Growing plants in agar

My daughter is 3. She’s very curious about everything, particularly plants (my wife is a big fan too). I’d like to grow some carrots in agar; this would hopefully give her a view on the development of the plant and root system. Ideally, I’d like the agar to be clear, so as to observe this easily.

I have purchased some agar from a health shop and mixed it and filled up a glass jar. The resultant “rubbery jelly” was semi-opaque, which spoils the effect. I’d ideally like to have a completely transparent medium.


a) Is there a growth medium I can use that is mostly transparent (without delving into expensive hydroponics!) - maybe a different source of agar?
b) Will it support vegetable growth (rather than bacteria/mould like most lab culture media)?

You can Google for agar hydroponics on your own, however, addressing each question you have for us.

You can experiment with weaker agar concentrations, and narrow tubes, made of clear plastic and the like. That will let you see better what’s going on, and let the roots penetrate more easily. A carrot seed, in a narrow tube, will sprout in agar.

Agar itself is non-nutritive to nearly everything that lives outside of the ocean. So nothing, neither plants nor bacteria or molds will grow well in agar. For hydroponics, you will need to add soluble fertilizer for plants. Again, Google for recipe, however, you can use something like MiracleGro plant food. You make it at 1/10 or even 1/20th concentration, add agar, let it set and its good for watching a carrot seed sprout and grow.

You’re not going to get a crop out of a jar, for many reasons, that hydroponics sources will explain to you. Also, MiracleGro and other soluble fertilizer brands contains urea, which will grow bacteria and molds quite well. So you can try another brand of soluble fertilizer, again, make it way more dilute, for the plants, for a little while if the ingredients list only inorganic salts. Because as the plant grows, and creates biomass and organic molecules … molds will easily sprout and kill it. And bacteria as well – you won’t see them as easily as molds, but they will kill the plant.

Still, all in all, its all a fun way to learn.

You could go high tech and use:

Transparent Soil for Imaging the Rhizosphere

Opacity/translucence is going to depend on the type of agar you get. About 1.5% w/v agarose plates are going to be mostly translucent, while the more components you add to the agar (e.g. nutrient agar or LB agar vs. agarose) will bring you more towards the opaque end of the scale. So the question becomes how translucent do you want it?

The thickness of the pour will also contribute to opacity, with the thinner pours being more translucent.
There are agars that are good for growing plants (taking into account the limited space and nutrients that a plate provides - the plant’s not going to become full-grown with such a small amount of nutrients - so you’re going to have the plant grow for a limited time only.) I don’t use this myself, but my understanding is that a Murashige and Skoog medium (MS) is a good plant growth medium.

Also, most agars are pretty good media for growing bacteria and fungi. You may want to use an antibiotic/antifungal in the agar to prevent overgrowth of the plant by bacteria.

Also, consider the water absorbing crystals sold in garden supply. They’re essentially cross-liked acrylamide, and hydrate very well into huge chunks of soft plastic.* Roots will grow well in the spaces. Again, you want a narrow clear plastic tube to be able to see what’s going on. You want to hydrate in a dilute soluble fertilizer. And you want to remember – this plant will grow, but likely die before it gets very big, and definitely before you get a crop, because … reasons. Also, dust will gather over time, which contains enough mold spores, bacteria and organic material for a sudden mold bloom, so that’s going to happen, sooner or later, as well.

*Which is essentially what naita: said

Remember that roots grow in the dark…

You could try something like this

I’ve done a bit of plant growing in agar, but in sterile conditions in a fully equipped lab. Having seen the spectacular bacterial and fungal colonies that the improperly sterilised stuff gets, I must admit, I don’t think I’d try use it as a plant medium at home, especially as a demo for a kid.

Anything that will allow a plant to grow will also allow fungus and bacteria to grow. Though the contaminated ones do look pretty cool… I don’t think you’d get a fungicide/antibiotic that would actually kill everything that would land on it and not damage the plant. If there was something that worked, microprop labs would use it, and they don’t.

I grew a bean in some ‘water beads’ last year for a demo, and it worked pretty well, the clear ones are totally clear when submerged in water. I didn’t add nutrient and it ran out of steam pretty fast, as I only needed it to look good for a day. It only got a bit of algae, though you’d probably get a bit more with added nutrient.

Note that the dry balls are potentially a serious hazard to a little kid, as they’re easily swallowed and fit in places like ears when dry, then expand and get stuck.

For a three year old, why not do things easier? Grow some cress seeds on wet paper in a dark cupboard. Sow a dozen carrot seeds in a pot and pull up a plant every week to show the progress.

Or use the ant-farm approach and plant between two glass plates. At least some of the roots will grow along the glass. (Cover the glass plates up most of the time of course, and pull up the carrots before they grow enough to force the plates apart, unless they’re really solid.)

The ant farm approach is not something to dismiss out of hand. It highlights an important concept to learn: With a mix of some finely sieved soil, some vermiculite or perlite (which you can buy infused with miracle-gro), and some peat moss, you can create a very light, fine grained, growth medium. That way, you will see some roots and can also see the interplay between fine roots and soil. That’s what plant growth is really about. The water gel and agar mediums are kinda cheating – yes they highlight plant growth more clearly, but they’re not actually how the real world works.

Profoundly low tech and old school but when PT did this stuff in primary school the plant of choice was broad beans.
You took a jar, put in a layer of blotting paper or similar absorbent paper against the glass.
Then you filled the centre of the jar firmly with wet soil, peat moss, or use the agar.
Finally push the bean seed or two between the paper and the glass.
Place in a window that catches the sun.

You’ll get a robust growth of roots, stem and, if you don’t over water, leaves.
Once a kid was able to get theirs to flower but that was unusual.

Carrots need super loose soil to grow. Seems like agar wouldn’t work for that.

It seems like my idea would require both an intro to the “scientific method” and basic biology; that’s going to be beyond a 3yr old’s grasp.

I’ll give it a try using naita’s “ant-farm” suggestion of growing the carrot in actual soil.

(my previous experiments showed that agar is a very, very accommodating home for mould)

Thank you all for your suggestions.

… I wanted to cheat! :frowning:

Are you sure the bean with paper towel in a small glass wont work? The bean is right against the glass, there’s no haze due to agar and beans grow a whole lot faster than carrots. Beans are large and take 8-10 days while carrots take 14-21 and start off really small.

They sell kits that let kids grow vegetables between two plates of glass, it would not be the full experience but from online pictures they provide a decent view of the rooting.

This is something I have research experience with. It’s not an easy task for a home grower with no lab experience. The other posters have covered the basics of plant nutrition well. To be clear, you have to put everything the plant needs in the agar. It’s not a drop-in replacement for soil. The other thing is that the culture will become contaminated and grow mold unless everything, including seeds and tools, is sterilized ahead of time and handled in a clean hood. This is not an easy task for a home lab. Agar isn’t clear, either. You can use a more expensive and higher purity form of agar called agarose, or even more specialized media like Phytagel ($$$$) if you really want clear media. You will also need a solution of potassium chloride and pH indicators to adjust the pH of the agar after adding nutrients. Agar loses gel strength below pH 5.7 or so, most plants don’t like media that acidic, and MS media added in standard amounts will create a slush that will never solidify no matter how much agar you add. I expect the same if you were to try Miracle-Gro or any other nutrient mix.

Correction: potassium hydroxide (KOH), not KCl.

As a kid, I remember getting a potato to sprout roots just by suspending it partially in a jar full of water. No soil seemed to be necessary (though, I presume, the potato wasn’t going to be long for the world without some additional nutrients thrown in).

I suspect that there’s some clearish nutrient broth that you could add to the water periodically so that the potato flourished.

The avocado pit, held over a glass of water by toothpicks, was always a fun one for us. It just cracks and sits there, and drops a fat root. Then suddenly, a bunch of roots, and a sprout. Then you put it in dirt in a pot. Then it flourishes, and then dies, 'cause by now, kids have stopped caring.

My early grammar school science text suggested the “beans in moist towel” experiment. Given that my parents were recent Italian emigres, growing food in a tiny apartment garden plot was already done anyway, but they played along with the experiment I wanted to perform. At any rate, the experiment said I was supposed to keep the beans moist in a towel, and open the beans daily and observe and note the changes. But I didn’t do that, I just moistened the towel and forgot them. When I remembered a week later, 6 year old me let out an ultrasonic shriek that sent all adults running – “VERMS!”

“Er, no, Arkcon, those aren’t worms, those are roots, plants make those, because they need them.”

Heh. I got better at science as I grew up. Anyway, there’s lots of suggestions here to help your 3 year old learn. I think you know not to expect that plant growth holds her attention.