Vegetable Seedling Care Guide

Vegetable Seedling Care Guide

This week I am covering seedling care and in particular, what we can do if we need to delay planting out for any reason but have seedling plants ready to go.

Close up of sowing seeds

Now that we approach the end of March and early April, we are coming into prime seed sowing time if we are starting indoors in seed trays or pots for planting outside in May. I think we covered timing already but, as a rule of thumb, most outdoor vegetable crops will be ready to plant out 4 weeks after sowing.

We have looked at sowing and growing in seedling trays already but this week I want to cover looking after seedlings in more detail, especially what to do if your plants are ready but you are not able to plant out.

Plant spacing when planting seedlings

The most likely reason you will need to delay planting seedlings outside is the weather. In most years, the end of April or early May is about right but last year remained cold right up to the end of May meaning we either had to take extra care when planting seedlings or delay for a week or two. The main problem with delaying planting is roots becoming constricted in the plant cells and/or the plants running out of nutrients but there are a number of things you can do to manage this as follows:

Plant nutrition

Most vegetable seedlings sown in modular trays are in a cell about 4cm wide and 5.5cm deep. The compost in this cell will provide 4-5 weeks of nutrients after which time the seedlings will need some extra feed if they are not going in the ground. A light feed of around half the recommended dose of tomato feed will keep the seedlings going; we want to give the minimum possible to avoid a growth spurt which will further restrict plant roots in their cells but just enough to keep the plants growing.

True Leaves & Cotyledons

By the way, we often get questions about seedlings where the lower leaves are yellowing but this is completely normal. The first set of leaves or 'cotyledons' naturally yellow and drop off once the secondary or 'true leaves' take over. If the first (lowest) set of leaves are yellow, there is no cause for concern but if the rest of the leaves are yellowing, the plants do need a feed.

Temperature

The warmer it is, the faster your plants will grow. If you are trying to hold a tray of plants before planting they should be kept cool so definitely not in a centrally heated room and, if you have a geenhouse or polytunnel, not inside on warm days. I would leave trays outdoors as much as possible unless the weather is very cold but bring them in at night if frost is forecast. The trick here is keeping plants as cool as possible while avoiding frost, rain or wind damage.

Hooped fleece cover for protecting vegetable seedlings from frost

Horticultural Fleece
I have mentioned it plenty of times but horticultural fleece is extremely useful for protecting plants at this time of year. A layer of fleece placed over a tray of seedlings will protect them for harsh weather outside and will prevent frost damage in a light frost overnight. I would advise caution if a heavy frost is forecast as the entire rootball can freeze in the tray, in this instance I would bring them back inside.

If it is a little early to plant out (e.g. mid April) but weather conditions are good, most outdoor seedlings can be put in the ground provide they are protected, again fleece will do the trick. We always need to bear in mind that a spell of good weather in April is just as likely to be followed by a spell of cold so don't wait to protect plants, cover when planting and keep them covered until May.

Horticultural fleece protecting a bed of potatoes from frost

Fleece can be simply laid over the new seedlings but take care to weight down with large stones or bricks around the edges to prevent it flapping about which will damage plants. Planting early using fleece can pay dividends if managed properly and can give a head start of 2 weeks or so over seedlings planted in May. On warm days, conditions under the fleece will be akin to the greenhouse with the soil warming and plenty of room for roots to stretch their legs.

Chard growing in a raised bed

What can be planted out, what can't?
Lettuce, peas, broad beans, spinach, chard, kale, calabrese and cabbage are a lot tougher than you think. They will all easily survive a cold snap under fleece and can be planted a little earlier than normal (mid April) if need be. You definitely should not plant out French beans, runner beans, sweetcorn, courgette or any other tender plants as they will definitely be killed by frost and will seriously struggle in cold weather and will be unlikely to survive. This group of plants will need to be kept under protection until at least the first week in May.

Potting on

The other option, of course, is to simply pot your seedlings on to a tray with larger cells or into individual pots. I understand this option might mean purchasing more compost and pots but very often you will have some old pots hanging around and perhaps a half bag of compost from last year. Using old compost is fine, even if it has lost most of its nutrients, we are just giving more space for the roots to spread and can give a liquid feed for extra nutrients if leaves begin to yellow.

Potting on seedlings April 2022

Potting on is more suitable if you have a tunnel or a greenhouse because the more light your plants can get, the better. Easy for me to say because I have plenty of pots and compost, but transferring to a larger container is my preferred option. An extra two weeks or so under cover will give you very impressive large plants and an even better head start in the garden when you are ready to plant out.When potting up seedlings grown in a tray of 5.5cm seedlings (e.g. in standard 84 cell trays), I would avoid using a large pot as roots will be more likely to rot, the ideal would be a larger 6 cell tray with cells around 7cm deep. The key to potting on is that the new container should only be a little larger than the seedling root ball or plug, if you are using pots the new pot should be within the next two sizes up.

Deep planting tomatoes for a stronger root system

Deep planting
If your seedlings have had to grow in less than ideal conditions before planting out and have got a bit tall and gangly looking, remember that in many cases your planting method can bring them back around. 'Deep planting' refers to planting a significant part of the stem in the soil, usually up to the first set of leaves, which transforms a lanky seedling into a nice stocky plant.

Some plants are better than others at this, (tomatoes are particularly well suited as they quickly produce new roots on their buried stems) but nearly all other vegetables can be treated this way. The exception are plants with tender stems like lettuce which will be prone to rot if covered but, as I said, these are the exception.

Brassica Seedlings

A note on Brassica family plants

You will also be surprised just how much punishment most members of the brassica or cabbage family will take and still produce a very nice crop. I would be more careful with cauliflower or Brussels sprouts but cabbages can be left almost for dead with no feed and constricted roots for weeks but will take off like a rocket when they are planted in the soil. I am not saying you should torture your plants on purpose but if cabbage or kale plants have gone past their best you needn't worry, they will make a full recovery.

OK, that's it for today, I hope some of the above was helpful and relevant in your garden. Next week we will be looking at raised bed gardening and how to lay out a garden as I have had a few queries on the same.

See you all next week!

Andrew