Cucumber seedlings are quite finicky and sensitive. They need to be transplanted at just the right time.
If you transplant them when they’re too young, they won’t take, and if you wait too long, the roots will die out of exposure during transplanting.
When should you transplant cucumber seedlings? The best time to transplant cucumber seedlings is 3 weeks after germination and after the last frost. By that time, the seedlings should have two sets of leaves: two cotyledon leaves developed out of the resources in the seeds and two true leaves that indicate well-developed root systems.
If you get the timing of transplanting cucumber seedlings right, you improve the chances of a successful season and a high yield.
Read more to find out how and when to transplant cucumber seedlings.
7 Things To Check Before Transplanting Cucumber Seedlings
Before you transplant cucumber seedlings, you need to run through a checklist of conditions. If the seedlings meet all the conditions, then they’re ready to transplant. Otherwise, you should wait.
Here are the 7 items to check on your list.
1. Number of True Leaves
True leaves are those leaves that the plant developed with the help of moisture and nutrients its root system absorbed from the soil or growing medium.
Your cucumber seedling should have two fully developed true leaves.
This is a sign that the root system of the seedling is healthy and can handle the shock of the transplant and the brief exposure to the open air.
2. Age of Seedlings
Once the cucumber seeds germinate in the tray, mark the date on your calendar. When three weeks have passed, then you know that the seedlings are ready to transplant.
Some people wait 4 weeks before transplanting the seedlings, but if the weather conditions are right and the soil is warm or at least workable, you can transplant the seedlings after 3 weeks.
3. At Least Two Weeks After Last Frost Date
The threat of frost is real for cucumber seedlings. The seedlings are too fragile to handle the frost or frigid conditions.
Once the last frost is over, wait for 2 weeks before you transplant the cucumber seedlings. By that time, the weather will have improved and the soil will have warmed up.
4. Consistently Warm Nighttime Temperatures
In some Hardiness Zones, the temperature drops sharply at nighttime even in the early spring. These chilly conditions at night can stunt the growth of the seedlings or even damage the roots.
Keep the cucumber seedling indoors until the temperature at nighttime stays above 55℉ consistently.
5. Soil Temperature
Cucumber seedlings need warm soil to grow; otherwise, the roots will freeze, and the seedling will have stunted growth and low yield.
Before you transplant the seedlings, make sure the soil temperature is between 75 and 85℉ during the daytime and doesn’t dip below 55℉ at night.
6. Well-Developed Root System
The root system of the cucumber seedlings has to be well developed before you transplant them. That way you won’t have transplant shock and the seedling will take quickly.
Since you can’t uproot the seedlings to examine their root system, look for two fully grown true leaves as an indication of a healthy root system.
7. Seedlings Have Been Hardened Off
Once the seedlings sprout leaves and the weather outside warms up, take the tray outside for a few hours every day. This will harden the cucumber seedlings and will make transplanting them easier.
Hardened-off cucumber seedlings take quickly and are less likely to suffer from transplant shock or dramatic changes in the weather.
Thinning Cucumber Seedlings
Cucumbers typically germinate readily, but if you over-seeded, you’ll have too many seedlings competing for resources in a small space.
Thinning is critical to prevent scraggly, undernourished seedlings. Wait for the seedlings to grow to about 1 inch tall at least before you thin them out.
This allows you to determine which are the healthiest and strongest seedlings to keep. The others will be culled out and disposed of in the compost pile if you have one.
To thin out cucumber seedlings, use clean and sterilized scissors to cut the stem close to the soil.
Don’t try to pull out the seedling by hand. This might disturb the soil around the healthy seedling and expose the roots even if briefly.
Cucumber seedlings need plenty of space and resources to grow. Since the roots of the seedlings grow entwined, if you grow more than one seedling in one pot, they’ll compete over nutrients and moisture.
How To Harden Off Cucumber Seedlings
Hardening off cucumber seedlings is a crucial step for the success of transplanting the seedlings and preventing transplant shock.
The changes in the weather outdoors can be abrupt, which puts a lot of stress on the seedlings and stunts their growth.
It usually takes no less than 7 days to harden off cucumber seedlings, so you can start this process one week before transplanting them. Here’s how to harden off cucumber seeds in easy steps.
- When the seedlings are about 2 inches tall and the first set of true leaves has developed, it’s the right time to start hardening them off.
- When the weather warms up and the threat of the last frost has passed, take the seedlings outside and place them in a spot with dappled or partial sun for 2 to 3 hours.
- The next day, take the seedlings outside in the afternoon, and place them under the full sun for 3 hours.
- On the third day, increase the sun exposure to 4 hours before you bring them back inside. A light breeze won’t harm the seedlings at this point.
- Increase the sun exposure to 5 or 6 hours the next day. Make sure the soil is moist before you take the pots outside.
- By day 5, the cucumber seedlings are ready to take the full sun for a whole day. Bring them back inside at sunset.
- On day 6, bring the pots outside in the morning, and leave them for the rest of the day and night.
- On the seventh day, the cucumber seeds are hardened off, and you can transplant them.
How To Transplant Cucumber Seedlings
Once the cucumber seedlings have developed their third true leaf and have been fully hardened off, you can transplant them to a large pot. Here’s how to do it in easy steps.
- Choose small pots about 3 inches wide, and add 1 inch of potting mix to each.
- Label each with the name of the plant and the date of transplanting.
- Sprinkle the soil with little water, and mix it well to get it moist before transplanting.
- Extract the cucumber seedling gently out of the cell tray. Don’t squeeze the stem when pulling it out of the tray.
- Place the root ball of the seedling in the middle of the pot, and add more soil to a half inch from the edge of the pot.
- Firm the soil with your fingers to push out air pockets.
- Water the pot lightly to help the fresh soil gel with the old soil around the root ball.
Do Cucumber Plants Need Support?
Although support is not technically necessary, it does a good job of keeping the cucumber fruits off the ground and preventing many diseases caused by contact with the soil and pest attacks.
Because cucumber vines tend to crawl over the ground, stakes, cages, and trellises are all viable plant supports for cucumber vines.
How Tall Should a Cucumber Trellis Be?
A cucumber trellis should be between 4 and 5 feet tall. This supports the cucumber vine and allows it to grow vertically while keeping the leaves, flowers, and fruits off the ground.
A trellis this size is easy for one person to install and dismantle.
Cucumber seedlings should be transplanted 3 weeks after germination, a couple of weeks after the last frost when the nighttime soil temperature stays above 55 degrees consistently.
Start hardening off the cucumber seedlings one week before transplanting them by taking them outdoors and exposing them to sunlight.