Sweet peas (Lathyrus odoratus) are the undisputed royalty of the cottage garden. With their ruffled petals, honey-and-orange-blossom scent, and an almost endless palette of colors ranging from soft pastels to deep, moody maroons, they are a must-have for any gardener who loves fresh-cut flowers. While they might look delicate, sweet peas are actually quite hardy, provided you understand their specific needs for cool weather and deep root runs.
If you have ever been intimidated by the process of growing these climbing beauties, fear not. Mastering how to sow sweet pea seeds is a straightforward process that rewards patience and a bit of “tough love” early in the season. Whether you are starting them in the autumn for early blooms or hitting the ground running in the spring, this guide will walk you through every step of the journey from seed to scent.
Understanding the Life Cycle of Sweet Peas
Sweet peas are cool-season annuals. Unlike many of your summer favorites like zinnias or sunflowers, sweet peas thrive when the air is crisp and the soil is cool. In fact, if you wait until the heat of mid-summer to get started, you will likely find your plants struggling or “fizzling out” before they ever produce a significant flush of flowers.
The secret to a long-lasting sweet pea season is a strong root system. These plants spend the first few months of their lives focusing on underground growth. By the time the spring sun begins to warm the earth, a well-established sweet pea plant is ready to explode with vertical growth and abundant blooms.
When to Sow Your Seeds
Timing is everything when it comes to sweet peas. Depending on your local climate, you generally have two windows for sowing.
Autumn Sowing
For gardeners in milder climates or those with a greenhouse or cold frame, autumn sowing (typically October or November) is often considered the gold standard. Sowing in late autumn allows the seeds to germinate and develop a robust root system over the winter months without putting on too much top growth. These plants are often sturdier, bloom earlier, and tolerate the eventual summer heat better than spring-sown counterparts.
Late Winter and Early Spring Sowing
If you missed the autumn window, don’t worry. You can sow sweet peas indoors from January through March. Sowing in late winter gives the plants enough of a head start to establish themselves before the final frosts pass. You can also sow directly into the ground in early spring as soon as the soil is workable and no longer a muddy mess.
Preparing Your Seeds for Success
Sweet pea seeds have a notoriously tough, water-resistant outer coating. While many gardeners simply poke them into the dirt and hope for the best, a little preparation can significantly increase your germination rates.
The Nicking Technique
One of the most effective ways to help a sweet pea seed absorb moisture is to “nick” it. Using a small pair of nail clippers or a piece of sandpaper, gently scar the seed coat on the side opposite the “eye” (the small spot where the seed was attached to the pod). You only want to break the surface slightly to allow water to penetrate; be careful not to damage the internal embryo.
To Soak or Not to Soak?
Some gardeners swear by soaking seeds in lukewarm water for 24 hours before planting. While this can speed up germination, it can also lead to seed rot if the seeds are left in water too long or if the soil is particularly cold and wet. If you choose to soak, ensure you plant them immediately after they have swelled.
The Best Way to Sow Indoors
Because sweet peas have long, deep “taproots,” they do not like having their roots disturbed during transplanting. Traditional shallow seed trays are generally a poor choice.
Choosing the Right Containers
Deep pots are essential. Many professionals use “root trainers”—specialized deep plastic cells that open up like a book to release the plant without pulling on the roots. Alternatively, you can use recycled cardboard toilet paper rolls or “deep” 9cm pots. Cardboard rolls are particularly popular because they can be planted directly into the ground, where they will eventually decompose, leaving the roots untouched.
Soil and Sowing Depth
Use a high-quality, peat-free seed compost. Fill your containers, leaving about an inch of space at the top. Use your finger or a pencil to make a hole about 1 inch (2.5cm) deep. Drop one or two seeds into each hole and cover with soil. Firm the surface gently and water them in well.
Germination Temperatures
Sweet peas do not need a lot of heat to germinate. A bright windowsill or a cool greenhouse is perfect. Ideally, keep the temperature around 54 degrees Fahrenheit to 64 degrees Fahrenheit. If the environment is too warm, the plants will grow “leggy” and weak. Once the green shoots appear, they can handle much cooler temperatures, even dipping down toward freezing, as long as they are protected from harsh, drying winds.
Direct Sowing in the Garden
If you prefer to skip the pots, direct sowing is an option, especially in the spring.
Soil Preparation
Sweet peas are “heavy feeders,” meaning they love a nutrient-rich environment. Before sowing, dig in plenty of well-rotted manure or garden compost into the area where they will grow. They prefer a sunny spot with well-draining soil.
Sowing the Seeds
Sow seeds 1 inch deep and about 2 to 3 inches apart. Once they have reached a few inches in height, you can thin them out to a final spacing of about 6 inches apart. Be sure to protect your young seedlings from slugs and snails, which find sweet pea shoots absolutely delicious.
Essential Early Care: Pinching Out
Once your sweet peas (whether indoor or outdoor sown) have reached about 4 to 6 inches in height and have two or three pairs of true leaves, it is time for the most important step in sweet pea care: pinching out.
It feels counterintuitive to cut off the top of a healthy seedling, but using your thumb and forefinger to snip off the growing tip just above a leaf joint will encourage the plant to send out side shoots. Instead of one single, spindly stem, you will end up with a bushy, vigorous plant with multiple flowering stems. This simple act can double your flower production later in the season.
Planting Out and Support
If you started your seeds indoors, you must “harden them off” before they go into their permanent home. This involves placing the pots outside during the day and bringing them back in at night for about a week to get them used to the outdoor air.
Providing a Structure
Sweet peas are climbers that use delicate tendrils to pull themselves upward. They need something to grab onto. Popular choices include:
- Trellises and Obelisks: Perfect for a formal garden look.
- Wigwams: Built using bamboo canes or hazel sticks tied at the top.
- Netting: Plastic or jute netting attached to a fence or wall.
When planting out, space your plants about 6 to 8 inches apart at the base of your support structure. Use a soft garden twine to loosely tie the young stems to the support until their tendrils take over the job.
Maintenance for Maximum Blooms
To keep your sweet peas blooming for as long as possible, you must be diligent about two things: watering and deadheading.
Watering and Feeding
Sweet peas do not like to dry out. In the height of summer, they may need watering every day. A high-potash fertilizer (like tomato feed) applied every two weeks once the first buds appear will help sustain the flowering period.
The Power of Deadheading
The goal of a sweet pea plant is to produce seeds. Once a flower fades and a seed pod begins to form, the plant will stop producing new flowers. To prevent this, you must pick the flowers regularly. The more you cut sweet peas, the more they bloom. If you see any green pods that look like tiny pea pods, snip them off immediately to keep the energy focused on flower production.
Common Problems to Watch For
While generally hardy, sweet peas can face a few issues:
- Powdery Mildew: This looks like a white dusting on the leaves and usually happens in late summer when the air is stagnant or the plants are stressed by drought. Ensure good airflow and keep the roots moist.
- Bud Drop: If the weather suddenly shifts from cool to very hot, or if there is a dramatic change in watering, the plant may drop its unopened buds. This is usually temporary and will resolve once the plant stabilizes.
- Aphids: These small insects love the tender growing tips. You can usually blast them off with a sharp stream of water from a hose or use an organic insecticidal soap.
Frequently Asked Questions
Can I eat the peas from sweet pea plants?
No. Unlike garden peas, sweet peas (Lathyrus odoratus) are toxic if ingested by humans or pets. They are grown strictly for their beauty and fragrance, not for the dinner table.
How long does it take for sweet pea seeds to germinate?
Under ideal conditions, sweet pea seeds usually germinate within 10 to 21 days. If your soil is very cold, it may take a bit longer.
Why are my sweet pea seedlings turning yellow?
Yellowing leaves are often a sign of overwatering or poor drainage, which leads to root rot. Alternatively, it could be a lack of nitrogen. Ensure your pots have drainage holes and consider a light liquid feed if the plants look stunted.
Do I need to soak the seeds if I am planting them directly in the ground?
Soaking is not strictly necessary for direct sowing, especially in early spring when the ground is naturally moist. The consistent moisture of the earth will eventually soften the seed coat on its own.
How tall do sweet peas grow?
Most climbing varieties of sweet peas will reach heights of 6 to 8 feet. There are also “dwarf” or “patio” varieties available that only grow to about 12 inches tall, which are perfect for hanging baskets or window boxes.