When your seedlings are strong enough it's time to plant them out in their permanent home
If you’ve started your seeds off in a propagator indoors or in your greenhouse, you’ll reach a time when they’re ready to be planted in their permanent home outside.
If you haven’t got your seeds going yet, try our guide to growing seeds in a propagator:
The best time of year to transplant your seedlings outside will depend on the kind of plant you’re growing, so make sure you check the seed packet when you’re starting out.
The packet will also tell you if you need to harden off, or pot on your seedlings before they are transplanted. In this guide we’re going to show you how to pot on and harden off your seedlings.
Each plant also has its own environmental requirements which will help you choose where to plant it. For example, some plants prefer to be in full sunshine, whilst some need shadier areas, some plants prefer acidic soil, and some need alkaline. Your seed packet should give you all the important information about what your plant needs to thrive.
Something else to consider is what kind of soil you have so you can prepare it for planting. Some types of soil will need a little work before plants will grow well in it. Read our guide to get to grips with your soil type and find out if you need to do any preparation beforehand.
How to tell when your seedling is ready to be transplanted
You’ll need to wait to transplant your seedling until it is strong enough to withstand the change in climate and humidity. If you do it too early they’ll have a hard time surviving, but if you wait too long your seedling could become rootbound which means the roots grow in a circular nest around the inside of the seed tray as the plant grows.
As plants all grow to different sizes, you’ll want to monitor other signs that your seedling is ready to move, and not just how tall it is. One of the most reliable ways to tell if your seedling is prepared is watching leaf growth. Generally, if your seedling has 3-4 true leaves, it’s ready.
Cotyledons, known as seed leaves, are the first leaves to emerge from the soil when the seed starts to germinate.
True leaves will grow after seed leaves, they’re more textured and complex in shape. True leaves are important because they allow the plant to start gaining energy through photosynthesis, which they’ll need to be able to do to grow.
Some seedlings will need more space to grow but aren’t strong enough yet to be transplanted outside. Seed trays can become crowded, depending on how you’ve sown your seeds, and seeding compost will only give your seedlings a certain amount of nutrients. You’ll need to move them to larger pots to help them continue healthy growth, this is known as potting on.
You can use a clean terracotta, plastic pots, or natural plant pots made from wood fibre, as their new home. The advantage of the natural plant pot is that when you come to transplanting them you can just pop in straight into the ground.
When moving them over, make sure you never hold them by the stem as this can easily bruise, and gently squeeze the seed tray pocket to loosen the soil. If you need to, use a lollypop stick to gently prise your seedling out with it’s roots. Try to get as much of the root ball out in tact as you can.
Fill your new container with potting compost, and make a hole using a dibber. Place the roots of your seedling into the hole and carefully firm up the compost with your fingers. You’ll want to leave about 1cm between the compost level and the top of the pot for watering. Your seedling will now have enough room to grow strong enough for the next stage of its journey.
Hardening off your seedlings
Once you’ve decided your seedlings are ready to make their move they will need to get used to being outside before being planted there, this is known as hardening off. Neglect this step and the sudden change in temperature may kill them.
If you have grown them indoors or in a greenhouse, either place the seedlings outside in a sunny spot for during the day (before bringing them back inside at night) for a couple of weeks or transfer them to a coldframe.
When they are in a coldframe, open the top a little at a time, for a bit longer each day. Gradually increase the time the seedlings spend in the open for around 2 weeks. If there are sudden frosts, use horticultural fleece to protect the seedlings.
Transplanting your seedlings and plants
If possible, try to choose an overcast day to do your transplanting. If you can avoid bad weather, like wind or rain, that will make it an easier transition for your seedlings.
If it’s been particularly wet you’ll need to wait for the soil to dry out a bit, you want it to be moist but not saturated. If it’s been hot, water the soil before you start.
Use a hand trowel or transplanter to make a small hole for each plants.
If you haven’t potted on your plant you can gently remove them from the seed trays, be careful not to hold them by the stem. Try and keep the root mass and soil intact. If you have potted on unpot it by carefully supporting it in a cupped hand and turning it upside down. Try and keep the root mass and soil intact. Don’t pull your seedling by the stem as you’ll damage it. If you’ve used a natural plant pot, you won’t need to remove your plant from it. You’ll just need to ensure your hole is big enough to plant the whole pot in.
Wearing gardening gloves, put your plants into the holes, plant them so that their true leaves are above soil level, and seed leaves are below.
Gently firm the soil around the roots with your fingers and form a shallow basin around the base of the plant, this will serve as a moat to collect water which will drain directly down to the roots. Keep the soil moist whilst your seedlings become established.