When to Transplant Seedlings to Ensure They Thrive, According to Gardening Experts

Starting seeds indoors is a smart way to get a head start on spring gardening. But after weeks of ensuring your seedlings have enough water and sunlight to thrive, the last thing you want is for them to die when you transplant them outdoors.

While it can be tempting to transplant seedlings once they grow a set or two of leaves, various factors, including temperature, time of day, and plant variety, determine when to move your seedlings from their current home.

To ensure a smooth transition process, we spoke to gardening experts who shared their tips for when to transplant seedlings and how to do it.

When to Transplant Seedlings

When to transplant seedlings depends on the plant. "Cool-season crops like broccoli, Swiss chard, kale, cabbage, and cauliflower should be transplanted in early spring when temperatures are still mild," says Adam Weiss, gardener and founder of Pike Lane Gardens. Meanwhile, warm-season vegetables like tomatoes, eggplants, peppers, and squash should be transplanted once the soil has warmed to at least 70 degrees Fahrenheit. 

The next thing to consider is the time of day. The best time to transplant seedlings is in the morning on a cloudy day. “This helps prevent transplant shock caused by sudden temperature changes and keeps the soil from drying out too quickly,” says Weiss. Avoid transplanting during midday, especially on hot, sunny days.

Signs That Seedlings Are Ready to Be Transplanted

Seedlings are typically ready to be transplanted about four weeks after they are started indoors, says Paul Dysinger, gardener and co-founder of Seedtime. However, some plants may need more or less time than that. "Really, the ideal time to transplant is after the plant has grown enough to fill the current container but before it gets stressed with not having enough growing space," says Dysinger.

A plant should have at least two sets of true leaves before it is disturbed. Another way to tell if your seedlings are ready to be transplanted is by looking at the roots. When you pull a seedling out of its cell, the roots should hold the shape of the cell without crumbling.

How to Transplant Seedlings Outdoors

Consider these steps when it comes time to transplant seedlings outdoors. Doing so the wrong way can shock the plant and potentially kill it.

Choose a Location

To transplant seedlings outside, start by measuring the correct spacing in the garden and plotting where you will put each plant. Consider how much sunlight each plant needs when choosing a location. Once you've selected a suitable spot, delicately remove your seedlings from the existing tray or pot, says Weiss.

Dig a Hole

Dig a hole where you want to place each seedling. Be aware that the planting depth varies based on the type of plant. “For example, tomatoes should be planted 5 to 6 inches deep in the soil, while kale and broccoli can be planted in more shallow soil, only 2 to 3 inches deep,” says Weiss. Hence, you will want to pick your depth according to each plant.

Plant and Mulch

Place the seedling in the hole, backfilling with soil. Then, surround the plant with mulch—keeping it a 1/2 inch from the base of the plant—for moisture retention and weed prevention, Weiss says.

Water Deeply

Water the plant deeply at the base of the seedling. “Overhead water can overwhelm the baby seedling, so make sure to water the plant at the base,” he adds.

How to Transplant Seedlings Into Containers

When transplanting seedlings into containers, always use potting soil instead of garden soil. “Potting soil is a light, aerated soilless mix made from materials like peat moss, vermiculite, and micronutrients,” says Weiss. In contrast, garden soil is heavier and more compact, which can negatively affect drainage, root development, and aeration in containers. 

Fill the container with the potting soil, then follow the same transplanting process as you would for an outdoor garden. “However, keep in mind that container plants require more frequent watering, as potting soil drains faster than garden soil.”