Can You Grow Plants In A Sponge? (Don’t Forget This Step)


Maybe you want to start seeds indoors without using soil, or maybe you just want to grow microgreens for fresh eating.  Either way, you might be wondering how a sponge would work for growing plants from seeds.

So, can you grow plants in a sponge?  You can grow plants in a sponge.  As long as you keep the sponge wet, the seeds should germinate if the temperature is warm enough.  To grow the seedlings further, you will need to either transplant them into soil or a hydroponic system.

Of course, the time that seeds in a sponge need to sprout will depend on the type of plant and the temperature.

In this article, we’ll get into the steps you should take to grow plants in a sponge. We’ll also discuss how you can ensure a good germination rate for your seeds.

Let’s get started.

Can You Grow Plants In A Sponge?

You can grow plants in a sponge and they will germinate (sprout) just fine.  Just remember that seedlings will eventually need more than a sponge if you want them to continue growing into full-fledged plants.

When you germinate seeds, the first thing you need is a growing medium.  A growing medium is simply the material that the seeds grow in.

sponge
A sponge is one choice for a growing medium to start seeds, but there are many others.

You can use many things as a growing medium for seeds, including the following:

  • Topsoil in your garden
  • Potting mix from a garden center
  • A sponge

To encourage seeds to germinate in a sponge, the most important factors are:

  • Water
  • Oxygen
  • Warmth

A sponge needs to be kept moist (just like soil) in order to give seeds the chance to soak up water and begin the germination process. However, if the sponge is too wet, the seeds won’t be able to get oxygen from the air, and they will drown.

sponge
A sponge that is too wet will drown seeds, so don’t overdo the watering!

A sponge also needs to be kept warm enough so that the seeds will break dormancyDormancy is the state of seeds during the winter months, when it would be too early to germinate (since the cold would kill them!)

Temperature is the indicator that tells plants when winter is over.  When temperatures get high enough, seeds “know” that winter is over.  At that point, it is usually safe for them to start sprouting.

However, sometimes warm temperatures in early spring followed by a late spring frost can trick seeds into sprouting early and kill them.  This is why it is a great idea to start many types of seeds (such as tomatoes and peppers) indoors to transplant outside later in the spring.

The ideal temperature range for seed germination varies depending on the plant species. However, a temperature of 75 degrees Fahrenheit (24 degrees Celsius) will work well for most seeds.

You can learn more about what seeds need for germination (and how to speed things up) in my article here.

The best part about growing plants in a sponge is that you do not need to use soil, which can leave a mess in your house.  Let’s take a closer look at how to grow seeds in a sponge.

soil
Growing seeds in a sponge lets you avoid the mess of soil or potting mix.

How To Grow Seeds In A Sponge

To grow seeds in a sponge, we will need the following:

  • Sponge – choose a clean one that is free of mold and bacteria.
  • Container – choose one that is large enough to easily hold the sponge.
  • Seeds – choose newer seeds, since seed germination rates decline each year.
  • Good location – a warm spot will help seeds to germinate faster.

Let’s start off with choosing the right sponge.

sponge
Use a clean sponge for growing seeds. A used one may have mold or bacteria on it.

Choose A Clean Sponge

The first step to growing plants in a sponge is to choose the right sponge for the job.  Avoid old sponges that might have mold or bacteria on them.

After all, you don’t want the seeds to succumb to damping off or other plant diseases! Instead, choose a fresh sponge for starting your seeds.

There are a few different types of sponges to choose from, depending on what you have available:

  • Kitchen or Household sponges: these sponges are made from either a synthetic fiber, such as polyurethane foam, or a natural fiber such as cellulose (wood fiber).  Wood fiber sponges are biodegradable if you are looking for a more eco-friendly alternative to polyurethane sponges.  These sponges are probably the most widely available sponges to use for growing seeds.  You can find them in any garden center (including Home Depot or Lowe’s) or at a grocery store.
  • Sea sponges: these are the true original sponges, used by ancient Greeks and Romans for cleaning long before we were able to make synthetic ones.  They live underwater and soak up lots of water, and our modern man-made sponges are modeled after them.  You can find sea sponges on Etsy here.
  • Bio sponges: these sponges are specifically made for starting plants from seeds indoors.  They are a sterile growing medium that give a very high germination rate (the percentage of seeds that sprout).  They are used in Park Seed’s Bio Dome Seed-Starting System, which you can read more about here. If you like, you can also buy bio sponges separately (without the bio-dome) from Park Seed here.
sea sponges
Real sea sponges are one option for a growing medium if you want to grow seeds in a sponge.

Find A Container To Hold The Sponge

Once you have your sponge ready, it’s time to find a good container to hold it.  A container will avoid spilling water everywhere, and it will make it easier to keep the sponge wet.

Your container should have raised sides that are high enough to hold a little excess water (in case any leaks out of the sponge.)

An ordinary plastic or ceramic bowl would work well.  You don’t want the sponge to fit too snugly inside the bowl, so make sure that the bowl is a little bit larger than the sponge.

This will leave some space for watering.  With the extra space, you can pour water into the bowl without washing the seeds out of the sponge.

A spray bottle is an alternative watering method that helps you to avoid over watering.

spray bottle head
A spray bottle makes it easier to keep seeds wet without over watering.

Another good option to hold your sponges is a standard size (1020) seed tray – you can read more about them here.  If you have a seed tray without separate cells, you can put one or more whole sponges in the tray.

In a seed tray with separate cells, you can cut your sponge into pieces that will fit into the cells in the seed tray.  With this method, you may need water from above.

seed tray
Just replace the soil with sponges and you can start seeds in a tray without the mess!

You can do that with a spray bottle if necessary, to avoid washing away the seeds. You can also use an open container below the seed tray to hold extra water.

Of course, you might be planning to grow plants indoors beyond the seedling stage.  In that case, you will need to think ahead about how you will transplant them.

This means moving them into either a hydroponic system, into pots with soil, or directly into your garden.  Either way, you will probably want to grow the seeds in separate sponge pieces, rather than in one big sponge.

This will prevent the roots of the plants from getting tangled up.  It will also make your life easier when transplanting the seedlings to their new home.

Starting Seeds In Sponges

Now that you have a sponge and a container to hold the water, it’s time to choose your seeds and plant them!  Sponges already have holes in them, and these holes will work just fine for smaller seeds (such as broccoli, onions, and tomatoes.)

However, you might need to cut or poke some larger holes in your sponge to hold bigger seeds, such as pea and sunflower seeds.

If you want to grow microgreens in a sponge, some popular ones include:

  • Arugula
  • Beets
  • Broccoli
  • Mustard
  • Peas
  • Radishes
  • Sunflowers
sunflower seed sprout
Sunflower seeds will grow into microgreens with a distinctive flavor.

This list is not exhaustive – there are many more seeds you can use for growing microgreens!

In addition to finding places in the sponge to put the seeds, you should also pay attention to spacing.  Spacing will vary, depending on the purpose of the plants.

  • Germinating seeds for microgreens: in this case, you don’t need much space at all between seeds.  When you plant seeds for microgreens, the sprouted seeds will be harvested shortly after germination.  So, you don’t have to worry about the roots getting tangled up or seedlings competing with each other, since they are not going to be transplanted.
  • Germinating seeds for a garden or hydroponic system: in this case, you will want to leave some space between the seeds in your sponge.  This will be helpful in transplanting them later.  If you don’t leave enough space between seeds, the roots will get tangled, and you could easily damage or kill the plants when moving them out of the sponge.

An even better option in the second case is to cut the sponge into pieces and plant 1 seed per piece.  That way, you know that the roots of separate plants will not get tangled, and it will be easier to transplant them into another pot with soil or into a hydroponic system.

Choose The Right Location

Choosing the right location for your sponge and seeds can make all the difference in whether they germinate quickly, slowly, or not at all!

The ideal temperature range for seed germination varies by the type of plant.  For more specific information on seed germination temperatures, check out this page from the University of California.

Generally, for seed germination, you really cannot go wrong with a temperature of 75 degrees Fahrenheit (24 degrees Celsius).

If your house is not quite that warm in late winter or early spring, then you can use plastic to keep the seeds a little warmer.  One option is to use clear plastic wrap to cover the sponge and seeds.

The plastic wrap will help to retain moisture for the seeds.  It will also warm up the wet sponge faster if it is left in the sun.

Another option is to use a cloche to keep the sponge and seeds warmer.  A cloche is simply a covering meant to protect seeds or plants from cold.

You can make a cloche out of a plastic water bottle with the bottom cut out.  You can remove the cap on the top of the bottle to act as a vent if it gets too warm underneath.

water bottles
You can make a cloche out of an ordinary plastic water bottle and use it to keep your seeds warm.

After the seeds sprout, they should have some light so they can continue growing.  Consider putting them on a sunny windowsill or under grow lights.

That way, the earliest sprouts will healthy until the later ones germinate. At that point, you can transplant or harvest them.

Keep The Sponge Damp For Seeds

One of the most important factors for seed germination is the moisture level in the growing medium.  In order for seeds to germinate properly, they need plenty of moisture.

This is because seeds need to absorb enough water through the seed coating to know that conditions are right for germination.  If you keep the sponge damp, you will keep the seeds damp.

If the seeds dry out, they will die.  With this in mind, keep an eye on the sponge, and add water as often as necessary.  This might be every day if the sponge is in a warm, sunny location with dry air.

One option for keeping the sponge wet is to pour a little water into the bowl or container that holds the sponge.  Then, the sponge can absorb water from below.

Another method is to use a spray bottle to lightly spritz the top of the sponge with water.  This is an easy way to give you precise control over the amount of water that the seeds get.

I would not recommend pouring water directly on top of the sponge, since the seeds could be washed away before they have sprouted.  It is also more difficult to control the water this way.

You can also use clear plastic wrap to cover the sponge. This will help to retain moisture and keep the air more humid near the seeds.

plastic wrap
Clear plastic wrap will retain heat and moisture in a sponge.

Transplant Your Sprouted Seeds (Or Harvest Your Microgreens)

Under ideal temperature and moisture conditions, most seeds will sprout within 1 to 2 weeks.  Of course, the time to seed germination time depends on the plant species.

For example, turnip seeds can sprout in as little as 1 day. On the other hand, parsnips will take closer to 14 days.

If you are going to harvest your sprouted seeds for microgreens, use a pair of clean scissors (disinfect with alcohol) to cut them close to the surface of the sponge.  Use them in salads, soups, or as garnish for other dishes.

microgreens
Harvest microgreens with a sharp, clean pair of scissors.

If you are going to transplant your seedlings, you will need to remove them from the sponge first.  The sponge might biodegrade eventually, but it may not happen fast enough for the seedling to grow its roots out properly.

Without soil, seedlings will eventually stop growing unless they are in a hydroponic system.  If it is still too early to transplant the seedlings outdoors, then put them into pots with soil.

Then, wait for warmer spring weather to transplant them outside to your garden.

Can I Use A Sponge For Hydroponics? (Using A Sponge As A Growing Medium)

To continue growing beyond the seedling stage, you will need to provide nutrients to your seedlings.  If you want to do this without soil, then you will need to set up a hydroponic system.

You can learn more about hydroponics (along with aquaponics and aeroponics) in my article here.

hydroponics
Hydroponics involves growing plants without soil.

You can certainly use sponges to start seeds for hydroponics, but they will need a different medium to grow into full-fledged plants that you can get a good harvest from.  You can learn more about different growing media for hydroponics at the leaffin website here.

Conclusion

Now you know how to grow plants in a sponge, from choosing the right sponge and seeds to keeping the moisture and temperature at the right levels.  You also know how to transplant or harvest your seedlings at the right time.

You might also want to read my article on growing plants in a can.

I hope you found this article helpful – if so, please share it with someone who will find the information useful.  Now it’s time to get your seeds started in the closest available sponge!

If you want to read some of my most popular posts, check out the “Best of GreenUpSide” page here.  Enjoy!

~Jonathon

jonathon.david.madore

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

Recent Posts