The Short Version
Sow ½” deep in a 5” deep pot.
Use a good compost based seed starting mix.
Keep at 50-55 degrees for germination.
Move to 35-50 degrees in full sun for bulking up (cold snaps down to 20 are fine!)
Transplant in early spring into rich soil in full sun.
Provide a trellis of some sort.
The long version
Sweet Peas are not difficult to grow from seed, they are simply specific in what they like and quite adamant about what they don’t like. I have written some rather detailed information here as there is plenty of misinformation about sweet peas out there. Most of the information is geared toward English growers. Few places in the US mimic English conditions so you need to understand the needs of the plants in order to figure out how to give them what they want in your individual climate.
If you are new to seed sowing, or perhaps have grown tomatoes a time or two, you may have some preconceived notions about what it takes to start a seed. The hardest detail for new growers to grasp is that some plants simply prefer it cold, while others prefer it warm. Tomatoes love it warm, and should be started on heat mats and kept cozy, while sweet peas like it cold, and will be a disaster if started on heat mats in a warm living room. You may think keeping them warm is kind, but you are setting them up for failure. Keep these temperature ranges in mind.
50-55 °F for germination
35-50 °F for bulking up (cold snaps down to 20 °F are fine!)
45-70 °F for optimal flowering. 65 °F day with 45 °F night is ideal.
If you are starting just a few plants, find a container that is tall and narrow. That’s truly all you need to keep in mind. I will elaborate on containers a bit more below, but too much emphasis is put on the container. Even a plain old 4” pot can work just fine.
We prefer to use a pot that is 5” tall and about 2-3” wide (we use the Anderson Band pot, 2 ⅜ x 5” to be exact). Some people use the cardboard tube from the middle of a toilet paper roll. In England the standard tray of choice is a product called a Rootrainer. Again it is tall and narrow allowing the the long deep root system to grow, and then the tray opens like a book, exposing the root system without disturbance. You can find them from several online retailers. If you are only growing a couple dozen plants the cost is justifiable to have a reusable tray system that will produce good plants year after year. We don’t use them simply because we grow about 30,000 plants a year the tray cost would be substantial. A deep 50 cell tray will also work. My favorite I’ve found is the T.O. plastics Sure Roots tray. Again, don’t stress about your container, but I wanted to share what I’ve learned through the years.
Any well draining potting media will suffice. I used ProMix BX for years, but I have recently switched to a mix with more bark in it for better drainage. Sweet pea roots are big and can negotiate a coarser media than a tiny slow growing seedling can. Something compost based is also a good choice. I have had good luck with Vermont Compost Fort Vee. Again, this is not a point to stress over. If you have a very fine seedling mix on hand just add some pearlite to create better drainage and get on with things. We all have access to different mixes, so any general seed starting mix will work.
Sow seeds about ½-1” deep, making sure they make good contact with the soil. You can sow directly into your final tray or container, or you can sow a number of seeds in a pot for transplanting after sprouting. We now use the latter method as we can start many more in a smaller space and then space them out at time of transplant. We sow 25 seeds per 4” pot for germination. If you’re sowing a tray or two, sow directly into the tall narrow container of your choosing. Simply sow, water well, and keep at 50-55 °F until you see emergence.
Notice I didn’t mention soaking. It is entirely unnecessary and in fact can reduce your percentage of germination by spreading fungal diseases from seed to seed. Roger Parsons of the UK and Dr. Keith Hammett of New Zealand (the two foremost experts in the genus) don’t soak. So I don’t either, and I get near perfect germination every time.
Emergence and bulking up
After 10 or so days (longer if it has been cooler) you will see little sprouts starting to emerge. When most of the seeds seem to be up, you can move your plants to a cooler location. 45 °F is my ideal daytime temperature, but they can cool down to 35 °F at night and still keep growing. In the dark of winter, they may not go above 35 °F for weeks on end. No worries. They’re happy. A hard frost, right down to 20 °F is fine for the plants. If you aim to keep them between 35-50 °F, and accept that they will sometimes get colder you will succeed. You aren’t looking for active lush growth at this stage. You want short and stocky growth, meanwhile the roots are growing actively getting ready for spring.
Just before leaves start to unfurl is a good time to bump up seedling from a nursery pot into their tray.
Full natural light is best. If you live in a climate where temperatures are mostly above freezing in the winter I would just set your tray in a protected sunny area outdoors (protect from birds and mice). If you get a real cold spell you can move them into a garage or some place more sheltered. Here in northern Vermont we minimally heat a greenhouse to keep the plants barely above freezing. They get a little warmer on sunny days, but generally hang out around 35 °F and slowly build up those roots.
Artificial lights can be used as long as you keep those plants 50 °F or preferably colder. (remember, these aren’t tomatoes.) When cold grown, the plants will stay short, so you can keep them just a couple inches from the lights. If you try to grow them warm under lights they will stretch out in no time, resulting in spindly weak plants. Natural light is best (outdoors, greenhouse, sunny sunporch) but lights can be used ONLY when grown cold. I know I’m getting bossy here, but I want you to succeed. The biggest mistake people make is growing sweet peas too warm and too dark.
When to start
This will be a different answer for everyone. The US is simply huge with many different microclimates even within a single zone.
Mild winter growers in zones 7, 8, 9 or 10, can start in the autumn with good success. Wait until the days are shortening, and getting cool. When your daytime highs are routinely 55 °F or less you should be safe to sow. This will likely be some time in October for most..
Cold climate growers in zones 3, 4, 5 may be wise to wait until late winter to get started. You could start as early as November, but not everyone has a bright and cool environment that can remain consistently around 35 °F all winter. You may have to get creative, and shuffle your plants around to keep them from extreme freezing, but still keep them cool and bright. Again the plants have pretty straightforward and defined needs and you have to figure out how you will offer them these conditions. The plants are in charge!
Moderate climate growers in zones 5, 6, 7 can use either of the above techniques depending on facilities. It will be up to the individual to determine when they can offer the ideal set of conditions to the plant.
When grown slow and cold, sweet peas will naturally branch on their own. If you see that your plant has not branched but is getting tall, you may take out the top set of leaves, provided you leave 2 or 3 sets on the plant. This will encourage robust side shoots. We never pinch as our plants almost always branch on their own.
If you are growing in a greenhouse or high tunnel, you can transplant any time after you have a good sized plant and your tunnel is reliably above 20 °F at night. If you are planting outside, you are safe to plant any time the ground can be worked, and again, nights are above 20 °F. If you see your daffodils starting to peek out of the soil, it’s probably safe to transplant your sweet peas into their blooming position.
Use lots of compost and manure. You want a moist, well-drained location with plenty of organic matter. Sweet peas are heavy feeders. When in doubt feed as you would a tomato plant. They aren’t too fussy about what they climb on, but they will need some support. Many varieties can reach 6 or more feet tall, so plan accordingly. They use their tendrils to attach to wires or twine or twigs or even shrubs. They need something a pencil width or smaller to hold onto. They cannot cling to a wall like ivy, nor can they twine around a thick support like a morning glory. A google search will give you dozens of ideas on how to support your plants.
Now for the fun. As the weather warms in spring your plants, with their robust root systems, will jump out of the ground. The goal is to get as much growth as possible before the heat of summer comes. Spencer type sweet peas need 12 hours or more of daylight to flower and if you have reached that benchmark, and your weather is mild (somewhere between 45-70) you should start to see buds. Keeping spent blossoms trimmed off of your vines will keep them blooming as long as possible. Heat will be what shuts them down in most climates. 80 °F is tough on a sweet pea and once you start seeing hot days and warm nights, the end may be near. Plants can handle warm days as long as they cool down to 45-50 °F at night, but many warm climate growers simply can’t offer these conditions for long. Growers in the deep south may want to try “Winter” or “Spring” specific varieties. (See our post on different types of sweet pea for more on these types of sweet peas.)
Maybe you’re thinking that this all seems like too much, and you’re wondering if you can just pop some seeds in the ground and stand back. For some of you, yes you can! Plant when you would normally sow garden peas. Sow in the fall in areas where you don’t routinely drop below 20 °F. If you live in a cold climate you will either need to grow in a high tunnel, or not attempt direct seeding. Those of us here in zones 3 and 4 can probably direct sow as soon as soil can be worked, but the flowers will be later and less abundant than if you had transplanted a bulky plant at the same time.
There are several reasons not to direct sow. Good seed is expensive. Mice and voles will gladly eat sweet pea seeds before or after they germinate. Slugs and snails also enjoy tender sweet pea growth. For people with wet winters, you also risk your plants rotting over the winter if direct sown. When grown in pots you can control how much rain and moisture they receive.
Want to learn more?
I highly recommend Roger Parson’s book, Sweet Peas, an Essential Guide. He goes into more depth than I on all manner of sweet pea related topics.
Ready to give it a try?
We have a nice collection of various varieties and mixes available. The majority of what we sell was produced here on our farm in Vermont, rounded out by some seed produced by specialists in the UK and New Zealand. This it the best quality sweet pea seed you can buy. Shop sweet pea seed here.