How to Grow Sweet Peas Three Ways: Direct Sow, Root Train, and Winter Sow

Why Grow Sweet Peas?

Sweet peas (Lathyrus odoratus) are lovely. Like most flowers like ranunculus, lisianthus, and dahlias, you have to experience them for yourself before you know just how lovely they truly are. Pictures don’t do it justice so please visit us and pick up sweet peas for yourself when they bloom this spring. If you don’t live near me, please find your nearest flower farmer to get yourself these fragrant, delicate blossoms. Get a taste of our flower farm here.

Or better yet, learn how to grow sweet peas for yourself so you can choose your favorite colors and the varieties that will work best in your area. If you want to skip straight to buying supplies, click here. You will be amazed at the options you find, including pure-colored, two-toned, speckled, and striped sweet peas. 

Starting sweet peas is an exciting endeavor whether you’re a seasoned gardener or a newbie with a green thumb. In this blog post, I’ll relate my experiences using three different methods to give you a leg up in your sweet pea journey: direct sowing, using root trainers, and winter sowing.

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DISCLOSURES and THOUGHTS:
If there are links, chances are they are affiliate links. You will never pay more for affiliate links, it simply means a company is willing to pay me for sharing about a product I’ve used and can get behind. Thanks for being here, reading this journal, and learning and growing along with me! ♥ 

This journal entry is a compilation of my thoughts that I choose to sincerely share with you. I am always open to new insights and would love to hear yours. When you add your comments, I ask that everyone is civil and respectful of all beliefs, even if they aren’t the same as yours.

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Direct Sowing Sweet Peas in the Garden

Two years ago I ordered my seeds at the end of summer, barely in time for me to plant them for fall to overwinter. Even though I received them in time to start planting, I delayed actually starting them. Maybe it seemed too daunting a task because I didn’t want to make my own mix of soil for soil blocking or maybe it’s because I was trying to keep a newborn baby alive and still get a little bit of sleep. Either way, time kept ticking and I started getting worried that they wouldn’t be established in the root trainers and in the ground before winter hit. So, I did something I’ve done before that (both times) resulted in dismal germination; I direct-sowed my sweet peas. Direct sowing means to plant your seeds into the ground right where you want them to come up.

I got about 5 out of 60 that came up from direct sowing, which is about the same rate as the year before when 1 out of 13 came up. So, lesson learned, don’t start your sweet peas out in the field. That said, if anyone knows any tricks or has had success with direct sowing I would LOVE to chat. When direct sowing I did try to give them a bit of a head start by soaking them overnight or for up to 24 hours as some people suggest. Soaking, and even nicking or lightly scuffing the seeds with sandpaper, is recommended to help decrease germination time. However, it’s best to follow the advice of whomever you purchase your seeds from because they know best. For example, Roger Parsons (his book is great if you want to hone in on your sweet pea growing skills) along with Ardelia Farm and Co (one of my favorite places to purchase sweet pea seeds) recommend not soaking their seeds. Maybe soaking was my downfall or in some way contributed to the seed rotting and ultimately not sprouting. 

Direct Sowing Sweet Peas Tips

If you do want to give direct sowing a try, because it is definitely saves you money in not having to get potting soil and time in not having to transplant, then here are some tips:

• Timing: Sweet peas prefer cooler weather, so plan to sow them in the fall 6-8 weeks before your first frost or in early spring as soon as the soil is workable. If your sweet peas are not growing by the time it’s getting warmer, then it will likely get too hot (at least in Zone 7, Utah) before they start thriving and you get to enjoy them. 
• Site Selection: Choose a sunny spot in your garden with well-drained soil. Sweet peas thrive in full sun but can tolerate partial shade. I have found that the plants getting afternoon shade have grown tallest and continued producing flowers longest in my garden so I aim to plant them in places where there is natural shade. Put up shade cloth (frost cloth can double as shade cloth in the spring) to help protect them from intense summer sun and heat if you have no natural shade in your garden created by fences or other structures. It could be a fun experiment to see how long into the summer you can keep them producing. Mine are usually finished by end of July and, if I have the space, I’ll leave them to produce seeds to harvest and plant for the fall.
• Soil Preparation: Ensure your soil is rich and well-drained. Amend it with compost or organic matter to improve fertility and drainage. This cannot be stressed enough. They are heavy feeders. If you provide them with the nutrients from quality compost and fertilizer along with the consistent water they need, they will grow quickly for you.
• Planting: Create a trench about half an inch deep and sow sweet pea seeds along the bottom, spacing them about 2 inches apart. I have found that by spacing them so closely, that when (inevitably) a few don’t come up, they will still end up being close to the desired 6 inch spacing (typical cut-flower farming spacing) once you’ve got them up and growing. If you do end up getting very good germination in the field, you can always transplant any that start growing too close to each other. I cover more on tranpslanting in the Root Trainers section below.  As with any seed that you direct sow, you need to make sure it is watered daily, or as frequently as needed to ensure the soil doesn’t dry out. If the soil dries out at any time while the seeds are germinating then you’ve just lost that planting and will have to start over. 
• Support: Sweet peas are climbers, so provide a 6 foot or taller trellis or support structure for them to climb as they grow. You should definitely have this trellis up before they start growing because, if you’re anything like me, if it’s not up beforehand it gets forgotten until it’s too late. You avoid possible root damage by putting the trellis in before the seeds have been planted. Hog panels are another option that are reuseable and less plastic. If you’re local and want a custom amount of the hortvana plastic netting, contact me and I’m happy to cut you some by the foot to fit your space and save you the shipping costs of the big roll.

Sweet Peas Growing on Their Own!

My last note about direct sowing sweet peas is a positive one. I have found that when I let the sweet peas stay in the ground past the time I am actively harvesting them for cut flowers, the sweet pea flower petals fall off and you start to see little pods. The pods on the vine are very fun to use as cut flowers to add some texture and unique elements to your vase, but if you have some self control and leave some of them on the vine they will eventually have seeds that are mature enough for you to use the following year. You’ll know when they’re ready to be used as seeds for the following year when the seed pods turn tan and the seeds are hard and dark brown.

Be aware that the new plant may not have flowers exactly like the parent plant, especially if they aren’t heirloom varieties and/or you have several varieties of sweet peas planted in your yard. Even if the varieties get mixed by the pollinators in your garden, they will likely be just as beautiful as the ones you originally planted. In the fall, when I was pulling the dried, sun-bleached sweet pea vines off the trellising, the sweet pea pods popped open on their own and released their seeds all over the ground. The sweet pea vines started growing on their own when conditions became just right. The best part of leaving your sweet peas to make seed is the natural advantage they will have over any other purchased seed. The plants, and thus the seeds, have adapted to your climate and will handle your temperatures, soil, moisture levels, etc. 

If you find the self-sown sweet peas growing up a little too far away from the trellis or in undesirable or inhospitable areas simply move them to your desired location. I start with a hand shovel in my desired end location for the sweet pea. I push the shovel straight into the ground as deep as it will go and wiggle it around a bit to create a small space that should fit the transplant perfectly. Doing this step first allows for the sweet pea to be moved without being put down while the new space is prepared. Less time out of the ground means less root disturbance which means happier plants.

Next, go get the sweet pea and dig about two or three inches away from the base of the sweet pea. Dig as deep as possible to ensure as much soil as possible stays with the sweet pea root while transplanting. If the soil falls off around the root that’s fine. Just get it planted and give it a good watering immediately to help it survive the move.

Using Root Trainers

Last spring, I tried two different ways of sowing my sweet peas to see which would result in the healthiest, fastest, and highest germination rates. I’ll talk about my experience with root trainers first. Root trainers are special containers designed to encourage strong root development in plants. Using root trainers can help your sweet peas establish robust root systems, leading to healthier plants.

Root Trainer Options:

• Make your own using toilet paper rolls
• Purchase really fancy root trainers that open up so you have minimal root disturbance
• Use deep trays. These are right in the middle of DIY and the expensive root trainers so I opted for these.
• It’s also helpful to have some sturdy trays for holding the root trainer tray, a humidity dome or some saran wrap, seed-starting mix or potting soil, your chosen variety or two of sweet pea seeds, a watering can and seed starting lights and you’re ready to go.

Just like direct sowing, you plant the sweet peas about ½” deep. Make sure they are completely covered as they require darkness to germinate. I like to keep a humidity dome over all my seedlings, not just my sweet peas, until they germinate to ensure the soil does not dry out between my daily check-ins. Sweet peas prefer temperatures of around 50-55꙳F to sprout and even cooler temperatures once they’ve sprouted. I keep mine on the basement floor and they do well with that temperature. Once they germinate you need to immediately get them under a grow light or turn it on if you’ve always had them under the grow light to avoid weak and leggy sweet pea starts. Keep the grow light a few inches above the seedlings and move the light up as the sprouts get taller. 

While I tried the root trainers during early winter for a spring planting, you can also try sowing and planting them in the fall if your area doesn’t get lower than 20꙳F or you are planning to protect them from the cold with a low tunnel, high tunnel, or a simple cover of frost cloth as long as you can keep the cloth from touching the plants.

Once your plants are a couple of inches long, you can transplant them outside. Don’t wait too long (more than a couple weeks) because the tap root starts to wrap around the root trainer cells. You can see that in this picture. 

When you’re planting them into your garden from the deep trays, take care that you’re not disturbing the roots too much. Sweet peas have one main tap root. In the above picture the tap root is the curled, longest root. If you aren’t gentle with it while planting you could stunt the growth of the plant. One advantage of getting the most expensive root trainers is there is minimal root disturbance, however I’ve found that I can put a pencil or chopstick, or even random sticks in my garden, into the bottom hole of the root trainer and it will pop the plug, which is the fancy word for the plant and the surrounding soil, out easily. I try to have my holes pre-made so I can quickly pop the sweet pea directly into the well-amended soil without fumbling and unnecessarily damaging the plants. 

My favorite tool to make the little holes to do this with is a butter knife. I’d recommend going to your local DI, Savers, Goodwill, or your local donation shop in order to get yourself a few you can use in the garden. I bought one from my local Deseret Industries and couldn’t find it during planting time. I used one from my kitchen set and when I was done, the shine was completely gone. The knife was dull and covered in tiny scratches from the soil. I won’t make that mistake again. So, listen to me and buy yourself several so that when you end up losing one, or you leave it in the garden and it gets buried for the season, you’ll have a backup without resorting to ruining your nice ones.

Winter Sowing

The second way I tried last spring, winter sowing, is the least expensive and time-consuming way to plant them. I’m all about saving time and money. Please note that although it’s fastest to get it planted, they take one to two weeks longer to germinate compared to the root trainer method.  For this planting method, simply rinse out your gallon or half-gallon plastic milk container and save it until you’re ready to start your seeds. I start saving my milk containers late summer so I’ll have enough for all the varieties of sweet peas and other fall-sown, spring-blooming crops I want to grow. If I have enough seeds, I sow more than I have space for in the garden to account for imperfect germination rates. I do this with all seeds because it also helps to have extras in the likely event that some don’t survive the transplant into the garden.

Don’t worry about cleaning out and saving the lids to the milk jugs because I’ve never used them when I place the jugs outside. Once I’m ready to get my seeds started each fall, I use my razor blade to cut an opening almost all the way around the milk jug. Give yourself a couple of inches to the side of the handle and an inch below the handle. Cut until you get to the other end of the handle. Leave a 2-3 inch section directly underneath the milk jug handle still connected. If the top of the milk jug gets cut or torn all the way off, attach it with some tape. I also make a little notch in two sides of the milk jug that help me to more easily place the top section over the bottom edge. This helps to avoid the need for tape which doesn’t always stick well in wet and cold conditions when the milk jug is being opened to check on the sweet pea starts.

Then cut holes into the bottom of the jug to allow water to come out. Take the razor blade and simply push it up into the jug so that the slit is the same length and very thin just the size of the blade. I haven’t had any issues with soil coming out this way. It is an easier (and safer for me and my hands) method than making lots of little holes with the tip of your razor blade. An awl is an easy alternative to make small drainage holes that would work just as well as the slits from a razor blade.

Fill the bottom half of the milk jug with the pre-moistened seedling mix or potting soil and sow the sweet pea seeds 1/2” down into the soil. A good moisture goal for the soil before you plant is to get it wet enough that when you squeeze really hard a drop or two of water drips down your hand. Once the seeds are planted, feel free to add a little more water to ensure the seeds are surrounded by soil.

Place the top of the milk jug back into place to create a miniature greenhouse effect. The top and bottom don’t usually stay aligned well anymore and I do not worry about it fitting perfectly. Tape it shut with duck tape or try the two notches cut out of the sides (shown in picture above) if the top part gets bent backward and wants to stay backward rather than up in place. As mentioned before, I don’t put the milk lid on to the top. This allows any rain or snow to help water my plants while at the same time any serious damage from hail or winds can be avoided. I set mine out into the cold on my deck and try to get them the most sun possible to help them grow into strong starts. Sometimes that means moving them around the deck as the sun moves and building and fences cast shadows. On really cold nights, when it is forecasted to get close to or below 20°F, take a blanket out and cover all the milk jugs. It’s better to be safe than start all over, especially with seed ordering and shipping times. Any blanket or sheet will work to provide a little extra protection. If you already have some or plan to repurpose them at other times in the year, use your greenhouse plastic or frost cloth.

Pros and Cons of Winter Sowing

I love this method for several reasons:

1. Many seeds can be planted quickly. It takes me less than 20 minutes from start to finish including gathering supplies and cleaning up.
2. It takes up less space than root trainers. Some of the seed packets I purchased had 25 seeds per packet so I put them all into one jug which saves so much space compared to the root trainer trays.
3. It requires fewer supplies and thus, less money, especially if you don’t already have seed starting equipment and don’t want to invest right now.

The only downfall is increased germination time, so while it’s less work and money, you pay for it in time. It took at least two weeks after my root trainers germinated before I started seeing my sweet peas come up in the milk jugs. This may have been partly due to being planted an inch deep rather than half an inch, though I have heard from other flower farmers that waiting for germination is common with winter-sown seeds.

Overview

Starting sweet peas can be a rewarding experience, and these three methods – direct sowing, using root trainers, and winter sowing – provide different options to suit your gardening style and climate. Whichever method you choose, you’ll soon be enjoying the beautiful blooms and captivating fragrance of sweet peas in your garden. So, pick your preferred method and get started on your sweet pea adventure today!

While I prefer root trainers if I’m shorter on time before they need to be planted in the ground and winter sowing if I’ve got plenty of time when getting them started, both had near-perfect germination rates. I know you will be successful as well. I hope my experiences help you choose the best option to suit your gardening style and climate.

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As promised, here are my favorite sources for sweet peas and tools. If you have found this journal entry valuable, I hope you are willing to support me through my affiliate account if you are going to purchase anything from Amazon. It will never cost more for you!

My Favorite Sweet Pea Seed Suppliers

Ardelia Farm and Co

Floret Flowers Sweet Pea Collection

Geoseed – You can purchase from here even if you don’t have a business, best prices although not necessarily the best options. Great for mixed seed packets.

Roger Parson’s Book – Sweet Peas, An Essential Guide

Supplies for Planting in Deep Tray, Root Trainers

Potting Soil – ProMix is one of my favorites, although any will work. I prefer buying in bigger bags to save money if you have the room to store extras or have houseplants.

Seed Starting Mix – Either potting soil or seed starting soil will work. Seed starting mix is great if you are also going to be starting other smaller seeds at the same time so you don’t have to get two different bags of soil.

Root Trainers – Great for easy access with minimal root disturbance.

Deep Trays – If you sign up for Johnny’s Seeds emails, you can usually get a $5 off coupon.

Heavy Duty Seedling Trays – You place this under the Deep Tray to help you with watering. It helps protect your space from water dripping out the bottom of the deep seed trays and when the sweet pea vines have germinated, you can start adding water directly into this tray to help the roots grow deep. Just make sure you get the ones without holes if you’re using it for this purpose. Please note — These are NOT your average seed tray that is flimsy, cracks down the sides after one year, and is difficult to move around while transplanting your starts. Trust me when I say you will want these! They are worth every penny and more, especially when you consider not having to repurchase more trays every year.

Humidity Dome – These are so useful for any seeds that you will be starting if you use the deep trays.

Seed Starting Light – There are so many options when it comes to setting up a light for your plants, so find something that works well in your space. This set comes with everything you need for a super easy set up for a small amount of seeds. Whatever you choose, don’t skimp on the light or else the plants become leggy and weak.

Supplies for Winter Sowing

Razor Blade – Useful for cutting milk jugs 

Seed Starting Mix – Either potting soil or seed starting soil will work. Seed starting mix is great if you are also going to be starting other smaller seeds at the same time so you don’t have to get two different bags of soil.

Supplies for Keeping your Sweet Peas Alive and Thriving

Frost Cloth – You can use any blanket, but if you don’t want to get your bed sheets or nice blankets dirty then I recommend getting a frost cloth (or two, depending on the size of your growing space). This will only be needed for sweet peas if it gets to 20°F or colder). This can be reused as shade cloth in the summer as temperatures start rising to help your sweet peas produce longer into the season.

Warm Winter Gloves – Use these to keep you outside gardening longer. There’s nothing worse than cutting your time out in the garden in the winter or early spring because of numb fingers.

Sweet Peas, An Essential Guide by Roger Parson – Learn more about all aspects of growing sweet peas. The info is based on the seasons/weather in UK so I searched online to see how my weather in Zone 7 compares to the weather there.

Hortvana Netting for Trellising – Support your cut flower plants as they grow. It helps you get straight stems and prevents damage during windstorms or whatever else Mother Nature throws at you during the growing season. You can use this for all types of cut flowers and can be used vertically for sweet peas between posts or placed horizontally over flower beds to help other cut flowers like snapdragons, dahlias, campanula, asters, pansies and more. If you’re local and want a little less than a 250′ roll for this good quality netting, reach out to me via email or in the comments and we can cut you some by the foot.

Compost and Fertilizer – These delicate-looking plants are heavy feeders and require a good amount of compost mixed in before or while planting these seeds to give them a good start. It also helps to keep them fertilized regularly throughout their growing season in early spring into summer. I use a general fish emulsion fertilizer throughout my garden to keep everything growing.

May you experience the joy of growing sweet peas this coming spring!

Learn to grow your own sprouts and microgreens for a quicker harvest and actually edible crop — sweet peas are not safe to eat for humans or pets!

Check out our shop for more spring flowers, like another favorite, butterfly ranunculus!