I hope to start a small vegetable garden in a few planters on a rooftop in downtown Los Angeles, California. I was wondering what type of soil and what type of fertilizer I should use (would prefer organic), and what would be the best vegetables to plant right now (as either seeds or seedlings). Below are the characteristics of the garden in its first incarnation:
-- Planter on a rooftop (full sun) in Los Angeles, California (USDA Plant Hardiness Zone 10b)
-- Currently full of dirt from a lawn that someone dug up (the grass had been dead for several years and had never been treated or fertilized, but there is a lot of root matter in the soil)
-- The planter has three segments of about 2' x 2' with soil 8-12" deep
-- The building is an old concrete industrial structure so for the most part weight is not an issue
Any advice about how to get the most from this space (and how best to expand it) would be greatly appreciated.
That sounds like a really cool project. I've only ever planted on a balcony, not a roof. And that was a loooong time ago. Do my only advise would be that this sounds like a really good application for drip irrigation. Also, maybe a few flowering plants with a fairly strong fragrance to attract some polinators. I would also still plan for water runoff as water can be very heavy. Even though you are gardening on an industrial structure, faults could be there and water will find them.
After reading your post I received in the mail the Spring 2013 issue of "HEIRLOOM GARDENER" magazine and I found there the answer to your questions.
Half of the magazine is focused on "roof gardening", which materials/soil to use and how is operated. Very interesting.
You can check your local garden store and see if they carry this magazine or you can order here:
It is a wonderful publication. You can have a look here:
Thanks a lot for the advice about pollinators... never thought of that but it's a great idea. I may get some pots to go around the planter. I will look into drip irrigation as well.
I appreciate your concern about water runoff... there are spaces near the bottom of the planter where water will hopefully drain, and the planter itself is elevated on some cinder blocks. I'll post some pictures in a few days.
@drthor and Gymgirl
I had never heard of that magazine but it looks awesome, going to pick up a copy of the spring edition... thanks!
That magazine is great so is the catalog. I must ask where in DT LA you are talking about? I worked down there by the Music Center the last time I was there I was at Union Station. Looking forward to the pictures.
I'm always obsessed with drainage, but I think that 2' x 2' planters with "dirt from a lawn" is a scenario where drainage really is a crucial concern. Oh yes - are you sure the lawn was not treated with persistent herbicides, or nasty insecticides?
I think that many people would say that containers "need" a soilless mix rather than garden soil. Garden soil tends to pack down tight in a container. Lawn dirt would probably pack even worse, unless you're lucky and have sandy or gritty soil under your friend's lawn. Tight-packed soil has few air spaces, which prevents good aeration. Roots need good air.
Worse, packed-down soil also has few voids, channels or opening for water, which makes it hard to water adequately. Worse, once you get water into it, the water has trouble draining out. That means waterlogged soil, which means NO air pockets, which prevents any aeration.
Sorry to be Debby Downer, but it would be very easy to have drowned roots and dead plants with poor soil in a container.
It's easy to say "throw away your 8-12 cubic feet of dirt, and replace it with 75 gallons of super-high quality commercial potting mix (note MIX, not potting SOIL). But I'm guessing you're not eager to do that. Depending on what you buy, and where, that might cost over $100. But it would be the only way to be SURE you were not inheriting herbicides, soil diseases, insects and weed seeds.
Let's assume instead that you can amend that dirt to the point where it is healthy, living soil. It won't be a soil-LESS mix, or a soul-less mix. It will have to be healthy enough that "good" soil organisms can out-compete "bad" soil organisms.
- First, air must get in plentifully. (aeration)
- Second, water must be able to perk in and drain OUT freely. If too much water is held in the soil, it displaces the air. That's really bad because roots and beneficial soil organisms both need oxygen or they die. Lots of big drainage holes let water escape if the soil lets it. Once the water drains, air enters from the drainage holes, too.
Those are mechanical issues. If you can't solve them, nothing else matters. Roots and soil organisms need air and water in balance.
I'm going to suggest lots of pine bark nuggets, shreds, chips, coarse fibers and maybe some fines. And/or grit, Perlite, crushed stone, very coarse sand, coir, or manufactured products like "expanded clay and shale" pellets.
- Third in importance is that the "good" soil organisms must find the food they need, which means compost (organic matter). A good micro-herd is either very important or crucial to healthy plants (and crucial to out-competing whatever undesirable organisms you inherited from the "lawn dirt".
- Fourth in importance - irrelevant unless you solve the first two problems and uneccesary if the thrid prob lem is WELL solved - is fertilizer - something to feed the plants in case the soil microherd does not supply enough mineral nutrients from breaking down the food you gave the micro-critters. "Feed the soil and the soil will feed the plants."
*IF* the compost you fed to the soil does not provide enough Nitrogen, Phosphorus and Potassium (NPK), I say *IF* it doesn't, you could add something like fish emulsion and bone meal, seaweed products, or what-have-you organic plant food, to feed the soil and the plants. It's hard to kill plants with organic fertilizer unless you really pile it on.
Or add SOME soluble chemical fertilizer IN MODERATION and NOT TO EXCESS.
Insufficent mineral fertilizer might slow plants' growth IF severe. Excess nitrogen burns roots quickly. Excessive phosphorus blocks beneficial mycorrhizae. Excessive NPK makes the soil saline which prevents water uptake and may kill roots all by itself. Many micronutrients are quite toxic to plants in excess. Chemical fertilizers are fine and beneficial as long as you don't add TOO MUCH. Start with none, or 1/4, or 1/2 the recommended dose. Don't go higher unless all other soil conditions including pH are good, but leaves turn light green or yellow instead of dark green.
You'll never kill a plant with malnutrition, just slow it down. But it is quite easy to use four times as much fertilizer as you should have, and kill the roots hairs and then the plant in just a few days.
(End of over-fertilization rant.)
Both of the first two issues (air-water-soil mechanics) can be solved by adding 20-70% gritty or chunky stuff, depending on whether your lawn dirt is heavy clay or light and sandy. If the lawn dirt is sandy, ignore most of what I'll say and add instead fine bark fibers and coir or even grit. Use even more compost.
You're going to have to do some mixing. Can you spread out tarps and make a big pile? Can you count on it n ot raining for as many days as it takes to mix up n ew soil from what you have?
Maybe make a few more planters out of concrete paving stones stood on end to contain your new pile. Or use the existing planters as three walls to contain your working pile.
Since you need to dilute your dirt by around half, it will wind up taking up around twice as much room as it does now. Or screen the roots out, screen out big stone, and throw some dirt away.
Say you have room now for 12 cubic feet. You probably need to buy 4-6 cubic feet of amendments. The bark products I'm thinking of will cost around $2 per cubic foot, so say $12 there. You'll also need some soil-microbe-candy, like compost or fish emulsion and bone meal ... maybe $10-$20. (A lot of carrying things upstairs!
For drainage and aeration, I'm a big fan of bark nuggets and bark shreds and fiber (bark "mulch" or "pine bark fines" = PBF). Pine, fir or hemlock bark is great. Any conifer/evergreen is OK. You would want the smallest nuggets or medium mulch. Nuggets too big to pass through a 1/4" screen (0.2" to 0.25" or 4 mm) are getting to be bigger than is ideal. Save them and use them later as a top-dress mulch.
You want a lot of nuggets and shreds with small dimensions from 1/2 mm to 3-4 mm to improve drainage and aeration. Elongated shreds and fibers or chips are a little better than round BBs , but it all helps.
You may want some pine bark FINES like fibers and powder if you need MORE water retention - like if you have sandy dirt. Say, small dimension less than 1/2 mm. When it feels softer and finer than "gritty". This form of pine bark serves the purpose of peat moss (except that it has bigger fibers which is good, and it lasts longer than peat, which is very good). It serves exactly the same purpose as coconut coir, but you don't have to wonder if it contains salt.
Probably, if you have clay soil, small bark fibers will help the clay become softer and friable so it breaks up and mixes, like coarse sand would.
Whether bark powder and thin fibers are good or bad kind of depends on what kind of dirt you're trying too improve and what it has been amended with. Remember that the goal is open, fast-draining soil that contains a balance of air and water after you pour water through it.
Where I live, Lowe's has several clean dry products for almost the exact same price that Home Depot sells very dirty logyard trash.
Watch out for damp, smelly mulch. That has been fermenting anaerobically and you don't sound set up to flush it clean or compost it easily. If it wasn't stored dry, buy somewhere else. If you already have the bag and it smells funky, it would be better used somewhere it can air out and be rained on without washing moderately root-toxic fermentation products into YOUR soil.
You should mix in "enough" bark to make the soil "light enough" that it drains water and air "fast enough". That's pretty subjective until it rains. Maybe you can tell by eye or by making a snowball out of a double handful, whether it is too desne and clayey, or fluffy and open enough.
After a rain or heavy watering, if the water runs out the bottom as fast as it pours in the top, and you sense or feel that air pockets are opening up inside the soil, then congratulations, you've made the mechanical part of successful soil. If it stays soggy and drains slowly and a poked finger encounters wet mud, roots will drown in it and only anaerobic bacteria will grow, not plants. Add more bark or grit.
Instead of or in addition to conifer bark, you could add Perlite or crushed rock (coarser than sand, i.e. coarser than 1 mm grains). Perlite is like "puffed rock" and holds no water. Or add very coarse sand, if you can find any that isn't mostly medium like 0.1 mm. Coarse coconut coir might improve drainge, but it holds water. vermiculite is harfull since it holds water AND breaks down very quickly into fine powder that clogs the air pores and channels you've been trying to create.
Or if you can find the expanded clay/shale products they use on baseball fields and "green roofs" , that is made for the purpose of permanent drainage in containers. They are porous so they hold some water. They are expensive. A construction yard would call them "light weight aggregate" or expanded clay and shale. Turface, Haydite, Hydrock and Norlite are brand names. Some producers (not distributes) are
Carolina Stalite Company
Big River Industries, Inc.
DiGeronimo Aggregates LLC
TXI-Texas Industries, Inc.
I see I've been verbose and opinionated (again). Sorry! But whatever you add, get the dirt to drain well and sustain some "loft" so air can enter and roots can breath. Water must flow right THROUGH every so often to flush away excessive slats and unused nutrients. If not, salinization occurs very fast in containers.
Those were the mechanical issues. I should have mentioned the main chemical issue - pH and dolomite limestone, but I forgot. Without soil testing, a fair guess is that you could use a little - maybe 1/4 cup per cubic foot? If I were you, I'd look around online or ask around locally for better advice there.
The third issue is biological. Since you started with soil (or lawn dirt), it will never be a sterile, soilless mix . You need to encourage the good bacteria, fungi and mold. All we can do is be sure they have what they need to breath (aerobic conditions), what they need to drink (irrigation or a watering can or hose) and what they need to eat (organic matter like compost).
Mainly, the soil has to have organic food added on Day One and again every 3-12 months, since you've solved the air-and-water problems.
Since you don't have room for a compost heap, you'll probably be supplying most of your OM from bags. While mixing your soil, you can blend in compost and some coffee grounds.
Given air, water, food, and plant roots, somehow the good bugs always seem to win. I guess that evolution "figured out" that more life can live in healthy soil, so things have co-evolved in such a way that they favor plants growing, then dieing and feeding the soil soil again. Except for a relatively few plant pathogens, most of what thrives in aerated soil ,is good for plants and soil structure.
Did you know that soil life is so rich and complicated that, even when trying as hard as they can, research scientists can only grow TEN PERCENT of soil microbes in their labs? Simple, humble soil supports ten times as many species in its complex interstices as PHDs can coax in Petri dishes. The network of soil life is that complex and interdependent.
To participate in the cycle of soil life a little more, you can look for some friends with a happy garden and healthy soil. Or a healthy compost heap. Ask for a quart or a gallon of their soil (maybe offer a big bag of kelp nuggets or alfalfa pellets in return). Or flowers or cherry tomatoes after you get going. Then mix their healthy soil into your "lawn dirt" to inoculate it with more beneficial microbes than all the PHDs in the world can name. Or just top-dress the innoculant soil or compost onto your established beds (under the mulch) and let the microbes wash down into the soil as you water.
Hey, we can't ever call it "dirt" any more! It's living soil , and deserves respect. We should probably have a fancier name than soil, like "ecosystem".
If money is no object, you can buy packets of mycorrhizae fungi or spores, and add them to your soil system. But since you started with lawn dirt and it had roots, you probably already have them in plenty. Or buy a 4-cubic foot $40 bale of professional nursery mix with mycorrhizae, micro-nutrients, water crystals and fairy dust. But then it wouldn't be soil that YOU made by hand, from scratch, and breathed life into all by yourself.
At the very top of the beneficial food chain in the soil are the noble worms. If you can find them for sale, look for Red Wigglers. "Red Wigglers, the Cadillac of worms."
All the things they do for the soil are so impressive that it seems they must be genetically engineered super-organisms. If your soil is "pretty good", and has enough organic matter to fed them as well as your awesome collection of soil microbes, they will mix and grind and glue and mingle the soil from "pretty good" up to "great".
They grind up coarse organic matter so microbes can digest it more easily. They break down clumps of clay and mix it with organic colloids and sand to "till" the soil on a sub-mm scale. They drill tiny tunnels and pack their sides tight to make long air-and-water tubes. They go up to fill their guts with organic matter on the surface, then tunnel down and release it deeper in the soil. They're like science-fiction fantasies except for the fact that they really do all these things!
If you can keep worms happy in little 2'x2' planters, you'll know that you've successfully re-created the whole majesty of living soil on your roof. And if you make them happy, they'll make your soil great, and that will make your plants happy.
Personally, I like "Red Wigglers, the Cadillac of worms." I'm actually looking for those. I still do not have worm sightings in my soil. I read an article in Country World News about an organic farmer here in Texas who has the same problem. He is raising his own so I though I might give that a try. I don't find you too verbose, Rick. I enjoy a good turn of phrase myself. If you do become a writer in the future, consider throwing in a few well placed murders. Although there are several mystery series with a gardening/organic farming theme, many of them are nothing more than glorified romance novels...I digress.
Rooftop, I was wanting to make sure you have plenty of drainage for your containers and then off the roof. No pooling, that sort of thing.
That was from "WKRP in Cincinatti". It was part of a radio ad, with music. I wish I could remember which forums "mraider" posts in. He's a big advocate of vermicomposting, both to make compost and multiply the number of worms in his field.
Hmm, we do have a Vermicomposting forum, but it doesn't look super-active.
I get scolded so much at work for being too verbose that I slip off the reins in online forums. I figure readers already have their finger on the "Page Down" key, or the mouse on the scroll bar, and "click click" gets you past most long posts.]
Admittedly, some of mine might require "click click click" to get past, so I apologize about that much.
No reason to apologize and being verbose can be a virtue. Like you said everyone is free to use the page down button on their keyboard or their mouse but it is more like click, click, click, click, click, click, click. :)