How Long Do You Keep Seedlings In A Heated Propagator?

When you use a heated propagator, you will speed up the germination process but you are then faced with the question of how long do you keep seedlings in a heated propagator? If you leave seedlings in a heated propagator for too long you are likely to get faster growth but weaker seedlings. If you remove seedlings from the comfort of the heated propagator too soon any benefit in faster, more reliable germination could be lost by seedlings failing as they are not strong enough to continue growing in a colder environment.

Here we aim to help you get the timing right on taking seedlings out of a heated propagator so that they continue to grow strongly, and you get plants that are healthy and produce lots of blooms or bumper crops.

How Long Do You Keep Seedlings In A Heated Propagator? Getting The Best Results Means Getting It Right From The Start
Before we look at removing seedlings from a heated propagator, let’s take a quick look at what you should do when sowing seeds in a heated propagator to help you get the best results.

Step One – Get The Seed Trays, Cell Trays or Pots For Your Seeds
When using a heated propagator it will consist of a heated base, it might include a cover and it could come complete with seed trays or pots. No matter which type of propagator you are using, you need to make sure that you start off with clean containers for your compost. Small seeds are small enough to sow in seed trays or multi-cell trays, but for larger seeds you might want to sow them directly into pots – such as sunflower seeds or sweet peas. If you are re-using trays or pots just make sure they are thoroughly clean so they have no disease or pests which could contaminate your compost – use something like citrox to wash them before use.

Citrox is a strong, natural disinfectant based on the extracts of citrus fruit. It is ideal for cleaning pots and seed trays before they are reused. First remove as much dirt as possible before soaking in a dilute solution for a minimum of 10 minutes.

Step Two – Select And Fill With Compost
There are many different types of compost available for gardeners to use. In fact the term ‘compost’ is very generic, simply meaning ‘decayed organic material used as a fertiliser for growing plants’

When sowing seeds, especially smaller seeds, we would always recommend using a ‘seed and cuttings’ compost. Seed composts have been milled to a fine texture so even small seeds will be able to set down roots without any obstructions. Good seed composts are also specially formulated so they don’t contain too many nutrients – too many nutrients or mineral salts can inhibit seed germination and can be as detrimental as too little.  

Step Three – Prepare The Compost
This step sounds like it should be easy but it’s all about applying the right amount of water to the compost – too much or too little water can result in seeds not germinating at all!

Ideally, use tepid tap water – not cold water or stored rainwater – and you want to apply sufficient so the compost is moist rather than soggy and too wet. Cold, wet compost will slow down germination and could even rot the seeds before they have the chance to sprout. So, we normally use a fine sprayer to apply the water to the compost, rather than a watering can – but if you want to use a watering can make sure it is fitted with a fine rose.

Step Four – Sow The Seeds And Cover
You now have the important task of sowing the seeds. This can usually be done by hand, gently and evenly distributing the seeds over the surface of the compost. However, if you find this difficult to carry out by hand then specially designed seed sowers have been created to make this easier.

This step then changes depending on the variety of seeds you have sown, so check the seed packet for the supplier’s recommendation. This is because some seeds won’t need to be covered to germinate, some will want a thin layer of compost over them, and others will germinate best if covered with a thin layer of vermiculite.

Why Do We Use Vermiculite?
There are several benefits which vermiculite provides when it’s used to cover newly sown seeds, including:

Step Five – Cover Your Seed Tray Or Pots And Add To Your Propagator
Even though you are adding your seed trays or pots into a heated propagator, to help with germination and to keep running costs to a minimum, we always recommend covering your seed trays or pots. This can be done using a clear plastic bag – or a better choice if available – a clear, ventilated cover which is designed to fit – these tend to be available for most seed trays.

The advantages of using a pre-made seed tray cover compared to a plastic bag include:

Built-in adjustable vents - many pre-made seed tray covers include adjustable vents which help you to control the level of humidity and the temperature which your seedling will be growing in. Maximum growing height - seed tray covers usually have straight sides which ensure there is ample space for seedlings to grow without touching the cover.

The perfect fit – unlike a polythene bag which might be hard to fit and difficult to remove, a sturdy clear cover is made to perfectly fit its seed tray. This makes it simple to lift off the cover for watering or other plant care.

Your prepared seed trays or pots with their seeds are now ready to place within your heated propagator ready for germination.   

How Long Do You Keep Seedlings In A Heated Propagator? Setting The Correct Temperature 
Getting the correct temperature within your propagator for seed germination will depend on both the variety of seeds you wish to germinate and the type of heated propagator which you are using.

You should be able to check on the seed packet for the recommended germination temperature – as shown in the example opposite, where a temperature range of 15 to 20 degrees Celsius is given.  Now you need to be able to reach this temperature range in your propagator. Using a heated propagator where you can set the temperature you require on a thermostat this is very straight forward.

However, what you need to remember when using any heated propagator is that it will only give a lift above the ambient / room temperature of approx. 11 to 14 degrees Celsius. So, in the example above, to reach the minimum germination temperature for the seeds to germinate, the propagator would need to be situated in a room where the temperature is at least 4 degrees Celsius.

When you use a propagator which is controlled by a thermostat, then once the temperature you set is achieved the thermostat will automatically turn the propagator on and off in order to maintain the set temperature.

How Long Do You Keep Seedlings In A Heated Propagator? Maintaining Humidity 
Humidity and being able to control the level of humidity is one of the key factors for successful seed germination. But what exactly is humidity and how does it help in propagation?

Humidity is defined as being ‘the amount of water vapour in the air’. If there is ‘high humidity’ that means that there is a lot of water vapour in the air – so the ‘wetter’ the air will feel.

Humidity is very important to the health of plants and the level of humidity they will flourish under changes through their lifetime. At the seedling stage most plants want a high humidity - 80% or more. Once your seedlings start to grow, with their root systems developing and growing around two to three pairs of leaves then they favour a lower level of humidity – although still relatively high at around 60-70%.

So, in a heated propagator, how do you achieve the high level of humidity required? Here are a few suggestions which will help you to create high humidity.

Step One – Use A Cover
We’ve already discussed the use of seed tray covers in regard to them helping to increase temperatures. What they will also do, especially when used on seed trays in a heated propagator, is retain any moisture present. When the air temperature is warmer – in this case trapped within the cover over a seed tray of damp compost – it can absorb more water, so the humidity level increases.

Step Two – Provide Water
To maintain a humid atmosphere there needs to be water present but what you want to avoid doing is adding too much water to the compost in the seed trays or pots that it becomes waterlogged and the seeds rot. The easiest option is to use a piece of water matting or capillary matting within the propagator – on top of the heated base with the seed trays/ pots on top. Either material will absorb and hold water, so you can water on to this rather than into the seed tray. The heat from the propagator base will then combine with the water to create a humid atmosphere.

How Long Do You Keep Seedlings In A Heated Propagator? Be Patient
So, your seeds are sown and snug within your heated propagator and now you need to wait for them to germinate.

Different varieties of plants take different amounts of time to germinate and you will usually see the expected germination time shown on the back of the seed packet – as shown opposite. In this example it states that seedlings will appear within 7–21 days. This means that in a single seed tray you could be faced with seedlings appearing on different days so it will be up to you to decide when to remove the seed tray from the propagator – basically, when to make the decision that sufficient seeds have germinated that to maintain the health of the new seedlings it’s time to move them from their snug environment.

How Long Do You Keep Seedlings In A Heated Propagator? Once The Seedlings Emerge 
In an ideal world all your sown seeds would germinate and emerge through the surface of the compost at the same time. Unfortunately, this won’t happen.

Your aim when the seedlings do emerge should be to start the process of hardening them off after about 24 hours – as described below.

Like many things in gardening, it can be a case of 'trial and error' and using what you learn for the next set of seeds you sow. 

How Long Do You Keep Seedlings In A Heated Propagator? It’s Vital To Harden Them Off
If you want your seedlings to survive and thrive once outside of the protected, heated environment of your electric propagator then you need to spend some time acclimatising them to the cooler temperatures outside of your propagator. This is a gradual process as you don’t want the seedlings to go into ‘shock’.

Shock or Transplant Shock
When we describe seedlings or plants going in to ‘shock’ we are basically explaining what can happen to plants when they have an abrupt change – that could be a change in temperature (as is the case with our seedlings moving out of a propagator), but it could also be ‘shock’ from abrupt changes in watering, fertilising or light.

Seedlings affected by transplant shock are likely to wilt, loose leaves or simply collapse after you have potted them on. Some seedlings – like tomatoes or chilli seedlings – are more prone to experiencing shock than others. Most will recover over a few days whilst others may need to be discarded and you will have to start again.

This is why hardening off seedlings is so important.

Ways To Harden Off Seedlings Grown In A Heated Propagator
To help your seedlings acclimatise to the cooler temperatures outside of a heated propagator, here are the basic steps we would recommend:

Step One – Remove The Heat But Keep Them Covered
When you have started seeds off in trays or pots which have fitted covers, the first step is to lift these off the heated base of the propagator, so you are leaving seedlings in position in their trays / pots and leaving them covered up to retain some heat. 

You can see how simple this is if you have used a Window Sill Propagator to germinate your seeds. With this heated propagator seven seed trays and covers are used on a heated base. This not only enables you to raise lots of varieties of plants in the same propagator at the same time, it also makes it simple when it’s time to harden them off.

When a tray of seedlings are ready to harden off, simply lift the seed tray and cover off the heated base and place close by – so they are still in the same room with the same lighting etc. but have just been removed from the extra heat.

Step Two – Remove The Seed Tray Cover
Now it’s time to get the seedlings used to even lower temperatures by removing the protected cover from the seed trays. Again, this step can take between a few days up to a week. Start by removing the cover during the day and replacing it at night until you think the seedlings are strong enough to have the cover left off all the time.

Step Three – Transplant In To Pots
During this process, if everything has gone to plan, your seedlings will have been continuing to grow and should be ready to be transplanted from the seed tray they germinated in, into pots where their roots have more space to grow and they can develop in size until they are ready to plant in their permanent location.

When transplanting seedlings we usually use our economy pots. Not only do they come in a range of sizes, so are suitable for different plant varieties, but because they are sturdy and we can reuse them season after season this makes them very cost effective.

Our guide to answering the question how long do you keep seedlings in a heated propagator has been created from our personal knowledge, information gathered by speaking to other gardeners or manufacturers in the gardening industry, by reading gardening magazines and devouring information from books and the internet. We aim to be as accurate as we can, so if you find a mistake, please remember, we’re only human. if you have any queries you can contact us today!