Organic Sweet Peas: Seed to Bloom Growing Guide
Sweet peas are one of the most enchanting flowers of the cool season, loved for their delicate ruffled blooms, unforgettable fragrance and generous flowering habit. Whether grown in an organic home garden or for cut-flower production, sweet peas are surprisingly easy to cultivate with the right timing, deep pots, good soil preparation and strong support. From sowing in autumn or winter through to harvesting long stems for the vase, this guide walks you through everything needed to grow beautiful, healthy sweet peas in Australian conditions.
Sowing Sweet Peas: Timing, Pots & Propagation Mix
When to sow sweet peas
In Australia, sowing time isn’t just about frost dates. It’s shaped by two key factors:
- The high heat many regions experience in late spring and summer
- The number of daylight hours required to trigger flowering in each variety
Understanding both will dramatically improve your harvest window.
Why Timing Matters
Sweet peas (Lathyrus odoratus) prefer to germinate in cool conditions. Warm soil can cause:
- Poor germination
- Long, lanky seedlings
- Weak early growth
If your climate is still warm when you want to sow, you can vernalise your seed — storing it at 1–4 °C for 30-60 days before sowing.
It is good to note that once vernalised, sweet pea seed optimal germination temperature is 13-21C.
But germination temperature is only part of the story.
Day Length + Temperature = Your Harvest Window
Each sweet pea variety requires a certain number of daylight hours to trigger flowering. Some begin blooming at around 10–11 hours of daylight, while others need 12 hours or more.
This matters enormously in Australia.
If your region reaches 12 hours of daylight when average temperatures are already above 27°C, your flowering season may be short and intense. Heat shuts sweet peas down quickly.
The key question is:
What will your temperatures be when your chosen variety reaches its required daylight hours?
Regional Sowing Guide (Australia)
These are generalised guidelines — always observe successful local gardens and ask questions when you see a beautiful flush of sweet peas thriving.
Subtropical/ Tropical Climates
- Sow in autumn once temperatures begin to cool
- Keep seeds cool during germination
- Vernalise seed for at least 30 days below 1°C if soil temperature is still above 21°C
- If high heat arrives early in spring, choose varieties that flower at shorter daylight hours (10–11 hours) for a longer harvest window
In subtropical/ tropical regions, heat in autumn and early heat in spring is the limiting factor — not frost.
Temperate Climates
- Autumn to mid-winter sowing works well / March–July
- Be cautious with varieties requiring 12+ hours of daylight — if sown too early, plants may grow very tall before flowering
- Good airflow and staking are essential
Cold Temperate Climates
- Sow in autumn or late winter–early spring
- Protect seedlings during severe cold spells
- If autumn sowing, allow seedlings time to establish before the coldest weather
- Plants will sit semi-dormant through winter and resume growth as daylight and temperatures increase
- If late winter sowing, start in a protected cool area and transplant once the worst frosts have passed
Sweet peas are cold hardy, but transplant once nighttime temperatures remain above –5°C for best results.
Before You Sow: Check Your Conditions
Timing also depends on your soil and weather patterns:
- Sweet peas hate wet feet — heavy winter soil can cause losses
- Emerging buds and flowers may need protection from late winter storms, wind, and frost
- If heat arrives early in late spring, earlier sowing gives a longer bloom season
How to Work Out Your Ideal Sowing Date
A reliable method we’ve found after several seasons of trial:
- Determine when your chosen variety will reach its required day length to trigger flowering
- Look at your average temperatures at that time
- Count 10–12 weeks backward to determine sowing time
This aligns flowering with your most favourable conditions.
A Simple Frost-Based Guide
If you prefer a traditional approach:
- Start seeds indoors 4–5 weeks before transplanting
- Transplant 4–5 weeks before your last frost
- Or plant them out when you plant garden peas
INTERESTING NOTE - A Japanese study found that Sweet pea seeds vernalised at 1 °C for 30–60 days produced flowers sooner and at lower nodes compared to non-vernalised seed.* This simple trick of refrigerating your sweet pea seed ahead of sowing could help many gardeners to have a longer harvest window!!
Where to Sow?
This depends on:
- Winter rainfall
- Soil drainage
- Frost risk
- Whether emerging buds may be damaged by wind or storms
In wet gardens, raised beds or starting in trays may be safer.
In drier, well-drained soils, direct sowing in autumn can produce deep-rooted, resilient plants.
The Takeaway
Sweet peas are cold hardy — but they are not heat tolerant.
If you plan for:
- Cool germination
- Correct daylight triggers
- Moderate temperatures at flowering (Ideal growth and abundant blooms occur at temperatures from 15–24 °C
you will be rewarded with weeks of beautifully scented blooms instead of a brief flush before summer shuts them down.
Which Pots Are Best?
We prefer transplanting sweet peas, so we always sow into pots.
Because sweet peas are deep-rooted, choose deep pots such as:
- Forestry tubes (50mm × 50mm × 120mm)
- 5" root trainers
If sowing in late winter, these are usually deep enough.
If sowing in late autumn, keep an eye on the base of the pots— If roots start poking out and the garden still isn’t ready, pot up to a 1L pot.
Once sweet peas become root bound, they never fully recover.
Propagation Mix
We use our seed raising mix (recipe found here) which includes coco coir, perlite, our own biodynamic compost and a base fertiliser with great results.
Essentially you want a mix that is not to rich that it stays overly wet while still providing enough nutrients to produce a strong seedling. Many seed raising mixes allow for good drainage but they lack the nutrient rich conditions sweet peas adore.
Sowing Sweet Peas
Depending on the germination rate listed on the packet, sow 1–2 seeds per pot, 5mm deep, and lightly cover with propagation mix.
Optimal germination temperature: 13-21°C
Avoid using bottom heat, as sweet pea seeds can easily overheat, reducing germination success.
Our Germination Method
We often germinate seeds using the paper towel method in a cool room. As soon as the rootlet emerges, seeds are transferred carefully into prepared pots.
This approach:
- Ensures no potting space or soil is wasted
- Allows us to monitor germination closely
- Produces germination within 4–7 days for most seeds
If a seed has not germinated, we gently nick or file the seed coat and leave it a few extra days. This improves overall germination, especially for seeds with tougher outer coatings.
We DO NOT recommend soaking sweet pea seeds. Our trials have shown better results without soaking.
Growing Out a Seedling
Once sweet pea seeds have germinated, provide plenty of bright light to prevent seedlings from becoming leggy, weak, or stretched.
Sweet peas are naturally cold tolerant and benefit from being grown hard. Grow seedlings outside to strengthen them, reduce legginess, and acclimate them for transplanting.
Just be sure to protect from:
- Heavy frost (below –5°C)
- Strong winds
- Extended periods of cold, wet soil
Pinching & Side Shoots
Pinching depends on when the seeds were sown:
- Do NOT pinch autumn-sown sweet peas - they will naturally branch themselves.
- For later sowings, pinch the growing tip when plants reach around 10cm tall to encourage bushier growth and more flowering stems.
- Dr Keith Hammett makes the distinction that Winter Flowering types tend to produce a single main stem that rushes up and produces flowers. This is accentuated when these varieties are grown in warm conditions (including a glasshouse or polytunnel).
Roger Parsons, leading UK sweet pea specialist and author of Sweet Peas – The Essential Guide, explains:
“Autumn-sown plants grown cold will have reached 75mm high by winter and side shoots will form naturally at this time… Left unstopped, those side shoots that are starting to form in winter will be just the right length for planting out in early spring. It is the cold weather that encourages natural side shoot development.”
If a plant has not begun forming side shoots by winter, this is the stage when pinching becomes beneficial.
Support & Trellising
Unless you are growing a dwarf or knee-high variety, most sweet peas reach 180–200cm and need support. Home garden options include:
- Teepee structures
- Arches or tunnels
- Netting on stakes
- Metal fencing or mesh
For later sowings, tall 2m+ stakes spaced every 3m can support two layers of Hortonova netting placed one over the other, creating around 180cm of vertical growing space for tendrils to climb.
A windbreak is extremely helpful—strong winds can damage stems and reduce flower quality.
Plant Spacing
For production, space seedlings 5-10cm apart, depending on your support. This is very close together and will create a green wall.
For seed production, we plant two rows, one on either side of the trellis, at 15cm spacing. This spacing ensures our seeds have plenty of airflow and dry naturally on the plant.
Continue to tie or clip plants to the support as they grow. Securing stems encourages:
- Upward growth
- Straighter stems
- Longer cut-flower length
Protection
After transplanting, protect young plants from:
- Hard frost
- Slugs & snails
- Mice
- Magpies
Sweet peas are cold tolerant, but a severe frost can damage or kill young seedlings and ruin forming flower stems.
Harvesting & Vase Life
Sweet peas have a naturally short vase life—around 4–7 days. To make the most of your blooms:
- Harvest during the coolest part of the day
- Pick when 2-3 flowers on the stem are open
- HYDRATE IMMEDIATELY and allow your flowers to rest in a shady, cool spot ahead of arranging.
- Use a commercial floral preservative, or make a simple solution of 1–2% sugar plus a tiny amount of bleach to feed the flowers and reduce bacterial growth.
- Change vase water every 1-2 days and recut stems.
- Coolness can extend vase life. You can refrigerate your vase with blooms overnight.
The remaining buds will continue to open in the vase, helping extend their display.
Sweet pea plants will bloom longer if continually harvested and temperatures remain below 27C. Continuous high heat induces the flower to set seed and stop blooming.
Important Warning
All parts of the sweet pea plant are poisonous if ingested.
Please take care around children and pets.
Recommended Resources
If you’ve been caught by the sweet pea bug (it happens!), these expert growers and authors are well worth reading and learning from:
- Dr Keith Hammett — world-renowned New Zealand breeder and sweet pea specialist -
- Roger Parsons — Sweet Peas: An Essential Guide
- Erin Benzakein of Floret Flower Farm
- Marryn Mathis — Sweet Pea School: Growing & Arranging the Garden’s Most Romantic Blooms
* Effects of Seed Vernalization and Photoperiod on Flower Bud Initiation of Summer, Spring, and Winter Flowering Types of Sweet Pea(Lathyrus odoratus L.)
Tomoaki Inoue