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What wrong with my Seedlings?

Monday, 5 February 2018  |  SimplySeed

Common Seedling Problems And How To Avoid Them - Reasons why seeds and seedlings fail 

Sowing plant seeds and then patiently waiting for them to germinate can be both personally exciting and very rewarding. For anyone who is new to growing plants from seed, there is no describing the sheer delight of seeing the first signs of tiny green leaves breaking the surface in a tray or pot of seed compost.

Over the coming days, and as the seedlings begin to quickly grow, the sense of achievement can be overwhelming. All of your efforts and preparation will have been duly rewarded, and many new and young lives will now depend entirely on you for their existence and survival. It’s quite a responsibility but one that many novice gardeners relish. Even many highly experienced, all-round, weather-beaten gardeners still secretly marvel at the ‘bursting into life’ process of seed germination.

But what of those that don’t make it? What of those that never see the light of day and those that start life with a spurt only to collapse and die a few days later? Is it due to a lack of care or attention? Or could it be put down to ignorance or sheer inexperience? As feelings of disappointment turn to despair and failure, let’s examine why your precious little plant embryos failed to put in an appearance or why they sadly perished at such a tender age. As some measure of compensation, you can be reassured that lack of success in the greenhouse is not always entirely the fault of the grower… although quite often it is.   

Old seeds and poor stock

There are several reasons why your seeds might not successfully germinate and why your seedlings never get to grow into adult plants. First thing to remember is to always sow fresh, quality seeds. Seeds that are collected and stored yourself could have a very poor germination rate if they are not stored correctly. They need to be dried, then sealed into airtight containers and kept in a dry, cool place that has a constant temperature below 4C. A fluctuation in temperatures or if the temperature is too warm then the aging process will affect the viability of the seeds. Also, poor and unhealthy stock plants are likely to produce poor seeds, which may germinate but produce sub-standard plants. For best results, it’s always advisable to buy fresh seeds each year from a reputable seed merchant, and especially a merchant that offers some form of guarantee.

Damping off

Damping off is quite a common problem and can affect some seedlings more than others. The term ‘damping off’ refers to a condition that affects the stems of seedlings, which begin to get thin and eventually collapse. The seedlings will then die.

There are a number of reasons for this condition, any of which or a combination of which, will usually be responsible. To avoid seedlings damping off always use a proprietary seed compost rather than mix up your own. Seed compost needs to be sterile so that no fungi or other soil borne diseases can grow and spread, any of which, could harm the germination rate or growth of the young and vulnerable plants.

It’s also important to ensure the seed compost or the newly germinated seedlings are not over-watered at this crucial early stage. Water should be clean and free of debris and insect larvae.

The circulation of air is also important and for this reason you should always sow seeds thinly in trays to ensure air can freely circulate around the base of, and in between young plants.

Finally, temperature plays a vital part in the early development of healthy seedlings, so a constant warm temperature is very beneficial. A cold and wet environment is almost always certainly fatal for delicate seedlings, especially sensitive plants like summer bedding varieties.   

Sowing seeds too deep or too shallow

Many gardeners make the mistake of sowing seeds too deep and then wonder why seedlings have not emerged through the surface of the compost. It takes energy for the tiny plants to break out of their case then grow upwards towards the light, so if they are sown too deep, they will simply exhaust their energy stocks and die on the way.

If sown too shallow they might struggle to get their roots down deep enough to support themselves and will keel over. Although they can recover from this they will end up having bent or twisted stems as they try to get themselves upright.

As a general rule, the bigger the seed the deeper it needs to be sown. For example a sunflower seed would be sown at a depth of 1.5cm, whilst a bedding begonia, whose seeds are as small as coffee grains, will only need to be gently sprinkled over the surface of damp compost.

Some seed merchants now offer the tiniest seeds in pellet form to ease the sowing problems associated with them blowing away as soon as you open the seed packet. Every packet of seeds bought from shops and garden centres should have clear instructions relating to sowing depths, on the back.    

Watering too much and watering too little

When sowing any type of seeds in pots or trays the compost should be wetted prior to the seeds being sown. This is to ensure there is adequate moisture available to encourage the seed to germinate without the need for further water being added. When covering seeds with a layer of compost, the top layer can be lightly watered with a watering can and fine rose attachment before covering with glass or polythene. No more water should be required before germination takes place. After germination the seedlings will only need watering if the compost is dry or begins to dry out.

If you apply too little water at this early stage, the seedlings can wilt and quickly die. If applying too much water, you will run the risk of the seedlings rotting off at the base. As a general rule it is best to place trays of seedlings in water for a few minutes, so that the young plants are watered from the base and not overhead. Watering overhead on a hot day, especially with fierce sun penetrating the greenhouse, will almost definitely scorch the delicate leaves. If you have to water overhead then carry out this task in the early morning or early evening when the sun is in a low position in the sky.

Problems caused by light deficiency and poor air flow

All young seedlings need adequate amounts of light in order to begin the process of photosynthesis. Basically, this is the term used to describe how they convert light into energy so that they can produce more green leaves and grow stronger. If there is insufficient light then the seedlings will become ‘leggy’ as the reach out towards whatever light is available. Tall, skinny plants will eventually topple over so always ensure seedlings are placed in a position, in which they can receive plenty of light all around. The free flow of air around young plants is also important in order to avoid fungi and mould growth on the compost surface, which could then spread to the stems of seedlings. It’s best not to leave germinated seedlings too long before transplanting them into modular trays or pots.    

Slow growing seedlings or stunted growth

When seedlings first appear on the surface of the compost they will grow at quite a brisk rate. However, when they are first transplanted they will initially slow down or stop growing for a short while. This is because they have been ‘checked’ by the transplanting process. Their root systems will have been slightly damaged and therefore will need to recover and regrow so that they can feed, and also to become anchored into their new positions. Generally, within a few days they will begin to grow again as normal. However, should they not, then it could be because of a nutrient deficiency.

Most seed compost mixes bought from garden centres will contain enough slow release nutrients to get the plants growing but if mixing your own then take care to ensure the quantities are right. Too little and the plants will be stunted. Too much and you risk damaging the roots of the young plants through ‘burning’. Temperature is also an important factor at this growing stage. If it’s too cold, the plants will not grow and there will be a similar reaction if the temperature is too warm. The ideal growing temperatures for most seedlings inside a greenhouse during March is 10C at night and a maximum 18C during the day time. Overwatering will also stunt the growth of plants and this can often be detected by yellowing on the tips of leaves. 

Summary, tips and other considerations

Growing plants from seed is not as daunting as it might first appear as long as you adhere to a few golden rules. Here are few reminders to help you get the best from your seeds and seedlings.

● Always use fresh certified seed from a reputable seed merchant. Sealed foil packs are usually best and highly recommended. Check dates and information about sowing. Remember, the germination percentage rate will fall dramatically as seeds age.

● Always use fresh seed compost for sowing seeds and then discard by adding it to the compost heap or spreading it over the garden. To ensure young plants grow healthily only use seed compost once.

● Ensure there is sufficient light and heat before sowing seeds. Greenhouses, propagators and growing lights all have distinct advantages over simply growing on a window sill or in a cold frame.

● Ensure the place you are growing your seedlings is airy. Good ventilation and circulating air will prevent humidity build-up. Humidity, although fine for mature plants, can cause fungal growth, mould and damping off in seedlings.

● Take care with watering. Seed trays are best watered from below. Transplanted seedlings can be watered overhead during mornings and evenings when the sun is low in the sky so as to avoid scorching the delicate leaves.

● Young growing plants will need nutrients in order to grow strong and healthy. Many proprietary potting composts will include a general slow release fertiliser but as the seedlings grow, a weak, general liquid feed, applied twice a week will also prove beneficial. * Carefully follow the instructions and recommendations on the label.

● Other things to consider when growing plants from seed are pests. Just one slug or snail could do untold damage to any number of juicy young seedlings, so ensure none have inadvertently entered your greenhouse. The same goes for mice, who will happily feast on plant seeds of all kinds along with seedlings of any type. Be vigilant against whitefly and greenfly.