides.