What Is A Seed Tray Used For? (9 Common Questions Answered)

If you are planning to expand your garden this year or grow plants in bulk, you might want to try using seed trays. They will save you time and effort when starting seeds, and there are lots of seed trays to choose from.

So, what is a seed tray used for? A seed tray is used to start plants from seed until they germinate. A large seed tray can hold dozens of plants (depending on the spacing or cell size). A seed tray makes it easier to move and care for lots of seedlings at the same time. A seed tray with individual cells makes transplanting easy.

Of course, seed trays are not magic. You still need to keep the soil wet enough and maintain the ideal temperature to ensure good seed germination rates in a reasonable time frame.

In this article, we’ll talk about seed trays and what they are used for. We’ll also answer some common questions about seed trays.

Let’s get started.

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What Is A Seed Tray Used For?

A seed tray is used in gardening to start plants from seed. I like to think of a seed tray as a germination factory – useful when you are trying to start lots of plants at the same time in the same place.

(If you want to buy trays, you can find some from True Leaf Market).

Depending on the spacing or cell size, a seed tray can hold dozens of plants (some can hold over 100 plants!) Depending on when you can transplant, you might want larger spacing or cell sizes to avoid overcrowded plants, tangled roots, and competition over resources.

A seed tray makes it easier to care for lots of seedlings at the same time (watering them, keeping them warm, monitoring them for disease, etc.).

spray bottle head — It is easy to water an entire seed tray at once with a spray bottle.

A seed tray with individual cells makes transplanting easy. However, there are also some good reasons to use open trays with no cells.

Let’s start off by getting a look at some different type of seed trays to give you an idea of what is available.

What Does A Seed Tray Look Like?

A seed tray is usually rectangular (unless you fashion your own from a round plastic container). Often, they are made of black plastic, but you can find them in other colors and made from other materials.

There are three basic types of seed trays:

Open seed trays – these have no cells or rows, and are basically just one big open area for planting seeds.
Row seed trays – these have no cells, but they do have rows. This makes it easy to sow different types of seeds in the same tray and keep track of what is in each row.
Cell seed trays – these trays have individual cells. Usually, you plant one seed per cell and aim to get one plant per cell. However, if germination rates are low for your seeds, you might plant 2 or more per cell (and thin the seedlings later).

Some seed trays have rectangular cells, while others have rounded cells. It really comes down to preference (and sometimes, what you can find or what is available).

You can see some row seed trays in this article from the University of Maryland Extension.

How Big Is A Seed Tray?

The standard size of a seed tray is 10 inches by 20 inches (often called “1020”). In reality, a tray has dimensions a little larger than 10 by 20.

For example, some seed trays are 11 inches long by 21 inches wide. Depth of the tray can vary, but 2 to 2.5 inches is fairly common.

There are also other sizes of seed trays available. For example, a 1010 square seed tray is approximately 10 inches long by 10 inches wide.

empty seed tray — You can also find square seed trays, such as 1010 (10 inch by 10 inch) trays.

In addition to the seed tray dimensions, you also need to consider the cell size and number of cells. A 1020 tray with more cells means that each cell has a smaller area.

More cells means more plants in a tray. However, the smaller size of each cell means that plants will become root bound sooner.

So, if you live in a cool climate and cannot transplant until later, don’t start your seeds too early! Otherwise, you will have to repot your seedlings from cells into individual pots before they go outside.

repotting plants — If you plant too early, seedlings may outgrow the cells in a seed tray. In that case, you will have to repot before transplant.

(For example, you can learn about repotting tomato seedlings here).

Of course, you can also fashion your own seed tray by repurposing common items (such as plastic food containers). The size of these seed trays is only limited by what you can find or make.

(By the way, you can find lots of ideas for DIY seed trays here).

How Many Plants Should Be In A Tray?

The number of plants in a tray depends on several factors, including:

Seed Tray Size (how large is the seed tray – what are its dimensions?)
Seed Spacing (how far apart do the seeds need to be to avoid overcrowding?)
Cell Size & Number (how many cells are there in the tray, and how large are they?)
Seed Germination Rate (what percentage of planted seeds do you expect to sprout and grow?)
Purpose (for example, you won’t need much space between seeds for microgreens, but you will want space between tomato seeds)

You can learn about growing microgreens indoors here.

If you are looking for a foolproof, soil-free microgreens growing kit, try out The Good Box (you can learn more here!)

The larger the seed tray, the more plants you can fit inside. A 1020 seed tray will fit twice as many plants as a 1010 seed tray.

For an open seed tray with no cells, you will need to pay careful attention to spacing. Follow the seed packet advice for spacing (unless you plan on thinning out some of the seedlings later).

seed package — Follow the spacing advice on the seed packet when sowing in an open seed tray.

The exception is if you are growing microgreens. In that case, spacing doesn’t really matter, since tangled roots are not a problem – you will just be cutting off the tops of the sprouts to eat!

(You can learn about how to grow delicious hydroponic microgreens here.)

For a cell seed tray, you should plan on one plant per cell. However, this doesn’t necessarily mean one seed per cell.

Remember that germination rates vary, and usually they are not 100%. For example, let’s say that the germination rate for a seed packet is 90%.

winter melon seeds — Seed germination rates vary, but you should never expect 100% of your seeds to germinate.

If you plant 100 of these seeds in a tray (one per cell), you would only expect around 90 (90% of 100) to germinate. The other 10 would be “duds”.

To improve your odds, you could plant 2 seeds per cell. At that point, you would expect 99 of the cells (99% of 100) to have at least one sprouted seed.

If two seeds sprout in a single cell, you can thin them (keep the healthier seedling). Most of the time, it makes sense to do this to avoid over crowding of plants.

seeds sprouting GU cover page — You might need to thin seedlings if two seeds germinate in a single cell.

Using a seed tray with more cells will naturally mean smaller cells. However, this is only a problem if you plan on getting an early-season harvest.

According to the Michigan State University Extension, the harvest size of pepper plants did not change with seed tray cell size. (However, there were differences in plant size earlier in the season, before harvest).

Just remember that smaller cells can dry out faster than large cells, and cell trays dry out faster than open trays. So, keep this in mind to make sure you water your seeds and seedlings properly.

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Where Do You Put Your Seed Trays?

Before germination, the best place to put seed trays is in a spot that is warm and easy to access. Easy access will allow you to keep the soil wet without trouble.

seedlings in seed tray — Before germination, put seed trays in a spot that is warm and easy to access for watering and care.

After germination, you should move the seed trays to a spot that is also bright (in addition to warm and easy to access). Seedlings need water, warmth, and light after they sprout (grow lights are probably your best bet for providing light to seedlings).

Some seeds need temperatures of 70 degree Fahrenheit (give or take) for ideal germination. If you don’t keep your house that warm, you might just want to heat up a small space.

Luckily, there are lots of ways to keep seeds and trays warm, including:

Heat Mats
Humidity Domes
Space Heaters

A space heater can help to keep seed trays warm if you don’t want to heat up your entire house!
Image Courtesy of user:
Roger McLassus 1951 via: Wikimedia Commons: https://commons.
wikimedia.org/wiki/File:Fan_heater_2005.jpg

You can learn more about how to keep seeds warm (both indoors and outdoors) here.

Do Seed Trays Need A Lid?

Seed trays do not need a lid. Seeds will still germinate just fine without being covered.

However, a lid might help to retain heat or moisture in the soil. This is useful if you want to save time on watering.

A humidity dome is one type of lid you can use for seed trays. A humidity dome is usually made of clear plastic.

plug plants — A humidity dome can help to trap heat and moisture in the soil in a seed tray.

The dome traps heat and moisture underneath (via the greenhouse effect) to improve conditions for seed germination. However, it still allows light through so that seedlings can grow after germination.

If you don’t have a humidity dome or the material to make one, you can use something as simple as a piece of cardboard to cover a seed tray.

cardboard strip — You can use an old piece of cardboard as a lid for a seed tray to retain water and warmth in the soil.

Just remember to remove the lid from your seed tray after the seeds sprout (remember that they need light to grow after germination!)

Do Seed Trays Need Sunlight?

Seed trays do not need sunlight before germination. This is because most seeds do not need light to germinate (there are some exceptions – read this article to see some of the common ones).

After germination, all seedlings will need light to grow. Sunlight is great, but if you cannot get enough natural light, a grow light will work fine.

sunlight through trees — All seedlings need light after germination. Sunlight is great, but grow lights will work too.

You can put a grow light on a timer to give seedlings the proper amount of light and darkness as they grow.

How Long Can Seedlings Stay In Trays?

Seedlings can stay in trays for several weeks. However, it really depends on the type of plant you are growing.

If you leave plants in a seed tray too long, their roots will become a problem. This is true no matter which type of seed tray you are using.

root bound plant — Any plant can get root bound in its container. The same thing can happen for seedlings in the cells of a seed tray.

In an open seed tray, the roots will get tangled as plants grow. This makes it more difficult to transplant, since you have to either

spend time to untangle the roots
break some of the roots (which can hurt or kill plants)

In a cell seed tray, the roots will eventually outgrow the cell. If a plant gets too big, it will become root bound in its cell.

This can stunt its growth in the short term (and sometimes in the long term).

In general, you should follow planting recommendations based on frost dates. If you start your seeds on the recommended dates, they should not outgrow their cells before transplant.

Are Seed Trays Necessary?

Seed trays are not necessary to grow a good home garden. However, like many other gardening tools, they make life easier by saving you time and effort.

I would say that seed trays are necessary if you are a serious gardening enthusiast and you have a large garden with dozens of each type of plant. They are also necessary if you have a small to large commercial operation.

Seed trays make it easier to move, water, and care for hundreds or thousands of plants. But don’t worry if you cannot afford to buy seed trays – there are ways to make your own.