What is better for your seedlings; top watering or bottom watering?
Lots of online sites and social media discussions recommend bottom watering for seedlings – are the claimed benefits real? If rain works in nature, why do we need to bottom water when starting seeds in the house? I am going to sort all this out in this blog.
Watering Seedlings vs Mature Plants
There is little difference between watering seedlings or mature plants, provided you water gently enough so that you don’t damage the plant. I’ve discussed bottom water for houseplants in Watering Houseplants – Top or Bottom? Which is Best?
What Is Top And Bottom Watering?
Nature uses top watering. Water falls from the sky onto the plant and soil, and slowly filters down with the aid of gravity. In the home you usually use a container with a spout to pour water on the soil.
Bottom watering, also called wicking, is done by setting up a tray, adding water to it, and setting the pot containing the plant(s) in the water. Water seeps up into the pot by capillary forces to saturate the soil. In some cases wicking is done by placing some string right in the pot so that some of it dangles out the bottom and sits in water.
Some people bottom water so that the top of the soil never gets wet. This is not proper bottom watering and can cause problems. For one, you never know how much of the soil is wet – the roots may all be dry after watering. Secondly, you don’t know when to water again. The drying process on the surface of the soil lets you know when its time to water.
Claims For Bottom Watering
Proponents of bottom watering make a number of claims as to why it is the better way to water plants.
Bottom Watering is Gentler
Top watering is also gentle, if you are careful. Just run the water down the side of the pot and let it soak across the soil with capillary action. Keep in mind that by the time you see leaves above the ground the root is already quite deep because the root develops first, then the stem and leaves.
Seedlings Need a Growing Medium That’s Constantly Moist
That is not really true. They can’t dry out completely, but they are not harmed with some drying of the soil. In fact it encourages a deeper root system. Keeping them constantly wet can harm them.
Bottom Watering Prevents Pests And Diseases
I have no idea why people make this claim. If anything top watering is better for preventing pests and disease. With top watering the surface of the soil is more likely to dry out, because the pot is not sitting in water for extended periods of time. This drying reduces damping off disease and makes the surface less appealing to fungus gnats. If done properly, the method of water has no impact on pests and diseases.
Bottom Watering Prevents Overwatering
Let’s first define “overwatering”. It has nothing to do with how much water is in the pot. Instead it refers to the frequency of watering – overwatering happens when someone waters too often.
Overwatering is much more likely to happen with bottom watering because the pot sits in a tray of water and if it is left too long, it will continuously water the pot for days on end. That is overwatering and can be detrimental to plants.
You can also overwater (i.e. water too often) with top watering, but that is easy to fix – just don’t water as often.
Promotes Healthy Root And Plant Growth
It is claimed that with bottom watering “the roots of the seedlings spread more uniformly”. No supporting science for this claim is given.
This makes no sense since the water profile in the soil (see below) is the same with both methods.
Prevents Root Rot
For some reason people think bottom watering adds less water to the pot, but that is not true, as explained below. The method of water has no impact on root rot. If the soil is too wet after watering, the soil type needs to be changed – not the watering method.
Works With Very Dry Soil
This claim is valid especially with peat based medium. When it dries completely, it is hard to re-wet especially with top watering because the water just runs past the soil. The best way to solve this problem is to stand the pot in some water and let it sit for several minutes until the root ball is totally wet.
Bottom Watering Prevents Soil Compression
This one was contributed through our Facebook Group. “If you’re using a peat based seedling potting mix, bottom watering keeps the mix from collapsing, hence it stays aerated.”
I think this is valid, but I am not sure it is important? Top watering does settle the medium in the pot, but if this compressed soil does not provide enough aeration for the seedling, the solution might be to change the media.
When I move small seedlings to new pots I always water from the top right away to settle the soil around the roots. I want roots touching soil and not be stuck in large air gaps.
Claims For Top Watering
There are also some claims for top watering.
Washes Fertilizer Salts Out Of The Pot
Watering from above allows you to use excess water which then runs out the bottom of the pot, taking excess fertilizer salts with it. This can be especially important when you are using hard water or when you are watering seedlings.
Top Watering Gives Better Control
Top watering gives you better control since you can treat every pot differently. Some pots may be dry while others are still moist enough that they don’t need to be watered. If you have a mixture of plants you can water each one differently. If you bottom water one pot at a time you also have such control.
Amount Of Water In A Pot
Does the method of watering affect the amount of water in a pot? A lot of people seem to think there is a difference, but there isn’t.
When water is poured in from the top it moves sideways by capillary action and down by gravity. Excess water comes out the bottom. Once all this movement of water is complete, the upper part of the pot has water soaked media with air spaces between the particles of media. The bottom of the pot has a layer of soil where both the particles and the spaces between them is full of water; something gardeners call a “perched water table”.
The amount of water in the pot is determined by the pot shape and size, the amount of soil and the type of soil used.
What happens with bottom watering? Water moves in from the bottom and creates the equivalent of a perched water table because the water level is above the bottom of the pot. Water then moves up by capillary forces until it reaches the top of the soil. When the pot is removed from the water tray, any excess water drains back out, but the perched water table remains.
The amount of water in the pot after bottom watering is exactly the same as with top watering.
Which Method Is Easier?
Some claim bottom watering is easier and faster while others make the same claim for top watering. The answer depends on your setup. If you have seedling trays with solid trays under them, it is faster to pour water into the bottom tray rather than water each individual cell. If you have a number of larger plants that are not sitting in a suitable tray, it is a lot of work to move each one to a tub or sink to bottom water.
Numerous people grow streptocarpus with a wicking system. Setting up the system is more work, but watering from below is faster once it is set up. Wicking is more work if you move your plants around a lot or grow a lot of seedlings.
Either system can be faster depending on your setup.
What do the experts do? They must know which method works best. Both methods are used, but top watering is the more popular method. If it didn’t work, commercial operations would not use it.
African violets are known to be very sensitive to having water sit on foliage, so I decided to ask an expert. I contacted HoltKamp Greenhouses, the producer of Optimara violets. Those are the African violets you see everywhere and they are “Earth’s largest grower of African Violets”. I asked them if they use top or bottom watering. I got a reply directly from their Vice President, Reinhold Holtkamp and he said, “In our greenhouses we use temperate water and overhead water our plants. Once they start to flower we water from the bottom using tables with a capillary mat that allows water to be more evenly distributed. “ Both systems work.
Working With Tiny Seeds
Starting tiny seed is done by sprinkling them on top of compressed media. You don’t want them covered with soil and you don’t want them too deep which is what happens with top watering. It is best to bottom water at least until the seed germinates.
Some claim you can’t water small seedlings from above, but I disagree. My streptocarpus seedlings are some of the smallest I have ever dealt with, and once they germinate, I water gently from above. Seedlings are tougher than you think.
Which Is Best – Top Or Bottom Watering?
I don’t think there is a best method. I water almost everything from above because I have more control over it. A pot that has a tiny seedlings dries slower than one with a large seedling and so I might skip the wetter pot. I can do that with individual top watering but could not do that with bottom watering.
My orchids grow in coconut chunks which soak up water more slowly, so I like bottom watering. I just let them sit for 15 minutes and then empty the pots.
The idea that only bottom watering works is wrong as are many of the claims for bottom watering. That does not mean it won’t work for you. It just mains that the reasons people use for bottom watering are wrong. Use it if you like it, or when you have a special situation. If you like top watering, do that because it also works.
Tips and Tricks for Watering Plants
Here are some more posts about watering plants.