Cool Flowers UK - The Complete Guide to Hardy Annuals for Autumn & Early Spring Sowing

Jane Westoby
Sep 7, 2024
7 min read

Updated: Dec 7, 2025

Hardy annuals — often called Cool Flowers — are the secret to getting earlier, stronger, florist-grade blooms in your garden or cutting patch. If you’ve ever wondered how some growers manage larkspur, orlaya, ammi, or stocks weeks before everyone else… this is how.

And the best part? These plants love cold weather. In fact, sowing them into cool conditions is exactly what makes them thrive.

Hardy annuals thrive in cooler conditions and can withstand cold temperatures and light frosts. Unlike annuals & tender annuals, which are planted in spring and complete their life cycle within a single growing season, hardy annuals are typically sown in autumn.

They establish themselves before winter growing strong roots and then bloom the following year, making them an excellent choice for extending the gardening season. These plants are often the first to bloom in spring or early summer, providing a vibrant display of colour when many other plants are still emerging from dormancy.

Why Cool Flowers Work So Well in the UK

The UK climate is practically made for Cool Flowers. While tender annuals (like zinnias or cosmos) struggle outside before summer arrives, hardy annuals prefer cool soil and cold nights. They germinate reliably, grow slowly but steadily over winter, and explode with growth the moment spring light returns.

Growing Cool Flowers gives you:

Earlier flowers (often 4–6 weeks earlier than spring-only sowings)
Longer, stronger stems — perfect for cutting
Heartier plants with better cold tolerance
A longer cutting season
A head start on your flower farming harvests
A Gap filler for the Hungry Gap - that awkward gap between spring bulbs and summer blooms.

If you’re a grower who wants armfuls of blooms by late spring, this is the method.

Hardy annuals can also be grown as annuals and can be started in springtime. This is particularly useful as staggered sowings can also help to extend the season and avoid a glut.

By understanding growing cycles, you can use hardy annuals to create a garden that bursts to life earlier in the season with minimal effort.

When to Sow Cool Flowers in the UK

The key to successfully growing hardy annuals lies in understanding the optimal timing for sowing and their blooms times.

Hardy annuals have two ideal sowing windows: autumn and early spring. Both work — they simply give different results.

1. Autumn Sowing (Aug – Nov)

The best option for mild and moderate parts of the UK.

Autumn sowing delivers:

the earliest flowers
the longest stems
the strongest overwintered plants
the most productive early-season harvests

Keep autumn seedlings in a cold frame, unheated greenhouse, polytunnel, or sheltered outdoor spot, depending on the variety.

2. Early Spring Sowing (Feb–Apr)

Perfect if:

you missed the autumn window
your garden is in a frost pocket
you prefer easier seedling care
your soil is wet or heavy in winter

Spring sowings catch up fast and still give excellent stems.

Each hardy annual has its own ideal sow time depending on a number of factors and also your individual conditions. Some need a period of cold stratification and others need a longer growing season. Here are a few of my top tips and timings based on my own observations and climate here in the south of England and this is also the order in which I sow some of my Hardy Annuals.

Centaurea (Cornflower)

End Aug/ Beg Sep - I sow my cornflower very early, It's doesn't need any cold stratification and it's very hardy so it doesn't need much frost protection. I do cover mine with a lower level hoop if I have one spare if there is a snow forecast but they should survive in most parts of the UK without it. I prick out in October and then plan out into their final flowering positions at the end of Nov.

Antirrhinums (Snapdragons)

I like to sow these early to - Beg Sep. The seeds and seedlings are tiny and are VERY slow growing so they seem to benefit from some extra heat at the beginning to be able to prick them out. I have a real problem with damping off as my greenhouse does not receive much sunlight in the winter months and the garden is so sheltered that there's not much wind either so by sowing then early I can avoid the worst damping off period and get them outside. Ideally they too can be planted into final flowering positions in November.

Snapdragon Grow Along

Orlaya Grandiflora

I pop the seed packet in the fridge at the beginning of September to give them a head start. They may need several periods of cold stratification to germinate so giving them a helping hand is useful. They will germinate one by one usually with a few every few days. Gradually, until they all germinate. Leaving them outside over winter will allow this process to happen naturally but I'll be honest I'm impatient! I leave these in their cell trays, bumping them up to pots when necessary and plating out in springtime.

Orlaya Glowalong

Larkspur

Beg Sep - These seeds go in the fridge along with the Orlaya until beg Oct. They also need several periods of cold stratification and will sometime take a couple of months to show any progress at all. Then a few seeds will appear. Which is honestly a relief, and then all of a sudden they will all germinate within a week - when the conditions are just right.

Larkspur Growalong

Daucus

Another seed packet for the fridge at the beginning of September . They also may need a couple of periods of cold stratification but aften just the one is enough. These also stay in cell trays until the new year if possible and then get bumped up to pots.

Sweetpeas

These are usually my last seeds to sow in mid to late October. They don't need any cold stratification but they are so quick to germinate and put on growth that by the end of January they are too tall for my greenhouse bench and are hitting the sloped roof. This is usually my sign to give them a trim and get some canes in the pots to support them. The trimmings can also be used as cuttings - learn all about taking sweet pea cuttings in our blog. I will usually plant them out in March as they are screaming to go out and have grown so big.

Sweet Pea Growalong

Autumn vs Spring Sowing: What Should You Do?

🌿 Autumn sowing is best if you live in:

The South, South West, London, Kent, or coastal regions
Sheltered gardens with average UK winters
Lighter, free-draining soil

🌿 Spring sowing is best if you live in:

Scotland, Northern regions, or areas with very hard frosts
Heavy clay gardens
Exposed or windy sites
High rainfall zones

My top 10 Best Cool Flowers to Grow in the UK

Orlaya Grandiflora
Bupleurum
Ammi
Larkspur
Nigella
Cornflower
Scabious
Cerinthe Major
Stocks (Matthiola Incana)
Sweet Peas

Biennials that Also Fit the Cool Flower Pattern:

Foxgloves
Hesperis
Sweet William

Perennials Often Autumn-Sown Like Cool Flowers:

Astrantia
Verbascum

What Not to Autumn Sow

With all the excitement around Cool Flowers, it’s just as important to know which plants do not tolerate cold. These are the crops that look tempting in late summer but will collapse the moment real winter arrives.

The following are not Cool Flowers and should never be autumn sown outdoors in the UK:

Zinnias — strictly warm-season; frost kills them instantly
Cosmos — hates cold soil and low light
Sunflowers — need warmth to germinate and grow
Marigolds — tender annuals with zero frost tolerance
Dahlias (from seed or tuber) — will not overwinter in cold, wet soil
Nicotiana — too tender to survive UK winters
Half-hardy bedding plants — all require warmth to establish
Tender vegetables — courgette, cucumber, pumpkin, beans, etc.

These plants are easily damaged by temperatures below 5–7°C, and true frosts will finish them completely. Attempts to overwinter them outdoors almost always end in failure — not because you’ve done anything wrong, but simply because they aren’t built for UK winter conditions.

Stick to genuine Cool Flowers for autumn sowing, and save these heat-lovers for late spring when the soil has warmed.

Do Cool Flowers Need Stratification?

Some Cool Flowers benefit from a natural cold period — a cue that tells the seed it’s safe to germinate. This process is called stratification, and in the UK it can happen either outdoors over winter or artificially in the fridge.

You’ll see the biggest improvement with:

Larkspur — notoriously tricky without a chill
Orlaya — much stronger germination after cold treatment
Icelandic poppies — prefer a cool, steady start

If you’re new to cold stratification (or want a quick refresher), I’ve written a full guide explaining the simple methods you can use at home. Unlocking the Secrets of Seed Stratification

Adding a cold period isn’t always essential, but it can dramatically improve germination and give you stronger seedlings — especially when autumn weather is mild.

Direct Sowing vs Transplanting

Direct Sow If:

your winters are mild
you have free-draining soil
slugs aren’t a major issue
the variety prefers cold soil (larkspur, nigella)

Transplant If:

your soil is heavy clay
your garden is exposed
slug/snail pressure is high
you prefer more control over spacing
you’re overwintering autumn seedlings

Why Cool Flowers Are Essential for Flower Farmers

If you sell bouquets or supply florists, Cool Flowers are your first profitable stems of the year. They fill the early-season gap before dahlias and zinnias start flowering.

They offer: