COLUMNIST: ELI APPLEBY-DONALD
At this point, we will have split into three groups of growers, all carrying out the same tasks, but divided by time or equipment. Some will be moving their young plants out into the greenhouse, some will be nurturing the tiny seedlings they’ve only recently planted, and others will only now be thinking about sowing tomato seeds - and not one method is better than the other.
If you started seeds indoors with the plan to move them out into the greenhouse later, the end of this month or the beginning of May is when you can do that.
The next step is about moving those plant babies to their final home for the season but we can’t simply plonk them into the greenhouse (or the garden) and hope for the best. Our carefully tended little plants need time to adjust to their new environment, a process known as hardening off. This process is also affectionately called the tomato hokey-cokey because you take them out, bring them in, take them out, and bring them in again.
Hardening off is all about acclimatising your tomato plants. It’s a way to transition from the controlled indoor environment to the harsher realities of outdoor life (well, unheated greenhouse life for the moment). The purpose of this process is to minimise stress and reduce the risk of transplant shock. So, let’s walk through how I spend a week getting my tomato plants used to the conditions in my unheated greenhouse before moving them out permanently.
Plants that have been grown indoors, whether on a windowsill or under grow lights, are used to stable temperatures, minimal direct wind, and stable levels of UV and sunlight. Moving them straight from this comfort zone to an unheated greenhouse where temperatures fluctuate significantly can be a shock. Variable strengths of light and UV can cause scorching on leaves, temperature variations can cause growth to stall, and in extreme cases, the plant may even die. It is less likely to kill the plants when moving from indoors to a greenhouse than it is moving from indoors to the outdoors, but in extreme situations, it is possible. The key is to ease them into their new surroundings gradually.
Over the course of a week, I take my young tomato plants through a step-by-step process to help them adjust to their new home in the greenhouse.
For the first couple of days, I usually put the tomato plants in a spot in the greenhouse that is sheltered from drafts (away from the door or any vents), somewhere that doesn't have direct light - usually at the back corner or under one of the shelves. I only leave them out there for a short time on the first couple of days, maybe for 2 or 3 hours. The goal is simply to introduce them to the idea that they are no longer in a controlled indoor space. I aim to do this in the morning so the light isn’t too strong and, importantly, I make sure to bring them back indoors before the temperature drops in the evening.
Now that the plants have experienced some instability over the first two days, on days three and four I start leaving them there for longer stretches, around 5-6 hours a day. I also bring them out of the shadier spot and let them experience a bit of sunlight. Again, I make a point of doing this in the morning and not in the afternoon when the sun is at its strongest. Even at this stage, they still spend their nights indoors, so don’t forget about them, go bring them back indoors before it gets chilly.
By the fifth day, I feel confident enough to leave my plants in the unheated greenhouse for the full day. The shelter of the greenhouse protects them from wind and rain, but they will now be getting used to the fluctuations in temperature and light levels. I’m still careful about leaving them out in the evenings and overnight because you never know how they might react.
Today is the big day. Today, I leave the plants out late into the evening so that they get their first experience of the temperatures starting to cool down.
By the seventh day, things get real: those little plants are going to have their first ‘sleepover’ in the greenhouse. I usually plan ahead, checking forecasts to pick a week when we are not likely to get any sudden cold snaps overnight, so there are no extremes to shock those little plants.
To be on the safe side, even by day seven, I use a little bit of fleece on the first couple of nights just to provide extra protection, just in case. I drape it over the plants to buffer them from any unexpected dips in temperature. In the morning, I check on them and remove the fleece.
After a full week of hardening off, your tomato plants are now officially greenhouse residents. Time to plant them up into their final home!
If you sowed tomato seeds in an unheated greenhouse in March, you will be nurturing little plants now to encourage them to grow strong whilst still being aware of the cold nights and the potential for frost in the garden.
Potting up those seedlings is going to be your main job for the next month or two, gradually moving small plants from one container into a slightly larger one to provide more space and nutrition. As I mentioned in the previous issue, there is a lot of success to be had in sowing your tomato seeds in an unheated greenhouse in March. You will get strong, sturdy plants due to the cooler temperatures and lower light levels but there will always be the risk, especially at night, for the temperatures to drop and perhaps even go low enough for frost or a thin layer of ice outdoors.
Tomatoes are a warm season plant which aren’t frost hardy, so when the temperature drops like that it can cause the plants to suffer a little from what’s called cold shock. Now I should clarify that it’s not as much of an issue as it would be if you had larger plants, plants that had been sown earlier. For March sown seedlings, these weather blips are pretty minor however, we can still make sure we protect those little plants, just in case.
The easiest way to protect little plants in the greenhouse is simply to cover them. Either an upturned plastic bottle (or something similar) or simply a piece of horticultural fleece is easy to remove when the temperatures are a bit warmer or to replace if we get a sudden cold night.
Like the group who started their seeds early indoors, you are aiming to have strong, healthy plants ready to be planted up into pots or a bed in the greenhouse at the end of April or the start of May.
For those of you planning to grow tomatoes outdoors, be sure to have read my column in the last issue about choosing the correct varieties, and preparing all the seed-sowing equipment.
If you plan to grow tomatoes outdoors, you will start things much later as our gardens in Scotland don’t see those warmer overnight temperatures until usually the beginning of June. We hold off, therefore, on sowing tomatoes until late March or early April so that the plants are not too big and unwieldy before then.
The benefit to holding off is that you can sow seeds in an unheated greenhouse and benefit from the warmer temperatures, and barring any weird weather, you won’t have the same worry of protecting the seedlings. You’ll find seedlings grow fast due to the increasing light and warmth available and they will catch up with those earlier sown plants very quickly.
From now on, it’s all about nurturing those seedlings into strong, productive plants. Over the coming months, they’ll be putting on plenty of leafy growth, and you’ll start seeing the first signs of flowers. To get the best harvest later though, there are a few key tasks to stay on top of.
Tomatoes grow quickly, and if left unchecked, they can become a tangle of leaves and stems. While bush varieties (determinate tomatoes) need little if any pruning, cordon (indeterminate) varieties benefit from regular tidying up.
If you are a keen internet gardener you will see a lot of folk talk about not following a traditional prune, or saying that it will reduce your harvest. Technically this is true, pruning side shoots (suckers) will reduce the harvest compared to leaving them on the plant, but we do this in the UK because we are trying to get the most out of a plant that does not naturally grow in our climate. If you live somewhere warm and sunny, you might choose to let your plants grow naturally and have a huge sprawl and lots and lots of tomatoes. Unfortunately for those of us growing in Scotland, we need to be a bit clever and grow the plants in a way that means they will thrive with our temperatures and season length - so we prune.
Side shoots grow in the leaf axils (the junction where a leaf meets the main stem). For cordon tomatoes, these should be pinched out regularly, as they are essentially the plant creating a whole new stem, which in turn will produce lots more greenery and eventually fruit. As you can imagine this will take a lot of energy and can cause the plant to be much slower at producing those lovely tomatoes that we want.
Simply snap them off with your fingers when they’re small, or use clean snips if they’ve grown a little bigger. Do this every few days to keep the plant focused on growing upwards, producing strong trusses of fruit. Ideally, you are aiming to only have that one straight tall stem on your indeterminate tomatoes.
Lower leaves should also be pruned. As the plant grows, the older leaves at the bottom can become shaded and start to yellow. These don’t contribute much to the plant’s energy production and can encourage damp conditions, increasing the risk of disease. Removing them helps with airflow and keeps the plant healthier— we aim to prune these off up until just above that first set of flowers (truss).
It might seem counterintuitive, but in the early weeks, it’s best to remove the first few flowers. Young tomato plants put a lot of effort into their first blooms, but this can come at the cost of strong root and stem development. If the plant is still small when flowers appear, snipping them off encourages it to direct its energy into growing a sturdier framework first.
A good rule of thumb is to remove flowers from any plant that is still under 30cm tall. If your plants are already strong and growing well, you can let them set fruit earlier. If they are struggling a bit, however, perhaps looking spindly or slow-growing, it’s worth delaying fruit production by pinching off the first truss of flowers.
Tomatoes are hungry plants, and they will need a lot of nutrients as they grow. If you are growing outdoors or in a bed, ensuring you’ve kept up with adding compost to your bed each year should mean your plants will get everything they need from the soil. If you are growing in containers however, you will need to provide regular feed for your plants.
Once flowering begins in earnest, begin feeding weekly with a high-potassium feed, such as a tomato-specific liquid fertiliser, to encourage fruit development. Feeding should start when the first flowers set and continue weekly throughout the growing season.
Supporting plants is something that really only those of you growing indeterminate plants (cordon) will need to think about and you will need to have a plan in place as these plants will get very tall, very quickly.
There are lots of options for supporting these plants, my favourite is to use string and clip the plant to that length of string at various points as it grows. You can either attach the string to the outside of the pot at ground level and then to something strong and stable at head height, or you can bury a tangle of string with the plant when you first plant it into its final position and the roots will grow through this, keeping it really stable. You can then use a length of string coming from there to head height, in the same manner, to attach the plant and provide stability.
Eli Appleby-Donald, one half of 'In the Garden With Eli and Kate' tends an productive urban garden in East Lothian. She considers herself a perpetual learner gardener, and has spent the last 11 years learning by experience, success, and failure.
Eli is a University digital educator by day, and due to the lack of good information about growing in Scotland when she started gardening, she put those skills to action and now regularly vlogs about gardening on YouTube where she has hundreds of videos about her gardening journey and her top tips, as well as appearing as a guest on other channels worldwide.
You can find out more on the fun of creating your own suburban garden paradise on either her YouTube channel, website, instagram, or her chat on the Scotland Grows Show.