Grow Three Plants Outdoors – Part 3: Planting Seedlings and Caring for Plants After Transplanting

Spring is in full swing, and the moment is slowly approaching when the weather will finally allow us to launch the outdoor season. In the third part of our series for outdoor growers, we will focus on planting seedlings in their outdoor location.

In the previous part, we showed how to germinate seeds and raise healthy seedlings from them. If you followed our advice, your plants have at least three sets of true (serrated) leaves and should be ready for transplanting outdoors.

When is the right time for planting

Popular wisdom says that in our latitudes, the right time to plant outdoors comes after the Three Ice Saints, that is, in the second half of May. During this period, night temperatures only rarely fall below 10 °C and during the day they often rise to 20 °C, which is ideal for freshly transplanted seedlings. What, on the other hand, does not suit them are morning frosts, persistent rain or daytime temperatures below 15 °C. Therefore, in addition to the calendar, follow the weather forecast as well, and if it is unfavourable, postpone planting a little.

Hardening off seedlings

The transition from warmth to outdoor conditions is a shock for seedlings. Plants grown at home or under artificial light in a growbox are not used to direct sun, wind or temperature fluctuations. If you plant seedlings outdoors without prior hardening off, their growth may slow down significantly or they may die. Hardening off is usually carried out 7 to 14 days before planting and should proceed roughly as follows:

• Days 1–3: Take the seedlings outside for 2 to 3 hours a day. Place them in a spot in partial shade that is protected from the wind.
• Days 4–7: Extend the time the seedlings spend outdoors to 4 to 6 hours a day and gradually begin exposing them to morning or afternoon sun.
• Days 8–14: Leave the seedlings outside all day. If night temperatures do not fall below 10 °C, you can leave the plants outside overnight as well.

You may also be interested in: How to raise seedlings at home for outdoor growing

Preparing the site

We covered the choice of site for your plants in detail in the first part. There is a difference between growing in a pot on a balcony, in a greenhouse or in a bed under open sky. In the following lines, we will deal in detail with preparing each type of growing site.

Beds

Growing plants in open ground in beds is probably the easiest method. If you have a garden with a patch of fertile soil where the sun shines, you will not have too much work ahead of you. All you need to do is clear the soil of plant remains from the previous season and, if necessary, enrich it with nutrients by mixing in compost, worm humus or a suitable organic fertiliser with slow-release nutrients, which you work into the top layer of the substrate (approx. 30 cm).

In addition to nutrients, soil pH is also important, and it should be in the range of 5.5 to 7.5. It is a good idea at the start of the season to measure the pH of the substrate with a rough tester. If the soil is too acidic (the pH is too low), add dolomitic limestone. You can lower the pH of an alkaline substrate by adding organic matter (for example compost, humus, peat) or a sulphur fertiliser.

You may also be interested in: How to prepare bed soil for spring planting

Pots

Outdoor growing in pots is not difficult, but unlike indoor growing it has its specifics. First of all, always start with fresh horticultural substrate. Soil from the previous season would be poor in nutrients and may contain pests or mould spores. For outdoor plants, highly fertile organic soil substrates are best suited (Canna Bio Terra Plus, Guanokalong Complete Mix, Plagron Batmix, BioNova Bio Soilmix, Atami Bio Growmix), but you can also use pre-fertilised substrates (Plagron Growmix, Plagron Allmix, Plagron Royalmix) or universal soil mixes (Biobizz All-Mix, Plagron Promix, Canna Terra Professional). Soil substrates of the “light-mix” type or coconut or inert substrates can theoretically also be used outdoors, but they require regular watering with precise EC and pH, which can be a problem outside.

You may also be interested in: Which substrate to choose for outdoor growing in pots

What size pot should you use outdoors? In general, the larger the pot you plant in outdoors, the better. A 10- or 15-litre pot for indoor growing is often too small for outdoor plants. A container that is too small has several disadvantages. First, the plants quickly exhaust the nutrients in the substrate and you will have to fertilise them often. In addition, the soil in a small pot will dry out quickly in the sun and need frequent watering. Last but not least, small pots outdoors often topple over in the wind. For outdoor growing, large fabric pots with a volume of at least 20 litres are best, or sufficiently large white plastic pots, which protect plant roots from overheating.

Greenhouse

The advantage of growing in a greenhouse is that under a roof the plants are always warmer and more comfortable than in wind and rain. This is especially true in spring, when seedlings in a greenhouse will grow noticeably faster than those under open sky. The disadvantage of greenhouses is the frequent problems with mould and pests, which thrive in humidity, low air circulation and stable temperatures. Therefore, when cleaning before spring planting, be as thorough as possible, especially if you had problems with mould or insects in the previous season. It is a good idea to place sticky arrows around the seedlings and treat them with a preventive organic spray.

Care for the soil in greenhouses in a similar way as for outdoor beds. What matters is a sufficient amount of nutrients and organic matter, which you can supplement with regular mulching, compost, worm humus or organic fertilisers with slow release. To revitalise greenhouse soil, soil improvers for organic growing are especially excellent, such as BioBizz Premix, BioNova Profimix or living soil substrates. As with outdoor beds, we also recommend measuring the soil pH every year in the greenhouse and adjusting it if necessary.

Planting seedlings and care after transplanting

As soon as your site is prepared and the weather warms up, it is time to start transplanting. It is best to transplant seedlings in the evening or in cloudy weather. Direct midday sun could kill freshly transplanted plants.

Seedlings transplant best when their substrate is partially dry and the root ball can be easily knocked out of the pot. Freshly watered or bone-dry soil tends to crumble. Plant the seedlings into pre-prepared holes. If they have elongated stems, you can bury them partially under the soil, where they will put out new roots.

As soon as you transplant the seedlings, water them with a suitable amount of water. The aim is to give them moisture, not drown them for several weeks in overwatered substrate that never dries out. For several days to weeks after transplanting, you can add a root stimulator to the watering, which will support root formation and help the plants recover more quickly from transplant shock.

In the first few days after transplanting, your plants may look somewhat wilted. However, if you have done everything correctly, they will recover quickly and within a week signs of new leaves should begin to appear. In the next part of Grow Three Plants Outdoors, we will look at fertilising in the vegetative stage and talk about how to prune and otherwise shape plants.