My Seed Trays Kept Drying Out in Spring – This Expert-Backed Bottom-Watering Method Finally Solved It
I’ve always watered seed trays from below – here’s why and how to do it
May can be full of ups and downs when looking after seed trays. One moment, you need to cover them if night-time temperatures drop, and on another, they dry out really quickly on sunny days. To help with the latter, I have always found that bottom watering seed trays is the most efficient way to keep them healthy and moist for longer.
When you water seed trays from the bottom, the soil soaks in all the water it needs, avoiding over- or underwatering. It also encourages strong root growth and avoids fungal diseases. It can be done either by placing them in a tray of water for 10-30 minutes or by placing trays on a matting to release moisture to them via capillary action.
I ran large propagation setups as a professional gardener, growing hundreds (if not thousands) of plants each year from seed to fill the gardens I worked in. To water seedlings, the setups used both the methods mentioned above to water seed trays from the bottom. It kept seedlings consistently watered and ensured they didn’t dry out as quickly when temperatures rose.
The Benefits of Watering Seed Trays From the Bottom
If you routinely water seed trays by dousing them with a watering can, that is all well and good. But it also comes with issues you may not have thought of, such as how far into the soil the moisture soaks, and what harm comes from water sitting on the foliage all day or night.
When temperatures rise on hot spring days, that small amount of moisture in the top inch of water can quickly disappear. That is not the case when you water seed trays from the bottom, where the soil slowly absorbs water and takes in as much moisture as it needs.
There are no dry pockets left in the soil. Provided you have soaked the tray enough, the soil will be consistently moist.
Indeed, gardening and greenhouse expert Lucie Bradley from Easy Garden Irrigation says that bottom watering is ‘more efficient’ than doing it overhead. ‘I’d always recommend bottom watering seed trays,’ she adds. ‘It helps you not to overwater or underwater your seedlings and saves you time.'
As well as being an efficient way to water seed trays, it encourages stronger seedlings with sturdier root systems.
Gently watering plants from below also reduces the risk of delicate seedlings being damaged. Sometimes the flow from a watering can is temperamental (we have all succumbed to that first moment where water floods out before it settles to an even flow), and it can bend seedlings. There is no such peril with bottom watering.
Finally, when you water seed trays from the bottom, they are less susceptible to common seedling diseases. This includes damping off, where seedlings collapse and die due to overwatering and a wet soil surface.
Also, mildew and fungal diseases are less likely, as the foliage doesn’t sit wet as it does with overhead watering, and there is reduced risk of mold on seedlings.
How to Bottom Water Seed Trays in 2 Ways
There are two different ways to water seed trays from the bottom. One is really simple and can be done by any home gardener on a windowsill, in the home, or in a greenhouse. And the other is more suited to kitting out a greenhouse. Let’s take a look at both:
1. Sit Them in a Tray
This is the simplest way to water seed trays from the bottom, and the method I would advise anyone new to this technique to start with. To do this, you need a large tray without holes, such as these sturdy plastic trays at Amazon.
Put an inch or two of water into this tray and then place your seed tray inside it. Leave the seed tray for 10-30 minutes to soak up the water. When it is ready to remove from the tray, the soil surface should look dark, and the tray should feel weighty.
Pick up the seed tray and allow any excess water to drain away before placing it back on the windowsill or table where it gets the light and warmth for developing healthy seedlings.
Do this routine every time the tray needs watering. Tell-tale signs that a tray is dry include the soil looking light brown, feeling light when lifted, or the soil coming away from the side of the trays.
Alternatively, you could use a soil moisture meter to quickly and reliably measure and tell you when to water plants. You can get a soil moisture meter at Amazon.
2. Use Matting
A slightly more advanced way to water seed trays from the bottom is to use matting designed to hold moisture and slowly release it to the trays.
This is a method I used in greenhouses I worked in to keep large numbers of seedlings watered efficiently. It was more time-efficient than soaking and moving lots of seed trays in and out of water baths.
Such products are known as capillary matting or moisture matting. They can be bought in sheets or rolls and trimmed to size for a DIY watering system. Made of soft, highly absorbent material, it draws up and holds water, releasing it to seed trays sat on top via ‘wicking’ to keep them hydrated.
‘If you place your seed trays on top of the matting, as long as the base of the tray is in contact with the surface of the matting so that the compost can touch the matting through the drainage holes of the seed tray, then when the compost starts to dry, it will naturally lift water automatically up from the matting,’ explains Lucie Bradley.
Gardeners can either regularly soak the matting to prevent it from drying out or install a water reservoir, such as a bowl or bucket, and a wick. This means the matting draws water from the reservoir to stay consistently moist.
You can get a plant watering mat at Amazon that can be cut to size to fit tables or trays and keep plants hydrated. A section of the mat can be trimmed into a wick and placed into a container of water to keep it moist for longer.
Encouraged to garden since she was old enough to walk, Lucie has been working in the gardening industry for over 28 years. This has meant she has been lucky enough to talk to both amateur and professional gardeners every day of the week and is always learning. She believes you can never know everything about gardening as it constantly evolves.
What to Shop
A roll of capillary matting that is three yards long, 21" wide x 1/16' thick. It is made of non-woven polyester, can be cut to size, and is machine-washable.
These capillary mats are 18.5 x 3.5 inches and pair with 12-cell seed trays. Each pack comes with eight pre-cut mats.
These heavy-duty shallow trays make ideal containers for bottom watering seed trays. Each one measures 20 in. x 10 in. x 1.25 in. Buy them in quantities of 2, 5, 10, or 30.
FAQs
Should Seed Trays Have Holes in the Bottom?
If you intend to water seed trays from the bottom, they must have holes. If they don’t, the soil won’t be able to soak up any water. If you make DIY seed trays out of yoghurt pots, food containers, or milk cartons, you need to add drainage holes in the bottom so excess water can escape, and moisture can reach the soil if you water them from below.
How Long Should Plant Pots Sit in the Water When Watering From the Bottom?
The exact time it takes for the soil to absorb sufficient water will depend on the size of the seed tray. When you place a tray or pot in a container of water, it should take between 10 and 30 minutes for the soil to take in enough water. If the soil surface feels moist, looks dark in color, and the tray feels heavy, these are the key signs that it can be taken out of the water.
If you grow plants in containers, you can also take advantage of ‘wicking’ by using self-watering planters. These pots have an in-built reservoir, from which the soil draws water to keep plants hydrated. Such containers offer an easy, low-maintenance way to keep plants watered during the hotter months. You can even make your own with this easy-to-follow guide to make a DIY self-watering planter.
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