Table of Contents
When to transplant seedlings into bigger pots?
Potting up your seedlings into a bigger container is an important step before they go out into the field, greenhouse, or into the garden bed. After a three to five weeks spent in a cell tray, seedling roots begin to expand and occupy more space, and if the roots do not have more space to expand, they will become root bound.
As roots become root bound and entangle themselves, the plants’ health will take a turn for the worse and will reflect this by the leaves turning yellow, the growth slowing down significantly, and eventually die back. Not to mention, if you are transferring a root bound plant into the soil, there is a good chance that this plant remains stunted and unhealthy…Yikes!
Troubleshooting a rootbound seedling
If you sense your seedling is becoming rootbound, you can first check under the leaves and at the base of the stem (the crown) before you take the seedling out to look at the roots. If you find that there is a purple hue under some leaves, it signifies a phosphorus deficiency. This color is created by an over production of anthocyanins which is the plant’s way of conserving energy due to the inability to carry out photosynthesis and distribute sugars properly with its roots being completely bound up.
If your seedling is still relatively young when you are seeing this (3-6 weeks old), your seedling has a great chance to bounce back. As your seedling gets older, the chances of it bouncing back get slimmer but it is not impossible.
Now that we have the negative effects of being root bound out of the way, let’s avoid this by transferring (potting up) our seedlings into bigger containers and letting those roots grow!
Potting up materials
Potting Mix
While a range of potting mixes can be used, it is important to keep the potting mix saturated before the young seedlings get transplanted to avoid any transplant shock. Keep it saturated by slowly adding water while you stir your potting mix. Periodically check the moisture level of the mix by taking a handful and squeezing it. This method is called the squeeze method. If two or three drops fall while squeezing, your potting mix is fully saturated.
* PRO-TIP! * A grower can add up to 5% of the potting mix with their outdoor garden soil to better acclimate your seedlings to the microbes they will eventually be growing with.
Containers
Bigger containers to pot up seedlings come in multiple sizes and shapes. Pots and containers sold by Harris Seeds can be found here.
Shapes
Forms
Singular container or six-packs (606 six-pack and 806 six-pack)
Sizes, diameter
1.5 inch (six-packs), 2.5” (singular, or pack), and 4” (singular). Larger containers can be used for larger seedlings and perennial plants and come in ½, 1-, 2-, 3-, and 5-gallon containers.
Transferring the seedlings into the bigger container
Pre-fill
Pre-fill your container with the saturated potting mix ¾ of the way to the top. Tamp the mix down. Form a hole with two fingers, deep enough for the seedling’s roots to be placed into but shallow enough for the top of the roots to be exposed.
Transfer from cell tray
This step can prove to be the most frustrating because the seedling doesn’t always want to leave the cell tray in one piece. Loosen the soil in the cell, and therefore the seedling itself, by pinching the cell from the bottom of the tray while gently pulling the seedling out of the tray by grabbing the base of the plant (the crown) with the thumb and pointer fingers.
Place seedling and exposed roots into the pre-fill container/pot. Once the seedling is in the pre-made hole, tamp the seedling down into the hole. Backfill with dry potting mix to cover any exposed roots.
*Be sure not to cover the seedling stem with potting mix, leaving the stem exposed is important for the plant to breathe.*
Water-in
Now the last step is to water all the seedlings that have been potted-up. This can be done with a watering can, a spray bottle, or a watering wand.
Everyday care
Watering
Seedlings that have been potted up in containers respond well to bottom watering. If the seedling pots are placed in a solid bottom tray, simply add water daily to the tray to make a water reservoir that the roots can go find on their own. This will make the roots learn, stretch, and grow.
Top watering can also be effective, but if bottom watering is the focus, be sure to keep the top of the potting mix moist with a spray method.
Warmth
If you are potting up warm season crops and your ambient temperature is below 75 consistently, keep the seedlings on a heat mat. Cool season crops do not require a heat mat.
Check out these blog articles on the differences between starting warm season crops and starting cool season crops.
Humidity
Potted up seedlings no longer need humidity domes, but it is important to maintain good humidity levels by misting the seedlings if the area around the plants is dry.
Airflow
It is important to maintain airflow to the seedlings, more so when they have been potted up than when they first germinated. As the seedlings get bigger and fill out their newfound space, airflow becomes scarce, and the leaves become the perfect breeding ground for plant diseases.
Keep a fan on in the direction of your seedlings. The fan should be oscillating or on a timer so not to cause continual stress on the seedlings.
Supplemental Light
If the seedlings are not in a glorious greenhouse, then supplemental light needs to be continuously supplied. Place your supplemental light 12-24 inches above the seedlings.
The space between the light source and the plant is dependent on the type of light being used and many resources online detailing this topic can be found. Keep the supplemental lights on a timer to reflect the daylight hours. 16 hours of light is best.
Now that the seedlings are set up for success, the next blog post will cover how to keep your seedlings healthy by adding organic nutrients and amendments.
Happy Planting!
Drew Hundelt
Product Manager & Content Specialist
Since stepping foot on his first urban farm in 2012, Drew found a passion that would eventually lead him here at Harris Seeds, Garden Trends Inc. Growing in multiple environments ranging from western New York to the bay area of California, and many environments in between, Drew has been and continues to build food production systems that yields food and community. Drew’s favorite projects so far in his career include carrying out a 12-acre trial of regenerative winter wheat production in western Pennsylvania and creating a horticultural program with three acres of market gardens, food forests, and greenhouses in the urban landscapes of Denver.
