When a seed sends out its first shoot and it rises above the soil level, germination is over and seedling development has begun. This is a particularly vulnerable time for the plant – it is running out of stored resources and needs to start collecting its own food. In this period of rapid growth it is also particularly at risk from pests and diseases.
As the plant continues to develop you will see stem growth and expansion of the ‘true’ leaves – they are smaller versions of the adult leaves, different for different species, and easily distinguishable from the plain ‘seed’ leaves that emerged first. Underground, the root system is branching and growing. The reason shoots go up and roots go down is that the plant is responding to gravity – it’s called geotropism or gravitropism. Tropos is the Greek word for ‘turn’, and it is used to describe the different ways that seedlings (and plants in general) respond to their environment.
At the tip of each stem and each root there is a growing point, called the apical meristem. These growing points need to be protected, as early on the plant doesn’t have many and damage to them can render it unable to continue growing. That’s why when you’re handling seedlings you should handle the leaves, rather than the stem. If you break the stem then the plant is unable to continue growing; damaged leaves are more easily replaced. When the plant gets larger it has more growing points, which is why it can respond to being pruned back by growing more bushy. (If you’ve ever seen an oddly shaped flower on a plant then you may have spotted an example of fasciation, which is where a damaged apical meristem causes growth to continue in a distorted way.)
What seedlings need
When you’re caring for seedlings the idea is to give them perfect conditions for growth, so that they don’t suffer a ‘check’ (stress, essentially) that could delay maturity, stunt growth and potentially reduce flowering or cropping. ‘Ideal’ conditions will vary according to species, but seedlings need the following things:
The high temperatures needed to trigger seed germination are no longer required, but many seedlings need a specific temperature range for their continued growth. The most obvious examples are the tender vegetables (tomatoes and peppers) and bedding plants we sow early in spring, before the weather has warmed up outside. They need to be kept indoors until the risk of frost has passed, but keeping them too warm now will encourage them to grow too quickly and become leggy – warmth, light and water levels are the important trio of conditions that regulate the speed of growth.
As soon as the seedling has spread its seed leaves, it can photosynthesize its own food if it has access to light. Seedlings on the window sill bend towards the light – a response which is called phototropism. Seedlings that don’t get enough light grow tall and leggy, with pale leaves and stems – this is called etoilation. You may see it in your tomatoes that are ready to go outside before the weather warms up, but fortunately you can replant them more deeply to offset some of the height and make them sturdier. Severely affected plants may never recover, at which point you may choose to sow another batch. Interestingly, some plants seem to have the opposite response to lower light levels. Pepper seedlings on a window sill may seem to hardly grow at all if they don’t have enough light – but then romp away if moved somewhere sunnier.
Ideally, keep the growing medium for seedlings continually damp but never waterlogged. Seedlings can dry out quickly, and become limp. While they’re limp they’re not growing – a classic check to their growth. You may be able to revive them with some water, but in the meantime they have slowed down. Clean water is your best option, as rain water that has been stored in a butt or barrel can harbour disease. Cold water can be a nasty shock to the system, especially for tender plants, so allow tap water to warm up to room temperature before you use it on seedlings.
If you have chosen an appropriate seed compost then seedling roots should have access to enough air, but you also need to consider airflow around the growing shoots. They need to take in carbon dioxide during the day while they’re photosynthesizing, and expel oxygen at night when they’re respiring – it’s done through tiny holes in the leaves called stomata. Good airflow around seedlings also helps prevent fungal diseases from attacking, and the movement of the air itself encourages seedlings to grow stocky and strong rather than tall and lanky, an effect called thigmomorphogenesis. Outside, seedlings are buffeted by the wind and other plants and animals. Indoors they benefit from being gently touched (try brushing them a sheet of card, or using a small fan to create air currents) if air flow is low.
Now that the seedling has used up the food stored in the seed, and has started photosynthesizing, it needs access to small amounts of nutrients. If your seed compost was essentially nutrient-free then you’ll need to prick your seedlings out into something a bit more nutritious. If it contained some nutrients then your seedlings should be ok for a few weeks, but keep an eye on them for signs of nutrient deficiencies (see the next section). Too many nutrients at this point can ‘scorch’ seedlings, so it’s not a case of applying a lot of nutrients in the hope that they will have more than enough.
If you’re moving from peat-based to peat-free growing media then the differences you’re most likely to encounter are in water retention and nutrient levels, so they’re both things you’ll need to pay special attention to during the transition.
Good care during seedling development means strong and healthy growth – which helps to protect plants against pests and diseases. If something goes wrong then you’ll need to be able to diagnose the common problems and apply a suitable solution, which is what we’ll be discussing in the next section.