An Easy Guide to Successfully Raising Seedlings in a Winter Greenhouse

We’re pretty lucky here in the Blue Mountains, our winters are mild compared to those that get covered in a blanket of snow… But still, winter can pose a challenge for gardening enthusiasts. But with the help of a greenhouse (and some other natural heating methods), you can continue nurturing your plants all year round. If you’d like to read more about the natural heating methods head to my previous blog post! Click here

Greenhouses provide a controlled environment that protects seedlings from harsh winter conditions while also giving you a nice cosy space to work in while its frosty outside. Below we will explore the key factors to consider when raising seedlings in a greenhouse during the Australian winter. Whether you're a seasoned gardener or just starting out, these tips will help you cultivate healthy seedlings in the colder months.

Here are some things to consider while raising your seedlings in your greenhouse!

Seed Viability: Before you begin, it's essential to ensure the quality and viability of your seeds. Check the seed packets for the recommended planting dates and expiration dates. Some seeds may have a shorter shelf life, so it's important to use fresh, viable seeds for optimal germination rates. Consider conducting a simple germination test by placing a few seeds between damp paper towels and monitoring their sprouting progress. This will help you determine the viability of your seeds and adjust your planting accordingly.

Temperature Control: Maintaining the right temperature is crucial for successful seedling growth. During winter, the warmer the better! While winter temperatures in Australia can vary across regions, aim to keep your greenhouse between 18°C and 24°C during the day and slightly cooler at night. You can put a thermometer in your greenhouse or take and educated guess on whether it’s warm enough, could you wear a t-shirt conformably? If yes then its warm enough! For some natural warming methods read my previous blog (they’re really cool).

Air Circulation: Proper air circulation is vital for preventing diseases and ensuring robust growth. Open the greenhouse vents during the day to allow fresh air to circulate and reduce the risk of fungal or bacterial issues. Air circulation strengthens those little seedlings too! You could use a small fan if you’re not in an area that a natural breeze is likely.

Watering: Watering seedlings correctly is essential, as overwatering or underwatering can lead to poor growth or root rot. Check the moisture levels of the soil regularly by using my ‘mud cake method’ sticking your finger or a small stick into the soil, if it comes out dry and clean your soil is dry, water asap, if its soggy like an undercooked cake, lay off the water and improve the ventilation. Use a watering can with small holes or a spray bottle with a fine mist to ensure gentle, even watering. Avoid wetting the foliage excessively, as it can promote disease development. Aim to maintain consistent moisture without allowing the soil to become waterlogged. I hydrate once a day in winter, but in the warmer months you may need to increase this!

Propagation Soil: Choosing the right propagation soil is crucial for healthy seedling development. Opt for a well-draining seed-starting mix that promotes good root growth. Avoid using garden soil, as it tends to be heavy with large particles like bark and wood chips and may also contain pests, diseases, or weed seeds. Seed-starting mixes, worm castings and finished compost are my go to! 2:1:1 ratio! Seed raising mixes are formulated to provide optimal conditions for germination and early growth and can be found at your local garden centres (support local where possible).

Managing Rot: Damp conditions and poor airflow in the greenhouse can increase the risk of rot and fungal diseases. To mitigate this, ensure proper drainage in your containers or trays and avoid overwatering. Provide adequate spacing between seedlings to allow air circulation and reduce humidity levels. If you notice any signs of rot or disease, promptly remove affected seedlings to prevent further spread.

Lighting: During the winter months, natural light may be limited. So ensure your greenhouse is placed int he best possible spot. If you’re really struggling for 6 outs of sun and your feelings are growing tall and skinny (leggy) consider supplementing with an artificial grow light to provide the necessary light intensity and duration for optimal growth. LED grow lights are energy-efficient and can be adjusted to match the specific light requirements of your seedlings. Send me an email if you need recommendations on this!

Hardening Off: As the weather begins to warm and spring approaches, it's crucial to gradually acclimate your seedlings to outdoor conditions. This process, known as hardening off, helps the seedlings adjust to the natural elements and prevents transplant shock.

Fertilising: Seedlings require proper nutrition to grow well. Begin fertilising your seedlings once they have developed their first true leaves. Choose a balanced organic fertiliser like Seasol or Charlie Carp, and dilute it to half the recommended strength! Apply the fertiliser every two weeks or as directed on the package, taking care not to over-fertilise, as it can damage tender roots. If you have used compost in your soil mix your seedlings will already have a nice biom to interact with. If not, consider adding a micro-organism fertiliser like EM1 or compost tea! Email me to learn more on this!

Here are some seedlings you can consider germinating during winter in the Blue Mountains, AND a guide of their recommended germination temperatures:

Vegetables: Winter is an ideal time to start germinating cool-season vegetables that thrive in colder temperatures and cane planted out in spring before the last frost. Some suitable options for the Blue Mountains include:

• Broccoli (germination temperature: 10°C-21°C)

• Cabbage (germination temperature: 10°C-21°C)

• Cauliflower (germination temperature: 10°C-21°C)

• Brussels sprouts (germination temperature: 10°C-21°C)

• Kale (germination temperature: 10°C-21°C)

• Spinach (germination temperature: 5°C-20°C)

• Lettuce (germination temperature: 7°C-21°C)

Perennial Herbs: Consider starting perennial herb seeds in July, as they require a longer time to establish before being transplanted outdoors. Some perennial herbs suitable for germination during winter include:

• Rosemary (germination temperature: 15°C-21°C)

• Thyme (germination temperature: 15°C-21°C)

• Sage (germination temperature: 15°C-21°C)

• Chives (germination temperature: 15°C-20°C)

Flowers: Winter-flowering annuals and perennials can add color and interest to your garden during the colder months. Consider germinating the following flower seeds:

• Pansies (germination temperature: 10°C-18°C)

• Violas (germination temperature: 10°C-18°C)

• Sweet William (germination temperature: 10°C-20°C)

• Primrose (germination temperature: 10°C-20°C)

• Iceland poppies (germination temperature: 10°C-21°C)

By starting these seedlings indoors or in a controlled environment, you can get a head start on the growing season and ensure stronger, healthier plants when it's time to transplant them outdoors in spring.

If you’d like to learn more about starting your own seeds, making the best compost and heating your greenhouse come to one of the Mountains Gourmet Workshops! Find them here!