When and How to Start Cucumber Seeds Indoors
Starting cucumber seeds indoors isn’t necessary, but it lets you enjoy crispy cucumbers earlier in the year. Vegetable farmer Briana Yablonski shares when and how to sow these seeds indoors.
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When the hot summer days arrive, biting into a chilled cucumber feels like a real treat. And if the cucumbers are homegrown? Even better!
Cucumbers flourish in warm temperatures, so you shouldn’t plant them outside until the cool nights are over. However, you can start the seeds indoors and later transplant the seedlings into your garden.
I’ll explain when and how to start cucumber seeds indoors. I’ll also cover a few common mistakes that gardeners make and help you avoid them.
Should I Sow Cucumber Seeds Indoors?
Cucumbers germinate and grow quickly, so you don’t need to start them indoors. However, since they require warm temperatures to germinate and the vining plants are sensitive to cold, sowing indoors allows you to harvest your cukes a few weeks earlier in the year.
While starting cucumber seeds indoors in the spring is a good idea, I recommend directly sowing them during the summer. Cucurbit roots are very sensitive, so transplanting is a stressful process that often sets the seedlings back.
When to Start Cucumber Seeds Indoors
The best time to start cucumber seeds indoors depends on your intended transplanting date. Ideal transplanting dates vary by growing zone, but you can use historical weather data and current temperatures as guides.
Cucumber plants are very sensitive to cold, so you should wait until both the soil and air are warm before transplanting. It’s safe to plant once nighttime temperatures stay above 50°F (10°C), which usually occurs a few weeks after the last spring frost.
Once you determine when it’s safe to plant your seedlings outdoors, you can decide when to sow them indoors. These seedlings are ready for transplanting approximately three weeks after you plant the seeds. That means you should start three weeks before you hope to plant them outdoors.
In many areas, you can plan on sowing indoors around the time of your average last frost dates. It’s fine to plant after this date, but starting earlier means the seedlings may be ready to plant while outdoor temperatures are still too cold.
How to Start Cucumber Seeds Indoors
With your ideal sowing date in hand, it’s time to move on to learning how to plant cukes indoors.
Gather Your Materials
Organize your materials before you sow. Preparing your supplies before your sowing date arrives will allow for a seamless planting process.
Although not everyone’s indoor seed starting setup will look identical, you should have the following supplies before you start planting.
Cucumber seeds
You can choose from dozens of cucumber varieties, including slicing cucumbers like ‘Marketmore’ and picklers like ‘Homemade Pickles.’ Cucumber seeds remain viable for five years when stored correctly, so you don’t necessarily need to purchase new seeds each year.
Seed starting mix
Choose a loose and well-draining mix designed for starting seeds. Avoid using native soil and potting mixes designed for houseplants.
Seed starting containers
Cucumbers grow quickly, so choose larger containers. Three-inch pots and four-cell trays are both suitable options.
Bright light
Once the seedlings emerge, they require a source of bright light to remain healthy. A greenhouse works well, and a grow light allows you to grow healthy seedlings indoors.
Heat mat
Although heat mats are optional, they warm the soil and promote quicker and more reliable germination. If your germination area is below 70°F (21°C), I recommend using one.
Fill Your Container with Seed Starting Mix
Grab your seed starting mix and slowly add water until it’s slightly moist. The mix should feel wet to the touch, but it shouldn’t be dripping.
Add the damp mix to the top of the containers, then gently tap them on a table or other solid surface. After the material settles a bit, fill in the resulting empty space with more soil mix.
Sow the Seeds
Use your finger to poke a half-inch-deep hole in the middle of each container or cell. Place one seed in each hole, then gently cover it with soil mix. If you’re worried about low germination rates, you can plant two per hole.
Gently water, then place the trays somewhere warm. These plants germinate best around 85°F (29°C), and cooler temperatures will lead to slower germination. If you don’t have a warm room or greenhouse, you can set the trays on a heat mat.
Check the soil every day and water if necessary. Placing a humidity dome over your trays helps trap humidity and decreases the need for water.
In ideal conditions, they will germinate in three to five days. However, they will take longer to sprout in cool soil.
Provide Bright Light
As soon as the seedlings emerge from the soil, move them somewhere where they receive 10-12 hours of bright, direct light. A greenhouse or south-facing window can work well, but a grow light is often necessary if you’re starting seeds indoors.
Position the light a few inches above the tops of the plants. Make sure to turn it off each night since seedlings require darkness as well as light.
Harden Off Seedlings
Your seedlings should be large enough to transplant outdoors about three weeks after you plant. However, don’t move them directly from their cozy indoor home and into the ground! This sudden change in temperature, light, and wind will stress the plants and cause transplant shock.
Instead, spend at least a few days acclimating your cucumbers to the outdoors. Move the potted seedlings outdoors for a few hours during the day, then back inside. Complete this process for a few days while increasing the amount of time they spend outdoors.
You can transplant them once they spend an entire day and night outside. Just make sure the weather is stable and the nights remain above 50°F (10°C). The best cucumber spacing depends on the variety and growing method, so consider these before planting.
Common Problems to Avoid
While starting cucumber seeds indoors isn’t complex, growers often make a few mistakes. Take note of these common missteps so you can avoid them yourself.
Planting Too Deep
Although you can plant cucumbers deeper than many smaller seeds, it’s still possible to plant them too deep. Tucking them a quarter-inch to a half-inch under the soil will keep them moist and allow them to quickly emerge once they germinate.
Not Providing Enough Light
Even if you think your bright kitchen window will provide enough light for your seedlings, it may not. Weak light can cause cucumbers to stretch toward the light source, resulting in weak stems.
Grow lights are your best option unless you have a bright, south-facing window. They provide overhead light which prevents plants from bending one way or the other. Plus, they’re a reliable light source and easy to set on a timer.
Keep the Soil Too Wet
Seeds need moisture to germinate, and plants need water to grow. However, too much moisture can cause them to rot.
Your goal is to keep the soil moderately moist but not wet. A well-draining potting mix is essential. Remember to water the soil only when it’s slightly dry, not on a set schedule.
