Potting on is the process of moving a young plant into a bigger pot. It’s often done in spring with tomatoes and other tender plants that have outgrown their seedling trays and modules but can’t yet go outside. You can tell it’s time for them to move on – they look ‘top heavy’ in their small pots and their roots may be trying to escape from the bottom. When you tip them out you’re hoping to find a healthy rootball, a network of white roots that holds the compost together.
A pot bound plant is one that has been in its pot too long – the roots will be snaking around and around, with nowhere else to go. Pot bound plants will be struggling for nutrients and water, and if potted on in this condition their circling roots may find it hard to break out into the fresh compost. You have to tease them out – literally pull on them to pull them out from the rootball slightly and give them a new direction in life. It’s a bit of a brutal process, but entirely necessary for pot bound plants. Ideally, they would never get in that condition in the first place.
When potting on you want to move each plant to a pot that is only one or two sizes larger than the one it is in now. They often don’t seem to thrive if they’re swamped in a big pot (although it does depend on the species of plant), and if you’re keeping them indoors you’ll probably be pushed for space anyway.
Water each plant a few minutes before you pot it on – it helps to keep the compost together, to make the pot slide off and to keep the plant happy while it’s settling into its new home. Meanwhile, get your new pots ready with a supply of fresh compost.
You may simply be able to slide the pot off easily. Sometimes they stick; turn the pot upside down and give it a sharp tap to the base. If that doesn’t work and it’s a plastic pot then gently squeezing the sides inwards can persuade the compost to come loose.
Add a layer of compost to the bottom of the new pot so that when you add the plant it will be buried to roughly the same level as before. As I mentioned, lanky tomato and pepper seedlings can be buried more deeply, but some plants don’t appreciate their stem being buried. Sit your plant on that layer and then fill the gaps around it with compost. Push it down with your fingers so that there are no large spaces and the plant is now firmly surrounded with compost. Water it in with a drizzle of water. If you’ve covered the leaves with compost then try and rinse that off so that the leaves can get some light. Again, don’t worry too much if your plant isn’t standing straight up, as it can correct that problem itself.
Depending on the plant, now may be a good time to add a cane or support for it to clamber up.
Earlier in the chapter we met the word thigmomorphogenesis, which describes a plant’s response to air movement and contact. Thigma is the Greek word for touch and appears in another big word associated with plants, thigmotropism, which describes their ability to respond to touching other objects – most easily seen in the way tendrils curl round and latch on. The time to pot on anything with clinging tendrils (like peas) is before it starts to cling on to its neighbours, but if yours have started to tangle then simply snip through the tendrils – there’s no point carefully unwinding them as they won’t be able to cling on to anything else once you have.
Whether you’re pricking out or potting on, you’re disturbing your seedlings somewhat and they will take a little while to settle into their new homes. Again, it very much depends on the species, but pretty soon you will notice that your plants are actively growing once more – getting taller, or sending out new leaves and shoots. Until you see that happening, you need to ensure that they aren’t overly stressed by drought, heat or too much light – they may struggle to get enough water until the roots are settled again. Never repot plants and then leave them out in the sun; pop them into the shade until they recover.
We also learned earlier than plants have holes in their leaves (stomata) through which they take in carbon dioxide and give out oxygen. Plants also lose water through these holes, in a process called transpiration. If the plant is losing too much water it has the ability to close its stomata and reduce transpiration losses. The flip side of this is that it can no longer take in or give out gases, and that’s why plants which are short of water stop growing.
The reason I’m mentioning this now is that your seedlings have learned to regulate their water losses in your lovely indoor controlled environment. Outside there is more light, and drying winds, and transpiration losses are much higher – it’s a harsh lesson for a young plant. The temperatures outside vary more as well, from very hot during the day to very cold at night. Thrust from the window sill out into the border it’s only a very strong seedling that will survive.
Hardening off is a kind of halfway house for young plants. When they’re just about ready to face the world we take them outside for a couple of hours on a nice day – not too hot, not to cold and not too windy. Then we bring them back inside. The following day we leave them outside a bit longer, and longer still the next day, and so on, but we always make sure we bring them back in before nightfall. Not only are nights colder, but they hold the risk of a slug and snail attack that can decimate a batch of seedlings in hours.
A cold frame or a greenhouse can help with hardening off, but neither is strictly necessary. Cold frames, in fact, can become overheated death traps if you’re not around to help regulate the temperature.
There are no hard and fast rules for hardening off. It depends on your plants, your climate and your dedication. Two weeks spent ferrying plants in and out at ever longer intervals is no doubt more than enough; two days is probably cutting it a bit fine. Somewhere in between you and your plants will both become accustomed to them being outside, and their life on the window sill will be over. Even then, remember to keep an eye on the weather forecast and nip out to protect your seedlings (or bring them inside) if a frost is forecast. And don’t forget to protect them from slugs and snails….