Compost Uses: How to Use Finished and Unfinished Compost in Your Garden

The transformation of kitchen scraps and garden waste into rich, dark compost feels like a small miracle every time. After patiently tending our compost piles, we reach that satisfying moment when it's time to put this "black gold" to work in our gardens. But questions often arise: When is compost finished and truly ready? And what can compost be used for when it isn't fully decomposed?

Reflecting on my composting journey, I definitely remember the first year of composting - having no clue what does finished compost look like. I waited for 1.5 years to start using my first batch of compost produced from our own back garden composting bin.

Over the years, I've come to appreciate that compost has value at every stage of the decomposition process and I'm no longer anxious about the right time to use my compost. In this post, I'll share techniques experienced composters use for recognising ready compost, explore the many ways to use compost in garden settings with both finished and unfinished materials, and discuss how to make the most of those intriguing liquid byproducts that sometimes appear.

How to Identify Finished Compost: What Does Finished Compost Look Like?

The line between "cooking" and "finished" compost isn't always clear-cut. Before we add compost to our gardens, it helps to assess whether it's fully mature or still actively decomposing. Here's what to look for when determining when compost is finished and ready for use:

Characteristics of Finished Compost

Appearance: Finished compost looks like rich, dark soil with a crumbly texture. It should be uniformly dark brown to black, with few recognizable materials remaining. Some resistant bits might persist (eggshell fragments, small twigs), but most of the original ingredients should be unidentifiable.

Smell: Well-finished compost has a pleasant, earthy aroma—similar to the smell of a forest floor after rain. It should never have a sour, ammonia-like, or rotten odour.

Appearance and smell combined are the most common ways for experienced composters to determine whether their compost is ready. But those could still be subjective and rely on experience. When someone is just starting out, there are a few more indicators to assist in determining.

Temperature: The pile will have cooled to ambient temperature—no longer generating significant heat—indicating that the active decomposition phase has completed.

Volume: The original materials will have shrunk considerably, typically to about one-third of their original volume.

Texture: When squeezed, finished compost should hold together slightly like a damp sponge, but crumble easily when disturbed. It shouldn't drip water or feel slimy.

Stability: If you bag or store finished compost, it shouldn't restart heating or develop unpleasant odours, indicating it's biologically stable.

For beginners, a helpful test you can try: Place a small amount of compost in a sealed plastic bag for a week. Upon opening, mature compost will maintain its earthy, pleasant aroma. If it develops a sour or rotten smell, the decomposition process is still ongoing.

Unfinished Compost vs. Finished Compost: Understanding the Difference

Unfinished compost still contains recognisable material (visible food pieces, woody chunks) and may reheat if turned or piled. It might have a slightly acidic or sour smell, though not necessarily unpleasant. The decomposition process is ongoing, meaning some nutrients aren't yet stabilised.

Finished compost is fully broken down, biologically stable, and has a balanced nutrient profile immediately available to plants. It's safer to use in direct contact with plants and seeds because it won't compete for nitrogen during further decomposition.

Compost Uses: Applications of Finished Compost in the Garden

What can finished compost be used for? It might be one of the most versatile materials we can work within our gardens. Let's explore the different ways we can put this nutrient-rich resource to work:

Understanding Topdressing vs. Mulching

Before diving into specific applications, it's helpful to understand two terms that often cause confusion:

Topdressing refers to applying a thin layer (typically 1/4 to 1/2 inch) of compost over existing plantings without disturbing the soil or plant roots. The primary purpose is to add nutrients and beneficial organisms that will gradually work their way into the soil through watering, earthworm activity, and natural settling. Topdressing is particularly useful for lawns, established perennial beds, and around delicate plants.

Mulching involves applying a thicker layer (usually 2-4 inches) of material around plants. While compost as mulch can be highly effective, the primary goals of mulching are to suppress weeds, retain soil moisture, regulate soil temperature, and prevent erosion. Nutrient contribution is a secondary benefit when using compost as mulch. The thicker layer stays on the surface longer and breaks down more gradually over time.

Both practices have their place in the garden, and sometimes the line between them blurs. In general, if you're applying a thin layer primarily for nutrition, you're topdressing. If you're applying a thicker layer primarily for protection, you're mulching.

As a Soil Amendment: The Foundation of Health

Garden Beds: Before planting, add a 1-3 inch (2.5-7.5 cm) layer of compost to the top of the garden bed and work it into the top 6-8 inches (15-20 cm) of soil with a fork or tiller. This improves soil structure, adds nutrients, and enhances microbial activity.

For established perennial beds, simply topdress with a 1/2-inch (1.25 cm) layer of compost in spring or autumn, letting natural processes incorporate it.

Potting Mix Component:

When we blend finished compost with other ingredients, we create a nutrient-rich potting soil that supports healthy container plants. A reliable recipe to start with is:

• 1 part compost
• 1 part garden soil (or coco coir)
• 1 part perlite or vermiculite (for drainage)

This mix provides excellent structure, moisture retention, and nutrition for container plants. For seed starting, use a finer-screened compost and limit to 25% of the mix to avoid potential issues with germination.

Using Compost as Mulch

Around Trees and Shrubs: Apply a 2-3 inch (5-7.5 cm) layer of compost around trees and shrubs, keeping it a few inches away from trunks and stems to prevent rot. This holds moisture, suppresses weeds, and slowly releases nutrients.

Vegetable Gardens: A 1-2 inch (2.5-5 cm) layer of compost around established vegetables feeds plants while conserving soil moisture. For tomatoes, peppers, and other heavy feeders, a compost mulch is particularly beneficial.

As a Lawn Treatment

Topdressing: Sift compost through a 1/4-inch (6 mm) screen and spread a thin layer (about 1/4 inch) over the lawn. Water it in or apply before rain to help it settle between grass blades. This improves soil biology, adds nutrients without burning, and helps prevent thatch buildup.

Lawn Repair: Mix equal parts compost and sand to fill low spots or bare patches in lawns before reseeding.

Creating and Using Compost Tea: Liquid Gold

What is compost tea?

Compost tea is essentially a liquid extract of finished compost, created by steeping compost in water (often with a small amount of unsulfured molasses added) and aerating the mixture to cultivate beneficial microorganisms.

Basic compost tea recipe:

1. Fill a bucket 1/3 full with finished compost
2. Add water until the bucket is nearly full
3. Add 1 tablespoon of unsulfured molasses (optional, to feed bacteria)
4. Aerate with an aquarium pump for 24-48 hours
5. Strain through cheesecloth or a fine mesh

Application: Use within a few hours of brewing (beneficial microbes begin dying after aeration stops). Apply as a foliar spray (diluted 1:1 with water) or soil drench (can use full strength). Using compost tea as fertilizer is particularly beneficial for boosting plant immune systems, increasing nutrient availability, and enhancing growth.

What to Do with Unfinished Compost: Safe Applications

Not all compost reaches that perfect finished state before we need to use it. Whether we're clearing space in the bin or simply working with what we have, there are several effective ways to use partially decomposed compost:

Using Unfinished Compost as Mulch

Unfinished compost in garden settings makes excellent mulch when applied to the soil surface. Because it's not incorporated into the root zone, any nitrogen drawdown during continued decomposition happens above the roots, not affecting plant growth.

Application: Apply a 3-4 inch (7.5-10 cm) layer around established plants, keeping it away from stems. By the time it finishes breaking down, it will have naturally incorporated into the soil below.

Best for: Ornamental beds, established perennials, trees, and shrubs. Less ideal for annual vegetables unless applied well in advance of planting.

Trench Composting

Trench composting offers a clever way to use unfinished materials by burying them in the soil where they can continue decomposing without disturbing existing plants.

How to do it:

1. Dig a trench or hole about 12 inches (30 cm) deep between rows or in areas planned to plant in the future
2. Add the unfinished compost
3. Cover with at least 6 inches (15 cm) of soil
4. Wait 1-2 months before planting directly in this area

Best for: Preparing future planting areas or enriching soil between existing plants. This is also an excellent way to use Bokashi pre-compost after fermentation.

Curing Bins

When space becomes limited in active composting systems but the material isn't quite finished, a dedicated "curing bin" provides the perfect solution. This separate space allows almost-ready compost to complete its transformation without occupying primary bin space.

How to do it:

1. Transfer partially decomposed compost to a separate bin or pile
2. Cover lightly (with breathable material like burlap if needed)
3. Leave it undisturbed for 1-3 months
4. Occasionally check moisture and add water if needed

The compost will finish breaking down during this resting phase, becoming stable and ready to use.

Sheet Composting

Also called "lasagna gardening" or sheet mulching, this method uses layers of unfinished compost and other organic materials to create new garden beds in place.

How to do it:

1. Cover the area with cardboard or several layers of newspaper
2. Add a 2-3 inch (5-7.5 cm) layer of unfinished compost
3. Layer with other materials like leaves, straw, or grass clippings
4. Repeat layers ending with a brown material on top
5. Allow to decompose for 3-6 months before planting

This approach is excellent for converting lawn areas to gardens or rehabilitating poor soil.

Understanding Compost Leachate and Tea: Liquid Gold or Caution?

Liquid byproducts often appear during the composting process, sometimes intentionally collected and other times appearing unexpectedly. Understanding the differences between these liquids helps us use them effectively and safely:

Compost Leachate

What it is: Leachate is the liquid that sometimes drains out of a compost bin or pile. It forms when water percolates through the decomposing material, often carrying soluble nutrients, microbes, and sometimes byproducts of anaerobic decomposition.

Cautions: Raw leachate, especially from an unfinished compost system, may contain anaerobic bacteria and unstable compounds that could harm plants or carry pathogens. It's not the same as properly brewed compost tea.

Safe use:

• If it smells earthy or not offensive, consider diluting it 1:10 with water and use it on ornamental plants (not edibles)
• If it smells foul (sulfurous or like sewage), it's best not to use it directly on plants
• Aerating leachate for 24 hours with an aquarium pump can help to encourage beneficial aerobic microbes before using
• Consider pouring it back into the compost pile if it needs moisture instead of applying to plants

Worm Bin "Tea"

What it is: The liquid that collects at the bottom of worm composting systems is often called "worm tea," though it's actually a form of leachate.

Safe use:

• Dilute it 1:10 with water before application
• Best used on ornamental plants rather than edibles
• If it smells unpleasant, aerate it for 24 hours before use or return it to the garden compost pile
• Can be used as a gentle liquid fertiliser for houseplants when properly diluted

When to Use Compost: Seasonal Guide to Application

The changing seasons present different opportunities and needs for compost application. Here's how we can work with nature's rhythms throughout the year:

Spring

• Garden Prep: Work finished compost into beds before planting spring vegetables
• Perennial Beds: Top-dress around emerging perennial plants
• Lawn Care: Apply sifted compost as a top dressing to lawns
• Potting Mix: Create compost-rich potting soil for the season's containers

Summer

• Mulching: Apply compost as mulch to retain moisture during hot weather
• Sidedressing: Add compost around heavy-feeding vegetables midseason
• Compost Tea: Use during active growth to boost plant health and productivity
• Refreshing Containers: Top-dress potted plants that may have depleted their soil nutrients

Autumn

• Soil Building: Apply compost to beds after harvest to decompose over winter
• Lawn Repair: Top-dress and overseed any damaged lawn areas
• Tree Care: Mulch around trees and shrubs before winter
• New Bed Preparation: Use sheet composting/lasagna method to create new beds for spring

Winter

• Indoor Plants: Use finished compost in potting mixes for houseplants
• Planning: Site new compost areas for the coming year
• Protection: Use partially finished compost as mulch around sensitive perennials

Special Applications for Different Garden Types

Vegetable Gardens

• Use finished compost at a rate of about 2-3 liters per square meter annually
• Apply more to heavy feeders like tomatoes, squash, and brassicas
• Consider trench composting between rows for continuous feeding

Flower Gardens

• Top-dress perennials with 1/2 inch of compost in spring or fall
• Work compost into annual beds before planting
• Use compost tea as a foliar spray to boost flowering

Fruit Trees and Bushes

• Apply compost in a ring around the drip line (not against the trunk)
• Use a 2-3 inch layer as mulch to suppress weeds and conserve moisture
• Replenish annually in early spring or late fall

Container Gardens

• Replace about 1/3 of the soil in perennial containers annually with fresh compost mix
• Top-dress containers midseason if plants show signs of nutrient deficiency
• Use diluted compost tea to water containers every 2-3 weeks during growing season

Compost Benefits: Tips for Maximum Garden Impact

1. Screen when needed: For fine applications like lawns or seed starting, sift compost through wire mesh to remove larger pieces. The result will be a fine textured compost. But many experienced composters don't care about this.
2. Track results: Keep notes on application rates and plant responses to fine-tune your approach. This is a very textbook suggestion, but serious gardeners do keep a notebook documenting their gardening experiments.
3. Don't overdo it: More isn't always better—a little compost goes a long way. The truth is it's probably quite difficult to overdo it; most of the time, you don't produce enough compost for your garden! Compost is food for the soil and plants. It's gone before you even notice!
4. Combine approaches: Use compost alongside other organic practices like mulching and crop rotation - if you have enough compost, of course.
5. Be patient with heavy soils: Clay soils may need repeated compost applications over several years to show dramatic improvement - where I live, we have very thin topsoil, heavy clay and flint. Perhaps more on my story tackling this in a future post.
6. Protect the soil food web: Apply compost to soil surface when possible to mimic nature's top-down approach.

Common Questions About What Compost Can Be Used For

"Can we use too much compost in our gardens?"

While compost is generally beneficial, excessive application can create problems:

• Too much phosphorus may build up over time
• Very high organic matter can retain too much moisture for some plants. For example, plants that thrive in your dry garden, most likely aren't fans of too much compost.
• Fresh, unfinished compost mixed into soil can temporarily tie up nitrogen. A good rule of thumb: only apply unfinished compost on the surface as topdressing or mulching.

For most garden applications, stick to the recommended amounts (1-3 inches worked into soil annually, or 1/4-1/2 inch as topdressing).

"Is using compost as fertilizer enough for the garden?"

Compost is an excellent soil conditioner and mild, balanced fertiliser, but:

• Heavy feeders may need supplemental fertiliser, especially nitrogen
• Compost releases nutrients slowly, which is ideal for overall soil health but may not provide immediate boosts for hungry plants
• Consider compost the soil's foundation, supplementing with specific organic fertilisers as needed

"What to do with finished compost that has bits of undecomposed material?"

Yes! A few persistent pieces don't make compost "unfinished." We can:

• Screen it, returning the larger pieces to the active compost pile
• Use it as is in applications where appearance isn't important
• Bury the partially finished material as trench compost

"What are the benefits of compost for plants and soil over time?"

Compost provides both immediate and long-term benefits:

• Some nutrients in the humus become available to plants within days of application
• Structural improvements can last 1-2 years
• The biological benefits (enhanced microbial activity) can persist for several seasons
• Regular applications build cumulative benefits over time

Conclusion: The Continuous Cycle of Garden Health

Using compost in our gardens closes the loop in the ecosystem of our home and garden. What begins as kitchen scraps and yard waste returns as nourishment for new growth, reducing waste while building healthier soil and more resilient plants.

Whether we're using fully finished compost as a premium soil amendment or finding creative ways to employ partially decomposed materials, we're participating in one of nature's most fundamental cycles. With each application of compost, we're not just growing plants— we're cultivating a thriving soil ecosystem that will reward us with healthier, more productive gardens year after year.

As we continue on this composting journey together, it's worth remembering that soil building is a long-term relationship. Every shovelful of compost we add is an investment in our garden's future—one that pays dividends in abundant harvests, vibrant flowers, and the deep satisfaction of working in harmony with nature's processes.

Completing the Cycle: Wrapping Up Our Home Composting Series

With this post on how to use compost in garden settings, we've now completed the full cycle of our Home Composting Series. Let's take a moment to reflect on the journey we've taken together:

1. First Post: We explored the fascinating science and magic that takes place in home composting, understanding the transformation process from waste to resource.
2. Second Post: We introduced various home composting methods and helped you choose the approach that best fits your lifestyle and living situation.
3. Third Post: We tackled the common challenges and troubleshooting problems that arise during home composting, making the process more accessible for beginners.
4. Fourth Post: We discussed home compostable products and materials, clarifying what can safely go into your compost system.
5. Fifth Post (this one): We've closed the loop by exploring what can compost be used for and the wonderful benefits of compost for plants you've created, completing the waste-to-resource cycle.

Throughout this series, available in both written and podcast formats, we've progressed from basic composting concepts to practical applications. The journey from kitchen scraps to garden treasure is now complete. My hope is that you now feel confident and equipped with the knowledge to either begin or enhance your own composting practice.

Looking Ahead: The Next Chapter

As we move forward, I'm excited to announce that we're starting a whole new series focusing on the nutrients contained within compost and how they directly respond to plants' needs in our gardens. As the growing season unfolds, it's the perfect time to deepen our understanding of soil fertility and plant nutrition.

Stay with us as we dig deeper into the relationship between soil health and plant vigour. As always, the goal is to reconnect with nature and understand the beautiful simplicity of natural cycles that connect us more deeply to the living world around us.

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Ready to start a composting journey? Visit our shop for home compostable products and composting supplies to help create nutrient-rich compost for the garden.

What's your favourite way to use compost in your garden? Share your experiences in the comments below!