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If you are looking for seed trays to germinate seeds for your garden, you have probably seen the phrase “standard size” or “1020”. Although this wording is common, the exact meaning can vary a bit. So, what size is a standard seed tray? A standard size (1020) seed tray is slightly larger than 10 inches long by 20 inches wide. For example, a standard size seed tray might be 11 inches long by 21 inches wide, with a depth of 2 to 2.5 inches. Sometimes, seed trays have separate compartments (cells) for each seed or group of seeds. This makes it easier to separate the seedlings for transplanting later. Of course, there are lots of different seed tray sizes available, so a standard 1020 seed tray is not your only option. For example, there are 1010 seed trays and seed trays that will hold round pots, instead of having square cells. In this article, we'll talk about standard size seed trays. We'll also take a closer look at the dimensions available, along with some other common questions about seed trays. Let's get started. What Size Is A Standard Seed Tray (What Does 1020 Mean)? A standard size seed tray (also called a “1020” tray) is slightly larger than 10 inches by 20 inches (25 centimeters by 51 centimeters), with a depth of 2 to 2.5 inches (5.1 to 6.4 centimeters). A standard seed tray has a length slightly more than 10 inches and a width slightly more than 20 inches. The phrase “1020” refers to the tray length and width, which are approximately 10 inches and 20 inches. Often, a standard seed tray will measure a little bit longer in both length and width. For example, these seed trays from Johnny’s Selected Seeds measure 11 inches (28 centimeters) long by 21 inches (53 centimeters) wide. This gives a little extra space at the edges of the tray, and allows a slightly larger size for individual cells (if present). As you can see, the seed trays from Johnny’s Selected seeds mentioned above come as one large tray. The trays have no individual compartments (cells) to separate the seeds. A seed tray without separate cells is perfect for growing microgreens. This type of tray is a perfect option for growing microgreens. The reasons is that microgreens don't need their own individual cells. Instead, the seeds can all grow together on one big "field" of soil mix. (The point of microgreens is to harvest them by cutting off the tops, rather than being transplanted at a later date). On the other hand, these 1020 seed trays from the Bootstrap Farmer have separate cells for individual plants. This makes it much easier to space out seeds properly. It also makes it easier to transplant plant plugs without harming the roots of your plants. With each seedling in its own cell, the roots of separate plants will not get tangled up together. When each seedling has its own cell, roots don't get tangled and transplant is easier. There are 50 separate cells in this seed tray (which is 5 cells long by 10 cells wide). The cell dimensions for this seed tray are 1.75 inches by 1.75 inches (4.45 centimeters by 4.45 centimeters), with a height of 2.25 inches (5.72 centimeters). Each cell has a 0.25 inch (0.64 centimeter) drainage hole, which keeps the soil in the cell from holding too much water. The drainage hole also helps to remove the seedling and soil when it is time to transplant outdoors for spring planting. You can even place the entire seed tray in a corresponding 1020 seed tray (without compartments). Then, you can water the seedlings from below by adding water to the bottom tray. That way, you won’t have to worry about over watering any one cell and killing your seedlings before they have a chance to grow. How Deep Should A Seed Tray Be? A seed tray should be 2 to 3 inches deep, depending on what you are growing and when you intend to transplant. The University of Maryland suggests seed trays should be at least 2 inches deep. On the other hand, the University of Iowa suggests using at least 3 inches of soil for starting seeds. Seed trays should be at least 2 to 3 inches deep. Make sure to make holes for drainage if there are none. However, according to Johnny's Selected Seeds, you should avoid trays that are too deep. The reason? You want them to be able to drain well (if the soil stays too wet, seeds can rot, and seedling roots cannot get enough air.) Your seed tray should be deep enough to hold enough soil for starting seeds, plus a little space on top for watering. Otherwise, the water can run right across the top of the soil and over the edges of the tray. Seed tray depth also depends on the type of plant and when it will be transplanted. Deeper trays leave more room for roots to grow deep. They also give you more time before you need to transplant, which can be helpful in cold climates. Remember that larger seeds need to be buried deeper. You can learn more about how deep to plant seeds in my article here. Seed planting depth depends on the type of seed. Of course, you can also repot certain plants (like tomatoes) into larger containers after they outgrow their seed tray cells. This lets you keep them indoors until it is time to transplant outside. You can use this page from the Old Farmer's Almanac to find the last spring frost date in your area to get an idea of when to transplant outside. Do 1020 Seed Trays Need Holes? A 1020 seed tray should have holes to allow for proper drainage. Otherwise, the soil will stay too wet. This can lead to seeds rotting before they germinate, damping off of seedlings, or root rot (which occurs when wet soil prevents roots from getting air). Seedlings may suffer from damping off if they get too wet.Image courtesy of:INAKAvillage211 from:https://commons.wikimedia.org/wiki/File:Pinus_taeda_seedling_damping_off_(cropped).jpg What Other Seed Tray Sizes Are Available? There are lots of other options if you decide to buy a seed tray with different dimensions or cell counts. For example, you can find seed trays with as few as 4 or as many as 200 cells! You can choose from lots of different types of seed trays, including ones that hold several larger circular pots. Let’s explore a few of these seed tray sizes in more detail. 1010 Seed Tray A 1010 seed tray has dimensions slightly larger than 10 inches by 10 inches (25.4 centimeters by 25.4 centimeters). These trays are square, and they are often compatible with 1020 trays. That means you can fit two 1010 seed trays side by side on a 1020 seed tray. A 1010 seed tray is slightly more than 10 inches long and 10 inches wide. These 1010 seed trays from the Bootstrap Farmer also have drainage holes to prevent over watering or to allow watering from a reservoir below the tray. The tray dimensions are 10.5 inches long by 10.5 inches wide by 2.5 inches deep (26.7 centimeters long by 26.7 centimeters wide by 6.4 centimeters deep). 1020 Net Pot Tray If you want to grow plants in separate round pots for either hydroponics or traditional soil gardening, then the 1020 Net Pot Tray is a good option. A 1020 net pot tray will hold 32 2-inch round pots - a good option if you want to give your seedlings more room and time to grow indoors. Instead of square cells, these trays have circular holes to hold individual round pots. These pots are larger than the standard square cells. The larger pot size allows you to give seedlings more room and more time for growth indoors. This helps to offset late spring frosts or a short growing season, which are common in northern regions. This 1020 Net Pot Tray from the Bootstrap Farmer fits 32 2-inch round pots. The tray is 4 pots long by 8 pots wide, with the cells spaced about 0.5 inches apart. These net pot trays fit perfectly into a standard 1020 tray, which allows for watering from below (if using traditional soil gardening). It can also be used in a soilless growing setup (such as hydroponics, aquaponics, or aeroponics). 6 Cell (2x3) Plug Tray Inserts These smaller 6 cell trays from Bootstrap Farmer are perfect if you want to keep different plants in separate trays, but grow them in the same place. Each cell in the tray is 1.5 inches by 1.5 inches (3.8 centimeters by 3.8 centimeters) at the top, tapering down to 1 inch by 1 inch (2.5 centimeters by 2.5 centimeters) at the bottom. The height is 2.25 inches (5.7 centimeters). The inserts are designed so that 12 of them will fit in a standard 1020 tray, allowing for watering from below. What Are The Best Seed Trays? In my opinion, some of the best seed trays are made by Proptek. Their seed trays are made from a durable black plastic that won't decay or break down in sunlight. This is helpful for dedicated gardeners who want to start seeds indoors year after year. It is also nice for budget-conscious gardeners who do not want to pay for new seed trays every year. Proptek sells their sturdy 162-cell seed trays on their own website, but you will need to buy them in bulk if you go that route. You will need to ask about pricing, and they have a certain minimum order quantity. On the other hand, you can also buy 162-cell Proptek seed trays (and other sizes) from retailers such as Greenhouse Megastore. Humidity Domes For Seed Trays A humidity dome is a plastic cover used to trap moisture in a small area so that seeds can germinate better. Humidity domes are sized to fit the corresponding seed tray dimensions. For example, this 1020 humidity dome from the Bootstrap Farmer will fit over a standard 1020 seed tray. Humidity domes are helpful because they make it easier to maintain high humidity levels in soil for germinating seeds. They can also keep seeds or seedlings warmer by producing a “greenhouse effect” if you have a grow light setup above your seed trays (which might be a good idea, since seedlings need light to grow). For more information, check out my article on humidity domes. One word of caution is necessary. Be careful about mixing and matching trays and humidity domes. If you buy equipment from different companies, there can be slight differences in dimensions. Sometimes, these differences mean that a dome or insert will not quite fit a tray. Another (Non-Tray) Indoor Growing Option There is another option for growing indoors, which is a little bit different from the seed trays we have discussed so far. This alternative method is the grow bag. A grow bag is made of durable fabric material, such as canvas. It is used to hold soil and plants. A grow bag is made from a fabric material such as canvas or burlap. A grow bag can be used to grow either one large plant or multiple smaller plants. Larger grow bags can even be used to hold dwarf fruit trees, or to get larger fruit trees started indoors. Grow bags can be used indoors or outdoors, or even planted directly in the ground. Grow bags prevent plants from becoming “root-bound” as they do in standard plastic or clay containers. They are reusable, and can last for years if given proper cleaning and care. You might be able to use a shallow grow bag to start seeds indoors, as an alternative to a plastic seed tray. For more information, check out my article on why to use grow bags. How Many Seeds Should You Put In Each Cell In A Seed Tray? (How Many Seeds In A Seed Tray?) For seeds with a high germination rate, you really only need one seed per cell in a seed tray. In a 162-cell tray, a 95% germination rate means you will only have about 8 empty cells (where the seeds failed to germinate). Remember that older seeds will have a lower germination rate. Some seeds will only remain viable for a year! If your seeds have a low germination rate, I would suggest planting two seeds per cell in a seed tray. That way, there is a very high chance that at least one seed in each cell will germinate. This means that you won’t have many, if any, empty cells. However, in order to avoid competition between plants, it is a good idea to “thin” the seedlings by removing any extra ones. If two seedlings start to grow in one cell, wait for a few days and observe them. Then, leave the stronger one to grow and pull out the weaker one. If two seeds sprout in a cell, it is better to leave the strongest one and remove the others. It seems cruel, and it can be difficult for gardeners to do this! However, if you don’t thin the seedlings, then the roots of the two seedlings will become intertwined. This makes it difficult to separate them out for transplanting later. Even worse, the two seedlings will compete for resources (water and soil nutrients). Ultimately, this will lead to two plants that are both smaller and weaker than one dominant plant would have been. So, make sure to thin your seedlings when the time comes! What Happens If You Plant Seeds Too Close Together? If you plant seeds too close together, you will see overcrowding & competition in your seed trays. The roots of separate plants will become tangled together, making it difficult to separate them out later. This makes it much more likely that the roots of some plants will be damaged during transplant. In turn, this will cause more of your plants to wither and die when exposed to the harsher conditions outdoors when you try to harden them off in the spring. You can avoid this problem if you use seed trays with separate cells (as long as you thin the seedlings when two or more grow in a cell.) If your seed tray does not have separate cells, then you will need to make sure that you use proper spacing for the seeds. Check online or look at the seed packet to find the recommended seed spacing for the plants you are growing. This table from the USDA (on page 2) gives some suggestions for cell sizes for starting seeds of various types, including: BroccoliLettuceOnionsPeppersTomatoes The planting depth for seeds depends on the type of plant. Conclusion Now you know what size a standard seed tray is, and you also know what the cryptic phrase “1020” refers to. In addition, you know about some of the other options available for seed tray dimensions. If you want to learn more about seed trays, how to water them, and how to make your own, check out my article here. I hope you found this article helpful – if so, please share it with someone who can use the information. If you want to read some of my most popular posts, check out the “Best of GreenUpSide” page here. Enjoy! ~Jonathon