How to Plant, Grow and Care For Sweet Peas
Are you thinking about growing sweet peas this season? These popular flowers can fit just about any garden design. In this article, gardening expert and cut flower farmer Taylor Seivers examines all you need to know about growing sweet peas in your garden, including their maintenance and care needs.
Charming. Delightful. Romantic. The sweet pea has enamored gardeners for centuries with its vibrant colors, unique forms, and sweet fragrance.
To quote Louise Beebe Wilder, famous 20th-century horticulturist and garden writer, in her book The Fragrant Garden, “How delightful it is to walk along the rows of Sweet Peas in the early morning when the dew is still upon them. It is then that they are the sweetest and most refreshing to inhale.”
Sweet peas come in many different shapes, sizes, and colors, like blue, pink, red, white, purple, mauve, peach, and many shades in between (except yellow). These popular flowers are used in the garden to provide vertical interest and an old-fashioned cottage garden feel. And we can’t forget they make wonderfully sweet cut flowers!
Here is everything you need to know about growing this garden classic!
Sweat Pea Overview
Botanical Name Lathyrus odoratus
Plant Type Annual
Flower Colors Red, mauve, blue, peach, lilac, pink, white, purple, bicolor; all but yellow
Light Requirements Full Sun
Water Needs High
Height 3 to 10 feet, average height about 4 to 6 feet
Spacing 4 to 12 inches apart
Hardiness Zones USDA Zones 7 to 10
Soil Friable loam, high in organic matter, well-draining
Pet Toxic Poisonous to humans and pets
History of Sweet Peas
The sweet pea, Lathyrus odoratus, is native to the Mediterranean region, specifically Sicily, where it was first discovered and recorded. It is a member of the Fabaceae (bean) family and is a nitrogen-fixing legume like its relatives. Sweet peas have a luscious, vining habit. They tend to snatch onto nearby plants, fences, and walls with their tendrils.
In 1695, Francisco Cupani, a monk of the order of St. Francis on the island of Sicily, discovered and recorded the first sweet pea. The “original” sweet pea had small red and blue flowers with a strong scent. This is why the sweet pea variety named ‘Cupani’ is still cultivated today.
It is believed that Cupani sent seeds to a man named Dr. Robert Uvedale in England. Uvedale was known for collecting unusual plants. Many visited Uvedale’s famous garden, and he shared seeds freely.
By the mid-1700s, flower varieties were widely shared by nurserymen and in seed catalogs in England.
Around 1860, only 6 colors had been observed. Forced hybridization of the plants led to Henry Eckford’s work of breeding sweet peas. He started around 1870. There were about 15 varieties known at the time, and he eventually created over 100 new varieties.
Silas Cole, a gardener for the Earl of Spencer, also experimented with sweet pea cultivation and breeding. When he discovered a frilly, wavy mutation, the Spencer sweet peas were born and named ‘Countess Spencer.’
Around the same time, William Unwin was another famous sweet pea breeder that bred many varieties with variations in color, like striping and flaking.
As colonists settled in the New World, sweet peas traveled across the Atlantic to America. The flower was not particularly popular in America until the work of the English hybridists took hold in the later part of the 19th century.
Most of the American production today is done in the Western states. Sweet peas are grown commercially as cut flowers, mainly in states with mild summers, like California and Oregon.
Sweet peas are touted as easy to grow. I’m sure they are for some people, but I wouldn’t say that they’re particularly easy to grow. I’d rather say they are easy to germinate.
Sweet peas are propagated mainly by seed, although in rare cases, they may be propagated by cuttings.
Starting From Seed
Because sweet peas originated in a climate with dry summers, they’ve developed a seed coat that is thick and hard. This prevents moisture loss.
Some gardeners will suggest you nick the seed coat to promote water uptake. Others will say you should soak the seeds in water. There is still debate on whether you should soak seeds. Some suggest soaking for 1 to 3 hours in water or up to 24 hours.
Others say you don’t need to do anything. Just plant the seed in very moist soil.
I have had the best success and the fastest germination by soaking my sweet pea seeds for at least 12 hours in water. I did not nick the seeds before soaking (also known as scarification).
If you choose to nick the seed coat, use a sharp knife or rub the seed gently on sandpaper. Make sure you do this before soaking in water. The point of scarification is to help water penetrate the seed coat.
I have tried planting the seeds in a 50-cell tray and soaking the tray amply with water, but my germination was delayed significantly. I did not have less germination, but germination was at least 2 to 3 weeks behind.
When to Start Seeds
Start your sweet pea seeds indoors in very early spring or late winter, depending on your climate. An ideal time to start sweet peas indoors is about 8 to 10 weeks prior to your last expected frost date.
Plant sweet pea seeds directly into the garden about 4 to 6 weeks prior to your last expected frost. Many gardeners will say to plant your Lathyrus around the time you would plant your edible garden peas.
If you live in USDA Zone 7 and above, consider planting in the autumn. Fall-sown plants are more vigorous because they have cool temperatures to establish their roots. In warmer climates, some growers will start their seeds indoors in August and plant them out in October or November.
I use 50-cell or 72-cell propagation trays to start sweet pea seeds indoors. Since Lathyrus dislike root disturbance, consider planting them in small four-inch pots or peat pots. I have had success both ways. To minimize root disturbance, make sure you are transplanting seedlings when the roots are starting to grow out of your cell trays.
As long as the seedlings have been hardened off properly, you can transplant your seedlings into the garden a few weeks prior to your expected last frost. This means you take them outside for one to two weeks and let them acclimate to the temperatures and sunlight. Keep them in a shadier spot so they don’t fry in direct sunlight.
Sweet peas can handle light frosts, but make sure the temperature stays above 25 degrees F. They will need protection in colder weather.
Optimal Temperature for Starting Seeds
Sweet peas can be started indoors at a temperature of 68 to 75 degrees F, but after germination, you should make sure they are kept at cool temperatures. 50 degrees F is the optimal temperature to grow sweet pea seedlings.
Propagating by Cutting
Propagating Lathyrus by cuttings is not common, but it can be done.
If you’d like to try propagating by cuttings, select young shoots at least three inches long. Snip them from the plant, but make sure you leave at least one shoot on the main plant so it can grow and flower.
Place the cutting in a moist propagation mix of 50% peat and 50% perlite. Use a heat mat set at 70 degrees F. The shoots should take root in 2 to 3 weeks.
You cannot root cuttings from older shoots, so it is important to take cuttings from young, fresh growth (The Sweet Pea Book by Graham Rice).
How to Plant
After hardening off for 1 to 2 weeks, it’s time to transplant seedlings. Remember, it is acceptable to plant sweet peas outside before your last expected frost date. They can withstand colder temperatures but should not freeze for prolonged periods.
Plant seeds at a depth of 1 to 2 inches. Because this flower mostly grows vertically, you can grow plants as close as 4 to 6 inches apart (for cut flowers on a trellis) or opt for 12-inch spacing between plants in the landscape.
If you are directly sowing into the garden, plant 2 seeds per hole about 2 to 4 inches apart, and then thin the seedlings to the appropriate spacing.
How to Grow
Sweet peas thrive in climates with cooler summers but don’t like to freeze. Because I live in the Midwest, sweet peas can be challenging because we have such a short spring. For these reasons, I’ve found that providing winter protection has best for me. Once you know the requirements for growth, you can adapt your sweet pea planting to your climate.
Sweet peas require full sun (at least 6 to 8 hours of daylight) to thrive.
They will not do well when planted under a shrub. Some people try to do this with the low-growing varieties and are sorely disappointed. This flower will benefit from some afternoon shade to protect the roots from drying out or exposure to excess heat.
An ample water supply is essential for sweet peas. However, they will not tolerate waterlogged conditions. A generous soaking every 1 to 2 days will help immensely when they’re young and their roots are getting established. After that, you can irrigate once or twice per week if there isn’t any rain.
Sweet peas can grow in many soil conditions, even hard clays. But they will prefer a loose, friable loam soil that is well-draining. They do not tolerate waterlogged conditions (even though they love water), so well-drained soil is a must.
Amend your soil with organic matter like compost or well-rotted manure. This will help improve the drainage of heavier clay soil.
They also like plenty of fertility. If you have sandy soil, adding organic matter will help boost the nutrition of your soil. A pH of 7.0 to 7.5 is preferred.
Climate & Temperature
Optimal temperatures for sweet pea growth are between 32 and 60 degrees F. This is why gardeners in warm climates should consider planting in the fall instead of early spring. This is also why growing sweet peas in the Midwest can be so tricky.
Sweet peas are considered winter hardy at USDA Zones 7 and above. Winter protection, like a cold frame or unheated greenhouse, can help you plant earlier if you live in a growing zone with a cold spring season.
Sweet peas are hungry little plants. But, like many flowers, too much fertilizer can cause them to grow abundant leaves with few flowers.
A basic low-nutrient fertilizer should provide an ample amount of fertility. Mix this into the soil at the beginning of the season.
Many growers prefer organic fertilizer options because they are slow-release. I like to use a mixture of fish and seaweed emulsion applied weekly as foliar feed. I am a cut flower farmer, so it’s important that I feed my plants. However, if I was growing these flowers in the landscape, I would not fertilize them as much if I’d already amended my garden bed before planting.
Other growers like to make compost tea or alfalfa tea and regularly apply it as a foliar feed.
Once the flowers begin to bloom, I do not apply foliar fertilizer (if you’ve ever used stinky fish emulsion, you’ll know why!). You could consider mixing fertilizer in your watering can when the plants begin to bloom to lessen the stench of organic fertilizers.
Sweet peas must be supported in some fashion, whether that’s by installing a trellis, allowing them to clamber up a dead shrub or tree, or training them up a fence.
There are a few varieties that do not grow as tall. They can be planted freely as a ground cover in the landscape. But the vast majority will need support.
People go about this in different ways. The most popular trellis option is using bamboo canes and twine in the shape of a teepee. Personally, I use tall metal t-posts, and 6-foot-wide white hortonova netting hung vertically. The netting is attached to the t-posts using zip ties.
Sweet peas have tendrils that will grab onto a trellis naturally, but with their rampant, wild growth, it is ideal to tie them up as they grow so you can control their growth. Some people use twine to tie them. Others use flexible paper-covered twist ties (like the ones you see at the grocery store in the produce section), and others will use special horticultural trellis clips.
Sweet peas should be pinched when they are about 6 inches tall. This will promote branching and more flowers! Pinch off the top of the plant just above a set of leaves.
If you plant in the fall, cold temperatures usually do the “pinching” for you. It is generally not recommended to pinch fall-planted sweet peas because the on-and-off freezing temperatures will cause shoot-tip dieback. Essentially, the frost creates the “pinch.”
Once sweet peas start to bloom in early summer, they must be deadheaded or harvested regularly! This is important because the plant will otherwise start to set seeds right away, and your bloom time will be short. Some gardeners say that keeping up with deadheading is a job in itself during full bloom.
Once you’ve harvested a bounty of blossoms, you can let the plants set seed and collect the seeds for planting next year! Plants are self-pollinating. This means you can collect seeds from one variety, and it will produce seeds of the same type.
Sweet peas, like any plant, are susceptible to pest infestations. Here are a few of the most common pests to watch out for:
Aphids are small brown to gray to green insects with piercing, sucking mouthparts that feed on plants like sweet peas. Aphids secrete honeydew, a sugary waste that will eventually appear as black spots on the leaves as different fungi feed on the honeydew. These black spots are harmless but can be a sign of an infestation.
Most of the time the feeding alone will not cause the plant major damage, except in cases of severe infestation. However, because of the way aphids feed on the plant, they are known to transmit viruses. These viruses can be detrimental.
If you notice a few aphids, it’s likely there are a hundred more that you haven’t seen. They also reproduce rapidly. A small problem can become a big problem in a short period of time.
How to Remove and Prevent Aphids
The first step to remove aphids is to douse the plant with a strong stream of water periodically. This will knock the aphids off the plant and can be effective in itself.
Next, you can release beneficial insects, especially if you grow in a greenhouse setting. Insects like lacewings and lady beetles are natural predators of aphids and can be released to reduce the likelihood of infestations.
Lastly, there are some sprays out there that are effective against aphids. Some can be purchased at stores, while others can be homemade.
Neem oil can be sprayed on the plant, which will not harm the aphids directly but will discourage feeding on the plant and ultimately cause the pests to die. Insecticidal soap, horticultural oils, and pyrethrins can also be effective against aphids, but they must come into contact with the insects to be effective. Ensure you are thoroughly covering the plant with the spray because aphids hide underneath leaves and along stems.
Simply using a 2% soap solution (2 teaspoons of soap in 1 pint of water) can effectively kill aphids and thrips. This is not a preventative; the soap must come in contact with the insect to kill it. Make sure you coat the plant thoroughly and do not spray in the middle of the day when light and high temperatures coupled with the soap can cause damage to the plant.
Thrips are extremely tiny (about 1/16 inch long) cigar-shaped insects that cause stippling (light dots) and distortion or damage to the plant leaves and petals. The Western flower thrip is the most common type of thrips to infect Lathyrus.
Thrips also have piercing, sucking mouthparts that can transmit viruses.
If you suspect thrips, the best way to scout for them is by holding a bloom over a white piece of paper and shaking it onto the paper. You should be able to see the thrips crawling around on the paper. In controlled environments like greenhouses, yellow sticky traps will help catch flying adults.
Control is difficult for thrips, but using a homemade solution (like the soap solution above) or other commercial contact sprays like insecticidal soaps may be effective.
Two-Spotted Spider Mites
Spider mites are also extremely small and cause stippling on the leaves like thrips. Sometimes the damage is confused as leaf burn because the leaves will have a bronze appearance to them. If you look at the underside of the leaves, you will notice webbing, a characteristic sign of spider mite infestation.
Severe infestations will cause the leaves to turn brown and drop. Mite outbreaks often occur in hot, dry periods.
Spraying the plant with a forceful stream of water will help, but you can also use insecticidal soaps and horticultural oils.
When it comes to disease, there are a few diseases that sweet pea gardeners will need to be on the lookout for. Let’s take a deeper look at some of the most common diseases.
Root, Stem, or Crown Rots
Rots usually occur when various fungal pathogens infect and spread throughout a stressed plant. The foliage will become dull and yellowed. Plants may become stunted and die. If you cut through the stem or roots of the plant, you will often notice brown to black discoloration.
The best course of action is to ensure your soil is adequately drained and prevent plant stress from pests or weather. Purchase seed from a reputable seed source. Amend your soil with compost or manure to enhance drainage in heavy clay soils.
Powdery mildew is likely the most common fungus you will run into when growing sweet peas. Greyish-white spots appear on the upper sides of leaves that eventually spread, creating a “powdery” appearance. In severe infestations, the leaves will eventually turn yellow and drop off.
To reduce powdery mildew infection, promote good air circulation in your garden. This can be done by increasing the spacing between plants or increasing the size of your pathways. Stagnant, humid conditions are a haven for powdery mildew.
Avoid overhead watering to reduce the wetness of the leaves. Instead, water at the base of the plant in the mornings so that any leaves that become wet will dry off quickly.
Some fungicides are available to reduce powdery mildew, but many need to be sprayed often or as a preventative. Instead, promoting good cultural practices rather than relying on sprays is best. An organic option is a copper fungicide, but this is best used as a preventative.
Choosing powdery mildew resistant varieties can also reduce the incidence of this disease.
Viruses are a serious problem with any type of plant because there is no remedy other than removing and destroying the plant. Viruses are transmitted by insects with piercing, sucking mouthparts, like aphids and thrips. That’s why it’s important to first control aphid and thrip infestations and the weeds that may be hosts for plant viruses.
Clippers can also spread many viruses when you harvest sweet pea flowers from one infected plant and then cut into the next plant. Always ensure that any diseased plants are removed from the garden, and do not cut from them without sanitizing your clippers.
Types of Sweet Pea Viruses
Some viruses, like the pea mosaic virus, cause mottling and yellowing of the leaves and are transmitted by aphids. Flower-breaking can also occur. This is when the bloom has patches of discoloration on it.
Thrips will spread tomato spotted wilt virus, which causes reddish-brown streaks on the stems and yellow spots on the leaves. Control garden weeds that can harbor the virus. When thrips feed on the infected weeds, they can then spread it when they feed on your sweet peas, so manage weeds accordingly.
The pea enation mosaic virus causes translucent windows to appear in the leaves and flowers in the warmer part of the growing season. The leaves can also be puckered, and the top of the plant may appear distorted.
Red and blue-flowered varieties are particularly susceptible to streaks of a different color on the petals (known as color breaking). This is transmitted by the pea aphid which brings the infection from clover particularly.
Sweat Pea Uses
Sweet peas are typically used in the landscape to provide vertical interest or fragrance. They are also grown for cut flowers. They come in a range of colors and typically bloom from May to July.
Their vase life is relatively short, but they provide interesting texture, color, and fragrance to a flower arrangement or even a bridal bouquet. Vase life is typically between 4 to 7 days. Sometimes the vines are harvested to use as foliage.
Sweet peas are poisonous and cannot be eaten like garden peas. The genus name, Lathyrus, is indicative of the type of symptoms that occur after ingesting sweet peas. Lathyrism causes convulsions, slow and weak pulse, shallow breathing, and paralysis. Thankfully, high amounts have to be eaten to be deadly. Still, be mindful and plant Lathyrus away from garden peas to avoid confusion.
Sweet peas are classified into groups based on when they flower and the characteristics of their blooms. The following varieties were discussed in The Sweet Pea Book by Graham Rice:
- Original – Any of the type that was first discovered in Sicily and close relatives of it
- ‘Cupani’ – standard, vivid deep maroon and white at the base with purple, strongly scented
- ‘Painted Lady’ – pink with white wings, 1 to 2 flowers per stem, strongly scented
- Grandifloras – Larger flowers with clearer colors, more flowers on the stem than the original sweet peas, well-scented
- ‘America’ – red and white
- ‘Eckford’s Mix’ – strongly scented
- ‘Old Spice’ – a mixture of older varieties with strong scent, good in hot summers
- ‘Annie B. Gilroy’ – carmine pink with pale wings
- Spencers – Significantly larger blooms, ruffled and waved petals, bloom when day length is 12 hours (synonym is “Late Spencers”
- ‘Countess Spencer’ – the original Spencer, bright clear pink on white, waved petals
- ‘Mollie Rilestone’ – pink picotee on cream or light peach, rippled edges, strong scent
- Cuthbertsons – Usually bloom about 2 weeks earlier than Spencers at 11 hours of day length, known to withstand heat, 4 to 6 flowers on each stem
- ‘Danny’ – dark blue
- ‘Evelyn’ – salmon rose
- ‘Frank’ – lavender
- ‘Janet’ – white
- ‘Lois’ – rose-colored
- Multiflora – 5 to 7 flower stems, waved petals
- Dwarf – Typically only about 12 inches tall
- ‘Bijou’ – ruffled red, salmon, blue, rose, lilac
- ‘Cupid’ Series – pink, carmine bicolor, deep lilac, scarlet bicolor, white bicolor, white, rose, crimson, cherry, salmon rose, lavender
- Intermediate – Cross between Dwarf and Spencers, typically reaching around 2 feet tall
- ‘Knee-Hi’ – 6 to 9 flower stems, no to little scent, mixed colors
- ‘Dawn Chorus’ – waved white and purple, slight scent
- ‘Snoopea’ – tendril-free
- ‘Supersnoop’ – an improved version of ‘Snoopea’ with more heat tolerance
- Winter – Flowering – Developed for hot summer climates because they will flower in the winter when the day length is 10 hours long
- ‘Winter Elegance’ Series
- Early Multiflora Gigantea – Earlier than the Spencers and often grown as commercial cut flowers
- ‘Mammoth’ Series
- Royals – Improved Cuthbertson form with longer, stronger stems; naturalized in Southern Europe
- Includes only ‘Royal’ Series and ‘Royal Family’
- Colors of blue, crimson, lavender, maroon, mid-blue, navy blue, rose pink, salmon, scarlet, and white
Frequently Asked Questions
Do sweet peas like sun or shade?
Sweet peas require full sun to flower properly. Slight afternoon shade may benefit the plant if you live in a warmer climate so that the roots do not overheat.
Do sweet peas return every year?
Sweet peas, members of the species Lathyrus odoratus, are annuals. This means they grow for one year. There are some perennial sweet pea species, like Lathyrus latifolius, but these have a different growth habit and do not have the quintessential sweet pea scent.
Can you eat sweet peas?
NO. Sweet pea flowers (Lathyrus odoratus) should not be confused with garden peas (Pisum sativum). Sweet peas are toxic to both humans and pets if ingested in large quantities. In order to avoid confusion between plants, DO NOT plant your sweet peas near edible garden peas. Also, be vigilant with small children and pets to ensure they do not ingest any part of the plant.
Where can I plant sweet peas?
Choose a site located in full sun with moist, friable, well-draining soil that is high in organic matter. Consider planting them in a pot or raised bed to help ensure adequate drainage.
When can I plant sweet peas?
Sweet peas are cool season plants. In order for them to flower before extreme summer temperatures, you will need to start them indoors at about 8 to 10 weeks prior to your last expected frost or plant them outside in the garden about 4 to 6 weeks prior to your last frost. Provide protection if the temperatures are going to dip below 25 degrees F. Many gardeners recommend to plant sweet peas around the same time as edible garden peas.
Sweet peas are a classic cottage garden flower that warrants a spot in the garden for at least one season. You may never be without them once you’ve seen and smelled them. Their fragrance, colors, and wild vining habit have inspired gardeners for centuries. Although they can be a little finicky about high temperatures, as long as you provide adequate moisture and plant them early, a bounty of blooms will await you in early summer!