Why do gardeners emphasize the seed-starting process so much?
June 19, 2015 12:34 PM   Subscribe

This might be kind of a dumb question, but I'm a new indoor gardener, I've been thinking about giving growing cherry tomatoes indoors a go, and almost every guide I've seen puts a lot of emphasis on the *technique* of starting plants from seed. I have basically zero gardening experience and I don't completely understand why we do this.

Specifically -- using special seed starting soil, grow lights, starting in shallow pots/trays and transplanting to successively larger pots as the seedlings get bigger, shifting up fertilizer types/composition, and so on... why do we do it? Why can't we just toss the seeds into a 5gal pot, maybe use a bit of light to help speed things along, use appropriate fertilizer, and be done with it?

I feel like a lot of these guides are geared towards gardeners in cooler/more northern climates where the growing windows are shorter, so it's about maximizing what you can get out of your plants and making sure they're all ready to go when the season starts and temperatures warm up, etc. My impression's that if you live in a more temperate climate such as CA, maybe the whole seed starting hoopla becomes less necessary, and doubly so if you plan on keeping your plants indoors (I have south/west facing bay windows so there'll be plenty of sun.) Am I wrong?
posted by un petit cadeau to Home & Garden (11 answers total) 6 users marked this as a favorite
Yeah, that's a lot of it, and it's also the thing I think of as "the deep-dive fiddliness of a hobby". Also if you're growing an acre or more of plants for a large home garden or small commercial operation, seed-starting is a larger-scale project and you want it to work as well as possible the first time or you lose part of the season.

If you are growing less than 15-20 pots of tomatoes, you'd be fine with starting however you want to start or, my preference, buying established plants.
posted by Lyn Never at 12:47 PM on June 19, 2015 [2 favorites]

WRT to seed-starting soil, seeds need to be kept wet enough to sprout but not so wet that they will rot. The stems are tiny and delicate so they dry out easily and die off. It's also good to have very light, very fine soil so the tiny roots can grow easily.

The grow lights start out very close to the soil and are progressively raised higher and higher as the plants grow. Plants grow towards the light so if the light is too far away, the plants get leggy. Bushier plants with sturdier stems are better - particularly for plants with heavy fruit like tomatoes.

As far as transplanting goes, sometimes you actually don't want to transplant since it disturbs the roots. Carrots, for example, are best grown in place. Otherwise, you get carrots than are all tangled not straight. In that case, you would start them the pot/container/bed they are going to grow in. I think you can start seeds in a 5 gallon pot but since seed starting mix is probably more expensive, most people just use tiny pots for starting and then transplant into more all-purpose soil. Plus if you are using grow lights, it's better to get many small pots under the lights rather than just a few large pots.
posted by Beti at 12:51 PM on June 19, 2015

Not all seeds will germinate. By starting in small pots/trays you can easily get rid of the non-germinating seeds and grow out the remaining ones. If you planted everything into the soil you would end up with gaps where the seed didn't grow. So it helps to maximize space.

A lot of the rest is giving the plant the best chance to grow well. So you want to establish a good root system. In a small pot/tray you can water to the bottom without using too much water. With a small plant in a big pot you could be wasting a lot of water until the plant grows big enough to access it. For tomatoes you would pinch the tops off so that you would get more branches instead of just one tall stem.

Tomatoes will grow if you just put them into a big pot of potting soil, but your plants may not be as good as they could have been, and both hobbyists and farmers are not going to treat that as a good outcome.
posted by any portmanteau in a storm at 12:59 PM on June 19, 2015 [3 favorites]

You can't toss a handful of tomato seeds into a five gallon pot. They would grow inches apart and each plant needs its own bucket. You can't grow thirty plants in one 5 gallon pot because you tossed in a seed packet.

Many people starts their seeds in Jan-Mar to plant April-May. Because that's the dead of winter, even in California, you may need additional heat and light to get the seeds to germinate. Typically, you start with with seeding trays so that you know which seeds are viable. You don't want to prepare 10 5-gallon buckets and plant one seed in each and hope it germinates. Also, root development is more robust when plants are forced to spread roots downward instead of laterally, which is common when tiny plants are put in big pots. The roots grow shallow instead of deep, which can impact watering and nutrient absorption.

Plants needs different nutrients depending on their life cycle - seedling, growing/vegetative, and flowering. If you want to start from seed, plant an individual seed in a Dixie cup and put them in your sunny window. When they're as tall as the Dixie cup, move to a 1-gallon bucket and when it outgrows that, move to final 5-gal bucket.

You might want to consider just buying starter plants instead of doing seeds.
posted by shoesietart at 1:02 PM on June 19, 2015 [2 favorites]

In my case because I am buying in "fancy" seeds of the type that cost $5 for 10 or so. I am growing trays & trays of flats because I have a huge garden and even with all the equipment & time it works out cheaper than trying to buy pregrown flats, assuming I can even find seedlings of the types I want.

But mostly because it's a hobby, and in the dead of winter when my garden is under 3 or more feet of freaking snow it is the closest I can feel to gardening without my garden. Also planting seeds in the dead of winter is a very hopeful act and I rather enjoy that part of it. It says spring is coming.
posted by wwax at 1:06 PM on June 19, 2015 [1 favorite]

I have read none of these details you mention. I've had almost no success starting seeds in little pots on a shelf in my house; they sprout and they die soon after.
posted by amtho at 1:24 PM on June 19, 2015 [2 favorites]

Why can't we just toss the seeds into a 5gal pot, maybe use a bit of light to help speed things along, use appropriate fertilizer, and be done with it?

I mean, you could do that, but your seedlings will probably not grow as quickly as they would otherwise, and you'll have greater mortality / lower germination rates. People do all of those extra steps to ensure that the plants they put into their gardens are as successful and healthy as they could possibly be; the idea is usually just that it's a lot of work to go through all season just to care for some leggy, weak tomato plants that won't produce much, so you might as well start with great, healthy stock. This is even more of a consideration for indoor/container gardening, because your whole crop is usually just a couple of plants. If those plants lose all their fruits to blossom end rot right before you were going to harvest them, that's a big deal.

It's also worth remembering that the plants we grow for vegetable production have been profoundly shaped by their history of breeding by humans. We've reduced their ability to thrive under a variety of environmental conditions in exchange for greater production of the parts we like to eat - it's a basic evolutionary trade-off. As a result, these plants are now so fragile in a physiological sense that they require extreme care and tending from humans in order to stay within the very narrow bounds of their tolerable growing conditions, especially at the beginning of their lives. When I first started gardening, I couldn't believe any plants were able to grow outside on their own at all based on how much work my cultivated veggies seemed to require!

Anyway, this is mostly about seedling physiology. Seedlings are small and fragile, even in wild plants, and their undeveloped root & photosynthetic systems put them in greater danger of mortality. They are only able to access resources (water and nutrients) that are very close to them, since their root system is small and can only access a tiny volume of soil. They also have very little storage for resources, so losing resources like water or nutrients for even a short period of time can prove fatal. Finally, they are so undeveloped that even small injuries/inefficiencies in the plant's root system or photosynthetic system could make the difference between thriving and dying - and since they have no resource storage, they aren't very resilient at all to even temporary decreases in productivity.

So, getting the exact right amount of light is important: they have very little energy storage (i.e. roots) and few protections against excessive light, so they need the exact right amount of light - not too much, not too little. If the plant gets too much light, it starts to be damaged by photoinhibition, where excess radiation energy from light damages their photosynthetic apparatus. Too little light would cause slow growth rates and much greater susceptibility to fungal pathogens, which can easily kill a seedling because even small injuries can decimate their productivity.

Temperature is also really important: if the plant gets too warm in direct sunlight, it might undergo excessive photorespiration, which will decrease its photosynthetic rate and could seriously harm the seedling's growth. But if it gets too cold, its photosynthetic enzymes work a lot slower and it won't grow nearly as quickly either. Photorespiration & photoinhibition might not be a big deal for an adult plant in full sunlight, but they can be enough to significantly slow seedling growth enough to effectively kill the plant. Many folks try to balance these factors by keeping their seedlings under an adjustable grow light with an adjustable heat pad underneath. It's really helpful to be able to independently adjust the amount of light and heat you receive, and these are difficult to decouple in a more natural environment. Usually it's either hot and bright, or cool and dark.

But really, water is the most important factor here. Seedlings have tiny root systems and can only access a tiny volume of soil, so all the water needs to be at the surface of the soil where the seedling's roots are, and it can't dry out for very long at all because the seedling has very little water storage. Unless you're watering a very small amount, very, very frequently, you'll have a pretty hard time keeping a 5-gallon bucket moist enough at the surface to germinate e.g. carrot seeds well (some of them will do OK but your germination rate will definitely drop and your plants won't be as healthy) because all of the water will eventually sink to the bottom and the surface will dry out quickly. At the same time, your seedlings can't be waterlogged (roots need to breathe too) so it's important not to water them too much - that's why you have to water a very small amount very frequently.

In fact, water is where the 5-gallon bucket gets really difficult: a column of soil like that is going to hold an internal perched water table where the bottom ~third of the soil is going to be saturated no matter what you do (this is true of almost all container-grown plants, and it's one of the most difficult aspects of container gardening), and it won't drain very well as a result. So if you go to water your seedlings with anything bigger than a mister, you'll quickly saturate the whole soil, and it won't drain well at all so it will stay waterlogged for longer than you want. Even worse, when it does start draining, it will immediately dry out the surface of the soil where your plants are. So essentially, you'll be stuck cycling between dried-out and too-wet, with very little time spent in between at the optimal water level. In contrast, in a shallow seed-starting flat, the saturation level is a lot easier to control, it dries out well between waterings, and you don't have a big moisture reservoir to encourage fungal pathogens either.

I never really mastered fertilizer in my seedling starts, but the same logic applies as for water: seedlings can only access a very small volume of soil, so they need to find exactly what they need there but not too much of it. If there's too much e.g. nitrogen, its roots can get chemically burned and that destroys the plant's ability to gather water effectively, since that single root was like 50% of its total root system. If the pot is too deep, frequent watering will move nutrients down into the soil column where plants with tiny, shallow root systems won't be able to access them.

I started a few flats of seedlings in my closet (mostly tomatoes) and it was WAY harder than I thought it would be to get the plants to grow right. I mean, they survived and were acceptable as plants in the sense that they were alive, but they were in no way suited for growing a lot of vegetables. My plants were leggy as hell, weak in the wind, extremely sensitive to drought... it's a surprisingly difficult thing to do, is all, and a hell of a lot of work. Because I just have a tiny apartment patio garden and don't need to achieve any economy of scale or anything, I just buy starts now.
posted by dialetheia at 2:47 PM on June 19, 2015 [12 favorites]

A big pot of wet soil and a tiny root system is soaking wet, and seedlings die of too much water more than they die of too little. When they're bigger and stronger the balance is more fair - root system to medium. But when they're small the medium stays wet and radiates moisture up to these fine little cell structures and basically they rot.
posted by A Terrible Llama at 4:17 PM on June 19, 2015

Just as a point of reference: I am in coastal Georgia. I didn't feel like messing with starting tomatoes, I had a packet of seeds for some heirloom cherry tomatoes, I scattered the seeds in an area of my garden. That area of my garden is now the "tomato thicket".
If any of y'all would like some cherry tomatoes, I seem to have way more than I can eat...
posted by rudd135 at 6:26 PM on June 19, 2015

It helps to start by recalling that flowering plants are highly "r selection" organisms, meaning that they put out thousands of seeds each growing season in the hopes that a handful survive. Getting from seed to productive plant is not a slam dunk, and there's a lot of things that home and commercial gardeners do to up those odds. Some of those things make a HUGE difference (adequate light, a planting medium that delivers enough but not too much moisture), and some make a modest difference (a planting medium that is relatively sterile, fertilizer, soil temperature, etc.)

It's actually not my impression that seed-starting guidelines for the average home gardener recommend a particularly strict fertilizer regime or lots of transplanting (some people may transplant once before the final transplant outdoors, but a lot of people just start seedlings in pots that are small enough to fit under a light but big enough to support them until they're ready to go outdoors.

A few things you may not realize about vegetable plants:
(a) you essentially cannot grow fruiting plants indoors on natural light, even with the best of southern exposures. Because of the way the human eye works, you don't realize how much weaker the sunlight is in a sunny indoor location than it is outdoors in full sun. A sunny south-facing window is less than half the brightness of full sun, and for fewer hours to boot. I think you're going to be pretty disappointed if you are actually planning to grow tomatoes indoors on natural light.
(b) tomatoes specifically not only have lower temperature limits for growth, but upper temperature limits for setting fruit. So it's not just northern gardeners who have to take a limited growing season into consideration. Southern gardeners in a lot of places have to plan their tomato season around those upper limits, as tomatoes won't typically set fruit when daytime temperatures are above 90 for an extended period of time, so gardeners in Texas, for example, plan their tomato harvest for late spring and if they can keep the plants alive over mid-summer, they may be rewarded with a second harvest once temps cool off in the fall. All that is to say that it's not just northern gardeners who may want to start tomatoes indoors early to take advantage of the right window for outdoor growth and production.

Fertilizer composition matters because some components of fertilizer stimulate leafy growth (nitrogen), while others encourage fruit setting and growth (less nitrogen, more potassium).
posted by drlith at 6:35 PM on June 19, 2015 [3 favorites]

I live in a place that has plenty of growing season plus high humidity. If you just throw your seeds in a big indoor pot, the surface of the soil will grow an assortment of molds, which will support fungus gnats, which will reproduce and spend part of their lives as larvae that love eating tiny, delicate seedling roots. Because of this and other damp-loving pathogens, your precious seeds will grow to a promising size and suddenly begin damping off (keeling over). One of the ways to prevent this is letting the soil go a bit dry, which as others have mentioned isn't going to work in big pots.
posted by zennie at 7:35 PM on June 19, 2015

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