Feeding Your Plants in the Autumn
Autumn Fertiliser Guide: Prepping Your Garden for the Cold Season
As the days grow shorter and temperatures drop, your garden’s needs change dramatically. Plants becoming dormant in the winter doesn’t mean that no work is required in the autumn. The right fertilisers applied during this period can boost root strength, improve plant resilience, and prepare your garden for a vibrant spring.
So, if you noticed that you didn’t get the best out of your garden this year, you’re planting perennials for next season or just have old soil: here’s a comprehensive guide on the best feeds to use leading up to winter.
Why Fertilize in the Autumn?
Your garden will continue to grow during the milder months of Autumn, but most plants will shift their focus down into the ground and towards their root systems. Your plants having a healthy, established root system will boost their chances of survival through the cold and encourage larger blooms in the next season.
During this time, your plants won’t require high-nitrogen fertilisers, as nitrogen primarily helps with leafy green growth. Instead, you’ll want to focus on nutrients that strengthen roots and improve plant health while they become dormant.
The key nutrients in autumn fertilisers are:
- Phosphorus: Supports root development and helps plants store energy for the winter.
- Potassium: Enhances plant resistance to disease, frost, and stress, which is essential during colder months.
- Calcium & Magnesium: Trace elements that help with nutrient uptake and soil conditioning, crucial for healthy soil during dormancy.
Fertilising in Autumn
- Slow-release, Low-Nitrogen Fertilisers
By autumn, most of your plants have shifted to storing energy in their roots, especially perennials that die back down to ground level. The best fertiliser for this season is a low-nitrogen, high-phosphorus and potassium blend, such as an NPK 5-10-10 formula.
- Why? Nitrogen promotes leafy growth, but in autumn, you want to avoid pushing out new foliage that will be vulnerable to frost. Instead, phosphorus and potassium help plants build strong roots and prepare for winter dormancy.
- When to apply: Apply a slow-release fertiliser early in autumn, allowing it to break down gradually over the coming months.
- Bone Meal for Strong Roots
Bone meal, a natural organic fertiliser rich in phosphorus and calcium, is perfect for autumn. It promotes strong root growth, making it ideal for bulbs, perennials, and newly planted shrubs.
- Why? Phosphorus helps develop a strong root system, which is essential for surviving the cold and bouncing back in spring.
- When to apply: Sprinkle bone meal into the soil when planting autumn bulbs like daffodils and tulips or when planting/dividing perennials.
- Lawn Fertilisers: High in Potassium
Your lawn can also benefit from a specialized autumn fertiliser. Look for a low-nitrogen, high-potassium formula (like 5-5-15) that will strengthen the grass and improve its tolerance to frost and cold conditions.
- Why? Potassium fortifies your lawn against the winter chill, making it hardier and reducing the chance of disease and frost damage.
- When to apply: Fertilise in late autumn, once the lawn’s growth has slowed, but before the first frost. This ensures the roots absorb the nutrients for better overwintering.
- Lime for Soil pH Adjustment
In London’s climate, rainfall can cause the soil to become more acidic over time. Acidic soil can limit plant growth, as many nutrients become less available in low-pH environments. Applying garden lime in autumn helps neutralise this acidity but be sure to check the soil pH before applying, your beds might be fine just as they are. This one doesn’t apply to lime-hating plants like acers, rhododendrons and camellia.
- Why? Lime raises the soil’s pH, improving nutrient availability and soil structure.
- When to apply: Any time in autumn is an ideal time to apply lime as it breaks down slowly and adjusts the soil’s pH over the winter, preparing it for spring growth.
- Mulching with Organic Matter
While not an obvious ‘fertiliser’, applying organic mulch such as composted bark or well-rotted manure during the winter can greatly improve soil health. As the mulch breaks down, it enriches the soil with nutrients, increases soil temperature and helps prevent weed growth.
- Why? Organic mulch feeds your soil, improves its structure, and helps retain moisture, which is especially beneficial in winter.
- When to apply: Late autumn or early winter, before the ground freezes, to protect the roots of dormant plants and improve the soil for spring.
- Houseplants: Light Feeding Indoors
For plants that you’ve brought indoors for the winter, consider a light feeding regimen. Indoor plants in heated environments may still require nutrients but at a reduced rate. Use a water-soluble, balanced fertiliser (such as 10-10-10) at half-strength.
- Why? Indoor plants can continue to grow in heated spaces and benefit from occasional feeding to maintain health and vibrancy.
- When to apply: Fertilise once every 4-6 weeks, depending on the plant’s growth and light exposure. If it has new growth, feed it!
- Using Mycorrhizal Fungi
This one isn’t necessarily a fertiliser, but it can help. If your soil is already in good condition but you’re planting woody plants (such as bare root trees) to get established for next year, consider using Mycorrhizal Fungi. The fungus will create its root system around your plant, boosting your plant's ability to take in moisture and nutrients.
- Why? It’s a natural relationship that will benefit plants going into any soil you don’t intend on cultivating too much. The fungi will protect against drought and help draw in nutrients for them.
- When to apply: Once! One application will help your plant for a lifetime. Not for use on any plants in the Brassica family.
Key Tips for Winter Fertilisation
- Test your soil: Always test your soil before applying any fertilisers or lime, especially if you’re unsure of its pH. This will help avoid over-fertilising or applying unnecessary nutrients.
- Slow-Release Granules: Most specialised fertilisers like superphosphate will come in granular form. They can be worked into the top layer of your soil and the nutrients will gradually release down into the plant. They’re best used with already moist soil and watered in afterwards. Follow application instructions on the packaging carefully, over fertilising can burn your plants.
- Water wisely: Fertilisers should be applied to moist soil. If winter rains are frequent, they will do the job for you, but during dry spells, make sure to water after fertilising.
