Reusing Compost: 5 Ways to Recycle It

Compost is the life force of many gardens. Whether buying it in bags from the garden centre (hopefully peat-free) or making your own with composting bins, this black gold is a multi-purpose wonder feed for the garden. However, most gardeners I speak to don’t know what to do with compost once it’s finished feeding your plants or once plants in containers have died back.

This guide will show you how to save money and improve your growing potential by reusing compost, and I’ve added a few extra tricks below that I use myself at Garden Ninja HQ, including how to check spent compost for pests before you put it back to work, and how far you can push the same bag of compost before it’s truly done.

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Can I reuse spent compost?

Yes, you can! Compost is nature’s way of recycling dead plants and organic leafy matter into a rich dark growing media. Which, when broken down, is rich in nutrients and moisture retentive.

Compost isn’t the same as soil. It’s a much richer growing material for fruit, vegetable and container gardening, allowing plants to easily root and access water and nutrients compared to complex soil.

Whilst compost will deplete over time, there’s nothing stopping you from getting every last bit of goodness out of it. This guide will show you how you can reuse compost in a number of different ways, and after nearly two decades of client gardens and my own patch at home, I still reach for spent compost before I reach for a fresh bag most of the time.

Over time old compost will:

• Reduce its nutritional content as plants consume them as feed
• Start to lose its structure and become looser losing moisture quicker
• Lower microbial activity (the good bacteria and organisms living in it)

How long does compost last?

Most compost manufacturers or garden centres will urge you to replace compost each year. Of course, they will. They want to sell more! Most compost starts to lose its macro and micronutrients within a few weeks of being planted into. Those hungry plants will soon start to absorb the goodness of compost as it gradually degrades over time.

Reviving old compost

But did you know you can revive or boost old compost for another years use? There are a few tricks you can use to help increase the lifespan of old compost.

The best way to do this is by the following:

• Mix old compost with some new compost to help boost its nutrients
• Add shredded leafy green material such as some grass clippings or even better, comfrey
• Add well-rotten manure to it
• Mix with a handful of chicken poo pellets

Can I reuse compost after growing vegetables or last years pots?

Yes, you can. Though you need to check that your plants were healthy and had no diseases, viruses or soil bourne pests. Like lily beetle or mealy bugs.

When it comes to reusing compost from vegetables such as tomatoes, the likelihood is that most of the nutrients will have been used up. So either top up with fertiliser or reuse it for less intensive plants like herbaceous perennials or as a top dressing for your garden borders to help implode soil structure.

What’s the difference between compost and soil?

A number of people confuse compost with garden soil. The two are very different.

Soil is a mix of 5 key ingredients that are naturally occurring:

1. Organic material
2. Water
3. Nutrients
4. Minerals/ rock particles
5. Organisms.

Compost on the other hand, is a man-made material made from organic matter usually found in the garden. It is usually made of a mix of green matter (nitrogen-rich clippings) and brown (carbon-based) material such as twiggy woodier material.

Compost creates a highly nutritious material that breaks down over time. When this breaks down into the soil it becomes humus (not the same as the delicious chickpea-based snack).

Soil is a mix of 5 naturally occurring ingredients. Stable and able to support plants for their lifetime.

Compost is absorbed by the soil, used to feed plants, retain moisture and act as a temporary growing medium. It has a limited shelf life as it degrades and breaks down. So, the soil is the stable matter that plants live and establish in, whereas compost is a temporary growing medium that eventually needs replacing or replenishing, unlike soil.

Compost: dark, rich and nutritious! A short-lived fuel boost for plants and organisms to feed off.

Why recycle compost?

As conscientious, ethical gardeners, we should always seek to reduce, reuse and recycle wherever possible. There is no point in getting closer to nature to then, conversely, place more of a burden on poor Mother Nature.

By reducing our demands on the environment, we are also helping to protect and nurture our green spaces. It’s also far better to reuse things in the garden than have to pay someone to take them away. It’s a win-win!

Compost, believe it or not, can still have many uses after plants have been grown in it. It still helps improve soil structure, retain moisture and add small amounts of nutrition to the soil in our gardens.

So let’s have a look at how I check compost is fit for reuse, then my five favourite ways to put old spent compost back to work.

Removing old spent plants or weeds enables you to loosen old compost so it’s usable again.

Should I remove dead or annual plants from old compost?

You’d be surprised how often people ask me whether to take out dead or spent annual plants from compost. The answer is yes. You can prepare your spent composted containers by lifting out the old dead plants and weeds.

Shake off as much compost as possible, then either compost or dispose of the old plant rootstock by cutting it up with secateurs first so it breaks down easily.

Perennial weeds can be hot composted or placed in a bucket for 6 weeks and then strained to make weed tea (a liquid feed for the garden).

You can also use a griddle, riddle or garden sieve to filter out any bits of old dead root or debris from your spent compost.

Sieving old compost removes old roots or bits of debris shown above.

It’s worth noting that if your plants have had known pathogens, diseases or issues during the growing year, such as blight. You shouldn’t reuse this compost as it can further infect other plants. Use your judgement if you think your compost may be harbouring nasties. For most gardeners, your used compost will be fine as known issues will have been very apparent by looking at the previous plant’s health.

How do I check spent compost for pests like vine weevil before reusing it?

Before I put any spent compost back into service, I always give it a quick pest check first. This is one of those steps that takes five minutes but saves you months of grief later in the season, because vine weevil grubs in particular are notorious for hiding unseen in old pot and container compost right through winter.

🛒 Buy a garden tarpaulin or dust sheet from Amazon UK. Tip your spent compost out onto it in a thin, even layer rather than a heap, since the grubs are far easier to spot when they’re not buried under several centimetres of material.

I found my first proper vine weevil infestation a good few years ago now, tipping out a batch of spent hanging basket compost onto the patio at Garden Ninja HQ. What I thought was a fairly innocent pile of old compost turned out to be riddled with the tell-tale creamy white, C-shaped grubs, and I’ve checked every batch since. My trick now is to leave the spread-out compost on the tarpaulin for half an hour or so and let the local robins do the hard work. They’re remarkably efficient at picking through a thin layer of compost and clearing out anything that moves, and I’ve never once had to intervene beyond giving the pile an occasional turn with a hand fork to expose whatever they’ve missed.

Adult vine weevils themselves are less of a worry in compost, as they tend to be found chewing notches out of leaves rather than hiding in the growing medium. It’s the larval grubs you’re checking for, since they feed on plant roots below the surface and can quietly kill off whatever you plant next without much warning until the plant suddenly wilts. If you spot even a handful of grubs, don’t panic and don’t bin the whole batch. Spread it thinly, let the birds have their fill over a day or two, then give it one final check before you use it.

Beyond vine weevil, keep an eye out for slugs, slug eggs (small, translucent, pearl-like clusters) and the occasional overwintering leatherjacket if the compost has been sitting outdoors near lawn edges. None of these is a reason to discard good compost; they’re simply a reason to check it in daylight before it goes anywhere near your best containers.

Five ways to reuse compost

There’s a variety of different uses for recycling compost, so I thought it was a good time to share my top five uses for spent compost!

1. Mulching your garden borders
2. Improving lawn seed germination
3. Speeding up existing compost heaps
4. Top dressing and repairing your lawn
5. Giving pots and containers a second season

1. Mulch your garden borders

If you’ve been following my work for some time, you’ll know that I love nothing more than to mulch my garden borders. Mulching helps reduce weeds, improve soil structure and can help reduce watering. It’s also a lovely mindful low drama gardening activity.

Used compost is an excellent organic material to help lock in moisture and provide a slow-release feed to flower beds. Even the most tired compost will help improve rich soil texture and help provide a better ‘crumb’ structure to your garden soil. Every autumn at Garden Ninja HQ I empty the last of the season’s hanging baskets and patio pots straight onto the borders around the pond project, and it’s an easy way to get a final bit of value out of compost that has already done a full summer’s work.

If you need any more convincing, here’s my garden mulching guide in more step-by-step detail!

2. Improve lawn seed germination

One of my best secret tips is to use spent compost as a lawn seed hack to help germination. I’ve even trialled this method versus the traditional lawn seeding method where the seed is scattered and then pressed on the soil. However, this leaves lawn seed prey to hungry birds and can mean that the seed never gets the optimal moisture to germinate effectively.

My top tip after reseeding a lawn, whether as a new lawn or lawn repair, is to sieve used compost lightly over the seed. Then water well.

What happens is the compost holds onto the moisture far longer than the bare soil will, meaning that the seed imbibes water (needed for germination) far faster. I also find that by scattering compost over the lawn seed, it is easier to spot the grass seed as it sprouts.

As the used compost is low in fertility, it won’t affect the viability of the seed as a rich compost might. It also can be used as a wonderful lawn top dressing to help improve soil structure and lightly fertilise your lawn.

3. Improve your compost heaps

Whether you already have a compost heap or are about to start a new one, used compost is a great way to give it a boost. The beneficial bacteria already present in used compost can help speed up your home composting methods.

With a new compost bin, first mix some green (nitrogen-rich) material and brown (carbon-rich material) and then mix in some used compost. This will help create the perfect environment needed for the material to break down. As the used compost retains more moisture, it also stops your compost bin from drying out, which can then slow down the composting process.

Spent compost is a great compost heap booster! I’ve also used old compost to help improve compacted or smelly compost heaps. Usually, when too much nitrogen-rich matter, such as grass clippings, causes the compost pile to stagnate. You don’t need much, just enough to help your compost bin along. It can be beneficial to turn it into the existing pile to spread it around rather than just layering it on top.

Composting is a brilliant way to recycle all sorts of garden material from prunings, lawn clippings and growing your own vegetables. This in turn, ends up in a cycle of recycling, reusing, etc. Drastically reducing your garden waste! if you’re thinking of starting a new compost bin, my guide below should help set you up for composting success!

4. Top dress and repair your lawn

Beyond helping seed germinate, spent compost is properly useful once your lawn is already established. If you’ve spiked or hollow-tine aerated your lawn in autumn, brushing sieved spent compost into the holes left behind is one of the best top dressings you can use, and it costs nothing beyond the compost you already had sitting around. The compost works its way down into the aeration holes, improving drainage and soil structure exactly where the lawn needs it most.

I mix sieved spent compost roughly half and half with sharp sand for this job, which keeps the mix free-draining rather than claggy. Work it in with the back of a rake or a stiff broom so it settles down into the holes rather than sitting on top of the grass, and give the lawn a good water afterwards if the weather is dry.

Spent compost is also handy for evening out dips and hollows that develop in a lawn over time, particularly where a tree root has lifted an area or where heavy foot traffic has caused settling. Fill the low spot with compost, firm it gently with the back of a rake, oversow with a little grass seed if the hollow is bare, and let the lawn grow back over the top. It’s a simple fix that saves you buying a specialist topsoil or top dressing mix for what is often a fairly small job.

5. Give pots and containers a second season

This is the use I get asked about most, and the honest answer is that spent container compost has more life left in it than most gardeners assume. I currently have a large glazed pot on my patio that has grown tomatoes for three summers running on nothing more than the original compost topped up with a fresh mulch of well-rotted manure and a granular feed each spring. The plants have never looked back, and it’s saved me buying a full new bag of compost every year for that one pot alone.

Working a handful of granular feed into the surface of reused compost at planting time replaces most of what the previous season’s plants took out, and it’s the single easiest way to get another good season from a pot without emptying and refilling it.

🛒 Buy a slow-release fertiliser for containers from Amazon UK.

Some plants are far less fussy about growing in reused compost than others, and it’s worth knowing which is which before you commit your best containers. Salad leaves, herbs, potatoes and most hardy perennials will grow away happily in compost that has already had one season of use, particularly once it has been refreshed with a little fresh compost mixed through it. Hungrier, more demanding crops such as tomatoes, chillies, aubergines and courgettes are a little more particular, though I’d still encourage you to try them in reused compost if you’re feeding regularly through the season, as my own patio pot proves it can be done successfully year after year.

Growbag compost from tomato or cucumber growbags can be reused in exactly the same way, though I’d treat it as slightly inferior to potting compost from the start, since growbags are typically filled with a lighter, less nutritious mix designed for one intensive season. Mix it with fresh compost or well-rotted manure before reusing rather than relying on it alone, and it’s perfectly serviceable for a second round of less demanding crops or as a border mulch if you’d rather not push it further.

If you’re not ready to reuse a batch of compost straight away, keep it somewhere dry over winter rather than leaving it out in an open bucket to get waterlogged by rain. I keep mine in a stack of old compost bags in the corner of the greenhouse, loosely folded over rather than sealed, so it can breathe without turning into a soggy mess by spring.

When shouldn’t you reuse spent compost?

There are a few instances where spent or used compost is not be suitable for reuse. Whilst pretty rare here are some scenarios when you shouldn’t reuse spent compost:

i) Disease Presence

If plants grown in the compost suffer from any fungal, bacterial, or viral diseases, it’s best not to reuse the compost to avoid spreading the pathogens to new plants. This is particularly important for diseases that can persist in the soil, such as certain types of root rot or wilt diseases. Usually quite rare for most gardeners, this wouldn’t be a Garden Ninja expert post if I didn’t give you both sides of the compost story!

In particular, the following diseases can stay present in the soil and compost for years if not removed:

• Pythium Root Rot: Caused by Pythium spp., this disease affects the roots and lower stems of plants, causing damping-off, root rot, and stunted growth.
• Rose Black Spot: The most common soil-carrying fungus. If you have black spots on your roses and the leaves have fallen and rotten into your compost, do not reuse it as it simply spreads the black spot around.
• Fusarium Wilt: Caused by Fusarium spp., this disease causes wilting, yellowing, and eventual death of plants by clogging the vascular system.
• Verticillium Wilt: Caused by Verticillium spp., this fungal disease affects a wide range of plants, causing wilting, yellowing, and eventual death by restricting water flow in the vascular system.
• Rhizoctonia Root Rot: Caused by Rhizoctonia spp., this disease affects the roots and lower stems of plants, causing damping-off, root rot, and reduced plant growth.
• Phytophthora Root Rot: Caused by Phytophthora spp., this disease affects the roots and lower stems of plants, causing damping-off, root rot, and wilting.
• Clubroot: Caused by Plasmodiophora brassicae, this disease affects brassica crops like cabbage, broccoli, and cauliflower, causing swollen, distorted roots and stunted growth.
• Sclerotinia Rot: Caused by Sclerotinia spp., this fungal disease affects a wide range of plants, causing soft rot, wilting, and collapse of infected tissues.
• Blackleg: Caused by various species of bacteria such as Pectobacterium and Dickeya, this disease affects the stems and roots of plants, causing blackening, rotting, and wilting.
• Root Knot Nematodes: Caused by Meloidogyne spp., these microscopic worms infect the roots of plants, causing galls, stunted growth, and reduced nutrient uptake.
• Southern Blight: Caused by Sclerotium rolfsii, this fungal disease affects a wide range of plants, causing damping-off, root rot, and wilting.

ii) Pest Infestations

Spent compost that has been infested with pests like nematodes, larvae, or insect eggs should not be reused, as these pests can cause damage to new plantings. Common pests include vine weevils, root-knot nematodes, and various types of larvae. This is exactly why I always run through the pest check further up this guide before putting any batch of compost back into use.

iii) Weed Seeds

Spent compost may contain weed seeds or rhizomes from previous plantings, especially if you see mares tale, docks or nettles taking root in your compost. Reusing the compost without proper sterilisation or weed control measures could introduce weeds into your garden beds or containers. Always sieve your compost if its been particularly infested with weeds or simply dispose of to prevent spreading them around.

Frequently asked questions about reusing compost

How many times can I reuse the same compost?

There’s no strict limit as long as the compost is free of disease and pests. I have pots at home that have been refreshed and reused for several years running rather than emptied and replaced. The main thing that changes over time is structure and nutrition rather than any hard cut-off point, so keep refreshing with fresh compost, manure or a granular feed each time and there’s no real reason to stop.

Can I reuse compost that I grew tomatoes in?

Yes, provided the previous plants were healthy. Tomatoes are hungry feeders so the compost will need a proper top up of fertiliser and ideally some fresh compost mixed through it before you plant into it again. If the previous crop showed any signs of blight or root disease, it’s safer to use that batch for mulching your borders instead rather than growing tomatoes in it a second time.

Is it safe to reuse compost from a growbag?

Yes, though growbag compost tends to be a lighter, less nutritious mix than potting compost to begin with, since it’s designed for one intensive season of growth. Mix it with fresh compost or well-rotted manure before reusing it, and it’s better suited to a second round of less demanding plants, or as a mulch, than to another season of hungry crops on its own.

Can I use spent compost instead of topsoil?

Spent compost isn’t a direct substitute for topsoil, since it lacks the mineral content and long-term stability that soil provides, but it works well blended into existing soil to improve structure and moisture retention. Use it to top up raised beds alongside proper topsoil rather than relying on it as the sole growing medium for anything long-term like shrubs or trees.

How do I store spent compost until I’m ready to reuse it?

Keep it somewhere dry and out of heavy rain, ideally in a stack of loosely folded compost bags in a shed, greenhouse or under cover rather than an open container. This stops it becoming waterlogged and compacted over winter, and means it’s ready to sieve and use again the moment the growing season starts.

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Summary

I’m a great believer that the process of successful gardening involves understanding how everything is connected. Then working with these connections for a more harmonious gardening experience. That even our organic garden waste has a cycle back into the earth, which saves on waste to landfill.

By reusing your garden compost in one of these five ways, and giving it a quick pest check first, you can improve your garden with zero additional spending or unnecessary waste. So what’s not to love?

Do you have additional ways you’ve found to use spent old compost? If so, let me know below. Why not share your gardening stories with me on Social media?

You can Tweet, Facebook or Instagram me with your composting tips. If you need gardening advice, why not check out my YouTube gardening guides and subscribe if you haven’t already?

Happy Gardening!

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Lee Burkhill

Lee Burkhill, known as the Garden Ninja, is an award-winning garden designer and horticulturist with over 30 years of gardening experience and 15 years as a professional garden designer. A qualified RHS (Royal Horticultural Society) professional, Lee specialises in sustainable garden design and practical horticultural advice. He designs and presents on BBC1’s Garden Rescue and in leading gardening publications. Lee combines three decades of hands-on gardening knowledge with professional design qualifications to help gardeners create beautiful, functional outdoor spaces.

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