Growing seeds in Perlite

joannacalaAugust 8, 2007


I am a recent gardener and am about to try my hand at growing some perennial flowers from seed - echinacea, rudbeckia, helianthus, helenium, agastache. I have read that Perlite is a good medium for germinating seeds. Since I live in zone 10 and have a only a very mild winter (never any frost), I was planning to leave the seeds in a semi-shady spot on a balcony outside about Jan-Feb. time.

Could someone please advise how to use the Perlite method? I guess I should cover the seeds with a thin layer of the perlite? Should I also cover with cardboard until they germinate? Do I need to use liquid fertiliser? If so, at what stage? When should I move the seedlings to a soil-based mix? Can they go straight from the perlite to the soil in the ground?



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How to use perlite-There are different grades or sizes of perlite. You should probably choose a pretty fine size for smaller seeds. One thing many people do for seed starting is mix perlite with peat.

How much to cover. It depends on the seeds. Check the seed packets if you have them. They usually Tell planting depth. If the seeds need a shallow planting depth (1/8 to 1/4 inch) they usually want light to germinate. Smaller seeds usually need to be closer to the surface, some really tiny seeds just gets sprinkled on the surface and perhaps lightly pressed onto the soil for good contact.

Larger seeds that get planted like 1/2 inch to 1 inch deep want dark to germinate. It probably would help those seeds to place cardboard over the top of the tray. Make sure there is a little space between the top of your starter mix and the cardboard so that when they germinate, they are not squished. Check daily and once you seed some seedlings emerging, remove the cardboard.

The most important part for seed starting is keeping the correct amount of moisture. Make sure your perlite or seed starting mix is moist before you plant your seeds, trying to water it after you plant your seeds could wash your seeds away. This is why those seed starting kits often come with a plastic dome that you remove or prop open once the seedlings emerge. The cardboard will also help with the moisture. You need to keep the medium (perlite, peat, soil, whatever) moist but not too wet. You don't want your seeds to rot or drown but they can't germinate without some moisture. Until the plants emerge, they won't be using any water so the only water you should be adding is to make up for evaporation. The best way to moisten it would either be misting with a spray bottle or if the tray has drainage holes, you might be able to bottom water by placing it in another try without holes that has water in it and then lifting it out and letting it drain. The hardest part of growing seedlings is keeping them moist but not waterlogged.

You don't need liquid fertilizer until after the seeds germinate. Small seedlings need very weak fertilizer at first. You can slowly strengthen the fertilizer as the plants grow. If you are using strictly perlite, you might concider a hydroponic nutrient as your fertilizer, but something more like a potting soil or peat based seed starting mix might support enough soil bacteria that would make using regular liquid fertilizer acceptable.

At what stage can you transplant. Well that will vary depending on 1-what type of plant, 2-the starter container, 3-where you are transplanting too, 4-your desire to manually care for seedlings in captivity.
Many people will transplant as soon as the first or second set of true leaves emerge (the first leaves you see when the seedling comes up are not ture leaves but the next set will be.) Other people will say transplant when the seelings are 4" or 6" tall. It kinda depends on what you start them in, how crowded they are (remember it is usually good to thin the ranks to appropriate spacing since two seedlings sharing too close of quarters will both suffer.) If your seed tray is simply an egg carton (The paper egg cartons are really great for seedlings you will be putting into the ground since most roots will grow right through them, you simply cut them apart and plant carton and all right in the soil), you probably have to transplant pretty quickly since the seedlings will outgrow the space. If you have them planted in larger containers then you can let them get bigger before putting them out on their own.
Also, the location you are planning to put them in after transplant will affect how big you will want them. It can be difficult for a really tiny transplant to survive in some environments like places where there is competition from other plants or careless gardeners who might pull a small seedling, thinking it is a weed.

Finally, your own time to be taking careful care of seedlings in small non-soil containing containers. Small containers with little seedlings need regular attention or they could dry out and waste your effort. Even in a location with automatic sprayers, you are gambling by leaving seedlings unattended for too long. What if the settings are off and things get too wet or too dry? The sooner the plants get situated into their long term arrangements, the less attention they will require from you provided the settle in well.

Good luck with your seed starting.

    Bookmark   August 10, 2007 at 9:48AM
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Just my 2 cents...if you plan on putting the plants in a sunny spot outside once the seeds have grew enough, you may want to transfer them to a heavier mix. This is if you are planning on putting all perlite inside the container. Just my recommendation, transfer the plant to like a mulch or bark chips with a drip method or a once a day small watering. I have seen large tomatoes growing inside 5 gallon buckets with bark chips and all of them inside a greenhouse setup with a drip system. Do some research on how perlite holds water. From my understanding, it is more or less used for aereation to the plant roots. I'm trying to picture your plants sitting in a darker colored container (with all perlite inside) sitting in a sunny area.
In the area that your located, the heat gets pretty high I imagine. With high heat and not so good water retention, plant roots can dry up fairly quick. Of course you don't want them to be water logged either. Just my 2 cents.

    Bookmark   August 10, 2007 at 11:07AM
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tailwheel(z9 CALIF)

Several years ago, I did a vegie garden using a kiddies play pool, I filled it with tap water and floated a foam disk that covered the top of the water surface. I cut several holes into the foam and inserted ordinary plastic 12 oz. cups with holes burned into the bottom into the holes. The cups were filled with perlite (size is'nt important) and I scattered lettuce, arugula, and other small seeds on the top of the perlite. I then spritzed the seeds a couple of times daily. They germinated quickly and the roots disapeared into the perlite. I did'nt use any nutrient in the water in the pool. The plants grew beautifully. Just remember that when a plant needs water and nutrient, it will find them itself.

I think I have pictures on a cd someplace that I've saved. If you have interest, email me at and I'll try to find them and send them to you.

    Bookmark   September 1, 2007 at 5:45PM
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I started an experiment a little more than a month ago. I have large sized perlite that I floated a couple inches worth in the top or a cooler. I put air stones in the bottom of the cooler. Filled with weak nutrient and and started lettuce seeds in bits of cotton ball on top. The bubbling acted to keep the top level moist and the roots will find their way down through the perlite.

It does work with the large perlite as it will support some wile floating. When the plants get large enough to get heavy they do start to sink but with small lettuces that just means it is time to harvest to make room for the slower growing lettuces.

I found that setting a few onion sets on the perlite and they will root and grow.

I am testing this method out for rooting cuttings as well.

    Bookmark   September 2, 2007 at 9:11AM
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I've been growing various herbs and veggies using hydroponic and soil. I see the hydroponic store offers rockwool cubes. However, I'm trying to keep the cost per plant as low as possible. As such, I'm starting seeds in perlite. The perlite is in some 2cup plastic kitchen tubs. 6 of these are on a plant flat. The assembly is under 4 fluorescent lights indoors. The tarragon, for example, barely has its true leaves showing after 2 weeks. I've found the perlite dries quickly. I really have to keep an eye on things. Meanwhile, an outdoor row of corn, zucchini and beans is ahead of the curve. I did add some diluted (about 1:4) leftover vegetative strength nute water from my tomato plant. Any ideas on germination in perlite would be great.

Ps- I did have great success with sunflower seeds. I pulled them out after about 10 days- they had roots about 4-6 inches long!

I'm photo logging some of my plants on facebook.

Here is a link that might be useful: facebook with photos

    Bookmark   June 10, 2009 at 5:28PM
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If you plan to transfer to soil, then why use perlite for germination in the first place? Unless you reuse the perlite, it is economically wasteful. At least, it is here. Perlite is not cheap in comparison to other germination methods and no more effective. Reuse of the perlite would require removal from the roots. There is no better way to facilitate transplant shock than to mess with the roots.

I started all my plants destined for soil in soil blocks made from materials with roughly the same density as the same medium the plants would be going in with the addition of worm castings (free for me). I tried a few ingredients and all worked well. This completely eliminated transplant shock as I was able to transplant without pulling the plant out of anything. I just moved the block into a hole and all is well. Some of the plants I started this year included the plants you listed. I've started a "bird garden" for my wife, an avid bird watcher. I often start the hydro plants in a perlite/coco coir mix as that is the medium they will be transplanted into or I just start them in the net cups to avoid transplant. I transplanted a few soil blocked seedlings into hydro, too, this year as an experiment. Well, it was because I started a ton of seeds expecting some to not germinate and all of them did so I had extras. Anyway, those plants are doing great, too. They were peppers and tomatoes.

When determining a germination method I feel that it is best to consider the final medium to minimize root damage or possible vegetative damage (like ripping leaves off in transplant accidentally, not that anyone has done that).

    Bookmark   June 11, 2009 at 9:46AM
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While Perlite is just perfect for growing Salads, and some other fast growing wedgies in deep water (small cups sticked into styrofoam), it really isn't the best choice for every seedling. Pepper seedlings seem to not grow at all in it, tomatoes also struggle. The other thing is that perlite produces algae very quickly if it's not covered by fast growing leaves (lettuce etc). I recently made a test with pepper and tomato in combination with a mix of rice hulls and rough (salt-sized) sand. My mix gave fine results, while all the seedlings growing in perlite only were not growing at all.

It is often said that seedlings in perlite should be started by water and eventually week nutrient. Well, in my experience different seedlings need a different strength of nutrient. I start my tomato seedlings in that special mix (containing perlite) with 0.9 EC straight away, and they grow better, faster and have stronger roots as the ones in 0.4 or 0.5. or 0.7 If the nutrient concentration is to week they just can't decide weather to to grow or not. Of course they will seek for humidity and nutrients (if in poor nutrient), and develop roots in the meanwhile, they surely will - but why torturing them!? ;-) Also, I use small (vertically) sliced cups, actually original perlite cups filled with my special mix and put them in a plate with 1 inch of water - they do not drown or suffocate they just do fine with that much water...

I don't really believe in these theories with week nutrient and just keeping enough humidity, I give them enough nutrient and plenty of water - and they do just fine.

Because later in my drip and recycle system, they get more than enough nutrient and are constantly supplied with dripping water. They (actually the roots) do not need to seek for any humidity or nutrients there at all - I mean you give them plenty of it, that's the plot, isn't it?

I have even tried to grow tomatoes in deep water. Seedlings in a vertically split piece of sponge, sticked into a floating styrofoam disk, actually completely emerged in water, and they do just fine and grow like hell! The only problem is when they grow bigger and have no support. well that's another story for another rainy day...



    Bookmark   June 12, 2009 at 7:38AM
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davemichigan(zone 6a (SE Michigan))


Hi Jean-Luc,

I have always worried that the plants might drown if I do that, but your experience shows that they are fine. How tall are the cups (roughly)?


    Bookmark   June 13, 2009 at 6:12PM
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Hi Dave!
Sorry for not calling back earlier, I've been busy and had a short trip to Burma. Here is a picture of the cups I used. Actually they are a little small and at the time the picture was taken, they should already be transplanted. Some of them are already transplanted into small Yoghurt cups, others were transplanted directly to the drip system and are growing better under the drip. Roots need to be trimmed anyway and they should be transplanted 1-1.1/2 inch deeper to develop new roots at the stem.

The cups I use are "pre-fabricated" but you can modify any cup easily with a standard metal saw. I'd rather suggest a size between the shown cup and a 150 ml Yoghurt cup. Von New Album 31/10/08 09:53



    Bookmark   June 16, 2009 at 2:43AM
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