How to Prepare Your Soil for the Healthiest Annual Flowers

Frosty soil in late fall with green shoots of oats and peas emerging
Peas and oats emerging in a new annual bed with a layer of compost on it. It was a fall planting and the soil is frosty.

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Soil has an ecology. The healthier the soil ecology the better it can feed plants and fend off soil pathogens (the pests and diseases). And all the things that go on in the soil is nothing short of amazing. Knowing about it helps you see how proper soil care for annual flowers is essential.

Annual plants need different nutrients than perennial plants.

Think of all the work annual plants do. They germinate from seed, grow large enough to flower, then develop and ripen fruits that contain seeds. Then they die. All in one growing season.

That’s a lot of growth.

So they need a good supply of nutrients. Annual roots need nitrogen in nitrate form, called nitrates. Perennial plant roots need nitrogen in ammonium form.

To get the nitrates annual roots need, they need soil that is “bacterially dominated.” What is that? It’s soil that is dominated by soil bacteria rather than soil fungi. The bacteria excrete nitrates.

Soil that is “fungally dominated” has higher populations of fungi, which is what perennials are designed to live in. Soil fungi excrete ammonium nitrogen.

To create the best soil ecosystem for your annual plants there are three things you need to do: no tilling, mulch with composts or green manure, use only organic fertilizers. Let’s dig in to these.

Do not rototill, turn, or otherwise aerate the soil!

That’s right, do not disrupt the soil by turning, digging, or rototilling. I know that’s what we’ve always been told. But with new science, we’ve learned that this approach is detrimental to soil ecosystems. So now tilling is outdated.

Think about what goes on in the soil:

  • There are worms and other organisms and microorganisms (microbes) in the soil who do not want to see the light of day. They live, eat, and excrete and build an ecosystem. Earthworms, for example, form tunnels through the soil which aerate it. Mini-ecosystems gather in and around the worm tunnels, feeding off the worm’s excretions and building the soil ecosystem. To till the soil is to destroy these organisms and what’s beneficial and already established.
  • In the soil are many miles of beneficial fungi called mycorrhizae. Most plants connect with them. They’re the extremely beneficial fungi that connect with plant roots to deliver minerals and water, which they’re best at finding, to the plants. These wonderful beneficial connections get severed and destroyed when the soil is turned or tilled.
  • Soil structure is destroyed and can take many years to build up again. Soil microbes coat soil particles to create a crumbly texture that’s full of life, absorbs and holds water, and keeps soil aerated. When structure is destroyed the soil becomes powdery and compacts easily.

The topic of soil ecology, called the soil food web, its benefits, and how to develop it is described very thoroughly in the book Teaming With Microbes*, by Jeff Lowenfels and Wayne Lewis, and in Plant Nutrients* by Jeff Lowenfels. The books describe how the soil food web works to develop soil the way nature intended to yield the healthiest plants and harvests.

If your beds are already prepared you can continue with just mulching and/or green manuring (see more below). If you’re starting a new in-ground bed, read my page How to Start a Garden Bed. I recommend growing in in-ground beds because the soil is a superior reservoir for soil ecology and holding water. Read more about advantages of growing in-ground here.

In short, when I dig a new bed I use a very good quality digging fork. Once I lay down the amendments on top of the soil I sink the fork into the soil to the top of the tines and gently wiggle it back and forth. This brings some amendments down into the soil. That’s it. From then on it’s only mulching and green manuring.

Use the right composts, mulches, or green manure

Fresh green stuff is good, in the form of a green mulch like lawn clippings, alfalfa meal, or garden compost, aged or composted horse or other animal manure (herbivores only), or green manure (see more below).

These materials can be laid onto the beds even during the growing season. There’s no worry about the soil getting robbed of nitrogen to break it down, this is not true. That only happens when raw material is worked into the soil.

But it’s best to lay the materials down after the harvest in fall. This is when the soil organisms get very active breaking materials down. That is, until the ground below the soil surface freezes. If it doesn’t freeze they just keep going, breaking down plant material and animal manure and feeding the soil food web.

You can lay the materials on thickly, 2-3 inches in the fall. This has the added benefit of preventing weeds by keeping them from germinating in the light.

Grow a green manure

Seed packet with illustration showing oats and peas growing in beds.
Botanical Interests Soil Builder Peas and Oats seed packet

You can  grow a green manure in your beds to add nitrogen and organic matter to your soil. It may be easier for you than hauling in manures, buying alfalfa meal, etc.

It means planting a mix of seeds that include legumes and annuals like oats to grow fast. When you’re feeding an annual bed in the winter you’ll need cold hardy plants, like the popular mix of oats and peas. If it’s in the summer, plant warm weather green manure seeds like buckwheat.

The legumes will host a soil bacteria that fixes nitrogen from the air into the soil. (There’s your nitrogen.)

The oats, like all grasses, develop extensive root systems, this adds lots of organic matter to the soil.

The plants must be cut or harvested just before they bloom. This is when their bodies have the highest level of nitrogen. Once they start to bloom the nitrogen goes to flower and seed production and the green material will break down too slowly. The green part of the rest of the plant is what you’re after here.

How to use your green manure

There are three ways to deal with the green manure crop in a home garden. It’ll likely be harvestable well before you plant your annual flowers and vegetables. Unless you got a late start.

Back of seed package with instructions on how to use it
Here are instructions on using the seed mix… but please skip the turning and tilling, the idea hasn’t caught on enough yet.

First, you can cut and shred the green upper parts and lay them on the beds as a mulch (I use a leaf shredder). This means leaving the roots and stubble in the soil.

This can be awkward for carrot seed planting, but for bigger seeds or planting from pots, it works well. The soil organisms don’t get disturbed and all the nitrogen slowly becomes available as the microbes work on it.

Second, you can pull the plants out, chop it all up or cut it to smaller pieces, and add it as a green in the compost pile. Some smaller roots will remain the soil and break down.

Third, you can cut the tops off and lay that green stuff onto the bed and layer compost over it to help them decompose. This method needs more time before planting. It works best for annuals planted from pots. It also helps prevent weeds from germinating and growing. This is my preferred method.

Look for seed mixes that are labeled as cover crop seed. The term green manure is usually used because it gets turned in. But we know we shouldn’t do that anymore.

Soil food web gardening practice requires that the soil be disturbed as little as possible when it comes to annual and vegetable gardens, unless you are trying to establish a vegetable or annual garden in fully fungally dominated soils.

Teaming With Microbes by Jeff Lowenfels and Wayne Lewis

Why you should only use organic fertilizers

To give your soil a boost, I recommend sprinkling an organic fertilizer on it before laying a mulch. I specify organic, and this is crucial. You want to support the soil food web. This in turn allows nature to do her work and feed the plants in the best way.

Chemical fertilizers give the plants nutrients only. This feeds the plants but it renders the soil food web useless so it dies off and even kills off microbes. This keeps the plants always needing more fertilizer. Soil biology suffers and plants become more vulnerable to pathogens because the soil checks and balances are no longer functioning. And you’re on a synthetic fertilizer treadmill, vulnerable to pest problems.

On the other hand, organic fertilizers supply natural materials that microbes can use if and when they’re needed. This is better.

So take care of the soil your annual flowers will grow in and you’ll have good growth and flowers from them. Do these things every year, or even twice a year. Grow green manure and/or lay down composts in fall and in mid-summer. It takes time to develop a high quality soil but starting right away will get you there.

Some flowers have lower nutrient needs so they can go into the newer beds. Flowers like strawflowers, tithonia, celosia, and marigolds, and even sunflowers are good for soil that’s not well developed yet.

Just use organic fertilizers, don’t till the soil, lay down green mulches and composts and/or grow green manures and you’ll build a healthy soil with a soil ecology that’s resilient to pests, diseases, and holds more water and nutrients.

This all means less work overall, healthier plants, and more abundant flowers.

Sources for Seed

Botanical Interests has packets of Peas and Oats for cover cropping. I use this for my beds in fall.

Johnny’s Seeds has a great selection of green manure mixes.

Note: For summer green manuring a popular plant is buckwheat. It’s not a legume but it’s fast growing and contributes high nitrogen organic matter for mid-summer bed prep for cool weather crops.

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