What Is A Plug Plant? (9 Things To Know)

Do you want to grow stronger and healthier plants in your garden with less effort?  If so, then plug plants might be just the thing for you.

So, what are plug plants?  A plug plant is a seedling that was sprouted and grown in a small cell.  Plug plants are often grown together in a large tray with many cells.  Plug plants are more affordable than established plants, but easier than starting plants from seed.

Of course, you can buy plug plants at various stages of development, from small “mini” plug plants to well-established ones.

In this article, we’ll talk about plug plants, sizes, and when to buy them.  We’ll also talk about how you can start your own plug plants at home.

Let’s begin.

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What Are Plug Plants?

A plug plant is a seedling that was germinated in potting soil contained in a small cell.  Usually, many plug plants are grown together in a tray, such as a standard size (10 inch by 20 inch) seed tray.

plug plants
Plug pants grow in individual cells, usually in a seed tray that supports dozens of seedlings at once.

Professional growers produce large numbers of plug plants in greenhouses for commercial use on farms.  However, small growers or gardeners can buy plugs for home use as well.

You can also start your own plug plants from seed (more on this later).  Buying plug plants is more affordable than buying established plants and easier than starting plants from seed.

A plug plant is easy to remove from a tray because each plug plant grows in its own separate cell.  This avoids the problem of tangled roots between neighboring plants.

It also minimizes root disturbance during repotting or transplant of the plug plant.  Another benefit of separate cells is that over watering a single cell does not hurt the other plants in the tray.

What Size Is A Plug Plant?

Plug plants come in three sizes: mini, standard, and established.  The size of the cells in a tray will determine how big the plug plants can get before they need repotting or transplanting.

According to the University of Georgia, smaller plugs take longer to produce mature plants.  Larger plugs produce plants with a larger root system, which improves plant health.

tomato plant roots
Larger plug plants from bigger cells will have better root systems.

There are three basic sizes of plug plants:

  • Mini plugs
  • Standard plugs
  • Established plugs

Mini Plug Plants

Mini plug plants are the first to appear on the market once the growing season begins.  These plugs are the smallest of the three categories, and are grown in small cells.

seed trays
Mini plug plants are grown in small cells and are still fairly young seedlings when you receive them.

Since they are so small, they need the most time and attention to mature.  After you get mini plugs, you will need to repot them and let them grow indoors for a while before transplanting to the garden (more on this later).

Mini plugs are a good choice for an experienced gardener who has cared for young plants before who does not want to start plants from seed this year.

They are also the most cost-efficient choice of plug plants if budget is a concern.

Standard Plug Plants

Standard plug plants are the next to appear on the market.  They are basically just mini plugs that have been allowed to grow a bit longer, though they may be grown in larger cells than mini plugs.

seed tray
Standard plug plants are larger than mini plug plants, since they are grown in larger cells and have more time to mature.

Standard plugs are a bit larger and more established than mini plugs.  As a result, they don’t need quite so much time to grow before you transplant them outdoors.

Still, you will need to repot standard plugs and give them time to grow before you put them in the garden.

Standard plugs are a good choice for intermediate gardeners who are familiar with repotting and are somewhat comfortable with raising plants.

They are more cost efficient than established plug plants, but more costly than mini plug plants.

Established Plug Plants

Established plug plants appear on the market later in the growing season.  Just as mini plugs grow into standard plugs, standard plugs will grow into established plugs.

plant cage
Established plug plants are large and may arrive ready to transplant into the garden.

Established plugs are larger than standard plugs.  As a result, you may be able to transplant them into the garden right after you get them.

Even if you cannot transplant established plugs right away, it still won’t take long for them to grow large enough for transplant.

Established plugs are a good choice for beginner gardeners who want to transplant immediately or soon after receiving the plugs.

They are the most expensive plugs, but they save you time by taking care of seed starting and seedling care.

What Does A Plug Plant Look Like?

A plug plant is grown in a cell.  Usually, many plug plants are grown together in adjacent cells in a tray.

seed tray
A plug plant grows in a cell, separated from other plants to avoid competition and tangled roots.

After you remove a plug plant from its cell, it will have a seedling growing out of soil, which contains the roots.  The size of the seedling and roots will depend on the type of plug plant you buy (mini, standard, or established).

Are Plug Plants Any Good?

If you buy plug plants from a nursery, they will only be as good as the care they were given.  However, many commercial growers have the expertise and facilities to provide excellent care for seedlings.

Proper care during the early stages of growth will lead to healthy plants that can resist drought, diseases, and pests.  When you buy plug plants, you also won’t have to worry about low germination rates or thinning out seedlings.

If you do decide to grow your own plug plants from seed, you will have more control over the care given and the health of the plants (more on this later).  However, there is a lot more work to be done in this case.

You can start your own plug plants from seed, but i takes more time and energy than buying plug plants.

If you like, you can always try your luck at growing your own plants from seed.  If it doesn’t work out, you can order plugs later in the season as a backup plan.

When Should I Buy Plug Plants?

Before you buy plug plants, check the last spring frost date in your area.  You can use this resource from the Old Farmer’s Almanac to find the last spring frost date in your area.

frosted leaf
When it comes to plug plants, frost dates will dictate when to buy.

Depending on the type of plant, you may need to wait until well after the last frost to transplant into the garden.  For example, the Old Farmer’s Almanac suggests waiting until at least a week after the last spring frost to transplant tomatoes outside.

You can check the planting dates for specific plants, based on your zip code, with this tool from the Old Farmer’s Almanac.

Once you know when to transplant into the garden, you can make your calculations to figure out when to buy plug plants.

For example, let’s say you are going to buy tomato plug plants that need 4 more weeks to grow before transplanting outdoors.

Let’s also assume that the date you want to transplant outdoors is May 15.  In that case, you would buy your plug plants 4 weeks before May 15, which is around April 17.

Before you buy plug plants, clear out some space in your house for them.  They will need light from a windowsill or an artificial grow light until they grow large enough to transplant outside.

When your plug plants arrive, check their condition.  Let the supplier know right away if there are any signs of disease or pests on the roots or leaves.

After you inspect the plug plants, you will need to repot them and give them time to grow until they can go outside.  The exception is well-established plug plants that are ready for transplant.

repotting plants
You will need to repot plug plants after you get them.

When Should Plug Plants Go Outside?

As hinted earlier, danger of frost is one of the biggest threats to plants after they go outside.  Some plants, such as tomatoes and peppers, are not frost hardy.

For them, you will need to wait until the danger of frost has passed.  At this point, you will need to harden off your plants gradually to help them adapt to the outdoors (more on this later).

Hey – you can learn about our seed starting course here!

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What To Do With Plug Plants

Some established plug plants are mature enough to transplant into the garden immediately.  For all others, you will need to repot them and give them time to grow.

According to the University of Massachusetts, you should water the plug plant trays 2 to 3 hours before you repot or transplant them.  This makes it easier to take the plugs out of their cells in the tray.

Before you repot, prepare a new pot that is larger than the plug itself.  Fill the pot with potting soil.

clay pots
Choose a pot that is wide and deep enough to hold the plant plug and enough soil for it to grow into.

Next, use a dibber or your finger to make a hole in the soil.  The hole should be the same size as the plug or slightly larger.

Then, carefully remove the plug from its cell.  Squeeze the cell or move the tray gently if the plug does not want to come out.

Once you remove the plug, be careful not to crush the plant stem.  The plant is very delicate in the early growth stages, especially for mini plugs.

Finally, put the plug plant into its hole in the soil in the new pot.  Aim for the soil to be at the same level as before.

The soil should not be so high that it covers the seedling’s stem.  The possible exception is a tomato plant, since it can grow new roots from the stem.

After repotting plug plants, water them to keep the soil moist, but not soaked.  Over watering can cause root rot and other problems for plants.

A spray bottle (plant mister) can also help to control moisture more precisely without over watering your plants.

spray bottle head
A spray bottle is a good way to keep soil moist without drowning your plants.

When the plug plants are large enough and the weather is warm enough, it is time for transplant.  If you have any reservations about a plant, don’t put it in the garden.

It is better to lose one questionable plant than to risk spreading disease to all of them.  You can learn more about repotting and setting out tomato seedlings in my article here.

Why Are My Plug Plants Dying?

One of the most common causes of dying plug plants is over watering.  After a plant is over watered, water replaces air in the spaces between the soil particles.

With the lack of air, the roots cannot breathe and they begin to die.  After a while, this “root rot” progresses to the point where the plants cannot absorb water from their roots anymore.

root rot
Root rot causes roots to turn brown and mushy.

At this point, the plant start to look like it is too dry, even though the soil is wet.  Watering the plant again won’t do any good, since the roots are already rotten.

Your best bet is to use a spray bottle (plant mister) to control the moisture and avoid over watering plug plants.

How To Grow Plug Plants From Seed

It is possible to grow your own plug plants for this year’s garden.  You’ll need to start at beginning: with seeds.

Buy Seeds

The first step is to buy seeds.  I suggest buying more seeds than you think you will need.

seed package
Buy more seeds than you think you will need, since germination, seedling growth, and transplanting never have a 100% success rate.

For one thing, you get a bulk discount when you buy more seeds.  You can then share them with family and friends or save them for next year (unless they are short-lived seeds, such as lettuce or spinach).

Also, you won’t get a 100% germination rate, and you might lose some seedlings due to over watering, under watering, or disease.  You may also lose some plants to transplant shock after you put them in the garden.

All told, if you germinate 90% of the seeds, and keep 90% of the seedlings alive, and keep 90% of the transplants alive, you will still only get a healthy plant from about 73% of the seeds you planted.

So, if you want 25 plants, you will need to plant around 35 seeds.  Buying and planting extra seeds acts as insurance against losses during seedling growth and transplant.

Buy Containers

You have two basic options for growing plant plugs.  One is to buy a seed tray with separate cells (compartments), one for each seed.

You can also use root trainers to produce plug plants.  A root trainer ensures a healthy root system and lets you see the progress of the roots as the plant grows.

According to the University of Georgia, larger cells produce bigger and earlier plants.  Larger plants with bigger root systems are less likely to suffer from transplant shock after you move them into the garden.

However, cell size will also depend on the type of plant you are growing.  For instance:

  • 1 to 1.5 inch cells: recommended for broccoli, cabbage, cauliflower, lettuce
  • 1.5 to 2.5 inch cells: recommended for cucumber, pepper, squash, tomato
tomato seedling
Tomatoes will need larger cells and pots to produce healthy plants.

Prepare The Growing Medium

A good seed-starting mix will contain enough nutrients for plants to get them from seed to transplant.  For example, this 512 mix from Johnny’s Selected Seeds contains fish meal, seaweed meal, and other ingredients that plants can use for growth.

Choose a good potting soil for your seeds, since the early seedling stages will determine plant health later in the season.

A lighter germination mix with extra perlite and sphagnum peat moss is also available from Johnny’s Selected Seeds to help smaller seeds to get the right start.

(If you want to try making your own mix, you can find a large bale of peat moss online from Ace Hardware).

Once you select your growing medium, fill the cells so that there is a little space at the top.  This leaves room for watering the seedlings as they grow.

Control Soil Temperature

Soil temperature is one of the most important factors for seed germination.  If the soil is too cold (or too hot), seeds will take longer to germinate.  They will also germinate at lower rates, which wastes seeds.

The ideal soil temperature for seed germination depends on the plant.  However, you cannot go wrong with a soil temperature of around 75 degrees Fahrenheit (24 degrees Celsius) for seed germination.

A temperature of around 75 degrees Fahrenheit (24 degrees Celsius) is ideal for germinating most plant seeds.

If your house is too cold, you can use a heating mat to warm up the soil to the proper temperature.  For example, this heat mat from Gardener’s Supply Company provides consistent heat for seed germination.

You might also be able to warm up plants with older light bulbs, which give off some heat.  Newer LED bulbs are super-efficient, meaning that they give off mostly light energy and nearly no heat energy.

Manage Light

Ideally, you can put your seedlings in a place where they will receive enough natural sunlight.  For example, putting a seed tray on a windowsill or under a skylight might provide the light your plants need.

However, if growing in a darker room in your home, artificial light may be necessary.  You can use LED grow lights that are suited for seedling growth.

LED lights
An LED gives off plenty of light, but not much heat.

For example, this Stack-n-Grow light system from Gardener’s Supply Company lets you stack up as many layers as you need for multiple levels of seed trays.

Ensure Proper Humidity

Besides temperature, humidity is another critical factor in seed germination.  The soil needs to be moist, but not wet, for seeds to germinate.

After seeds sprout, the seedlings need enough water to survive. However, too much water will “drown” them with root rot or cause diseases, such as damping off.

Watering seed trays from below will help to avoid over watering.  To do this, place the seed tray in another container.

Then, add some water to the outside container.  If the soil in the cells sucks up all the moisture, then add a little more.

As mentioned earlier, a spray bottle (plant mister) can also help to avoid over watering plants.  However, it is less useful for avoiding damping off of seedlings after the seeds sprout.

A humidity dome is another way to make sure that your seeds get the humidity they need to germinate.  If you want to use a humidity dome, pay attention to dimensions and look for a seed tray can fit inside your humidity dome.

Watch For Root Growth

You will know that it is time to repot your plants or transplant them outside by the roots.  Once the plant grows enough, its roots will start to climb out of the bottom of the cell through the drainage holes.

plant roots bottom of plug plant
When you can see roots coming out of the bottom of a plant plug or pot, it is time to repot or transplant into the garden -weather permitting, of course.

At this point, you may need to thin out your seedlings to remove any diseased ones.  If transplanting outside, you will need to harden off any seedlings that remain.


Now you know what plug plants are, when to buy them, and what to look out for.  You also know how to start your own plug plants from seed if you are inclined to do so.

On a related note, you can learn more about seed trays and how to water them in my article here.

I hope you found this article helpful.  If so, please share it with someone who can use the information.

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Jon M

Hi, I'm Jon. Let's solve your gardening problems, spend more time growing, and get the best harvest every year!

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