Starting Seeds in a Greenhouse [Hometown Structures]

One of the things I was most excited about doing in my Hometown Structures Greenhouse is seed starting. With summer upon us, you might be thinking “seed starting is over for the year!” But if you’re trying to grow year round like I am, July and August are actually when we need to start planning our seed starting for our fall and winter gardens. So the cycle is beginning again and while a greenhouse makes seed starting easier in so many ways - with it comes some challenges that are unique to starting seeds in a greenhouse that we’re going to dive into today.

What’s the benefit of starting seeds in a greenhouse?

A greenhouse like mine from Hometown Structures allows you to control the temperature and environment, which means you can start seeds earlier in the spring than you would be able to outdoors. This gives your plants a head start on the growing season and can lead to a longer harvest.Greenhouses provide a more stable environment for seedlings to grow in, with consistent temperatures, humidity, and light. This can help to prevent them from becoming leggy or stressed, and can lead to stronger, healthier plants overall. You're also less limited to planting seeds that thrive in your local climate. A greenhouse allows you to grow plants that wouldn't normally survive outdoors in your area. Seedlings are vulnerable to pests, diseases, and harsh weather conditions. A greenhouse can help to protect them from these threats. And with a greenhouse, you have complete control over the environment your seeds are germinating in. This allows you to fine-tune the conditions to match the specific needs of each plant variety.

What are the key steps and considerations for successful seed germination?

Before Planting:

Choose high-quality seeds from a reputable source. Consider factors like your climate, desired harvest time, and gardening experience when selecting seeds. Different plants have different germination requirements. Research the specific needs of your chosen seeds, including ideal planting time, depth, light requirements, and any pre-treatment needs (like scarification or soaking). Seed packets often have this information. Use a well-draining, sterile seed starting mix. Avoid using garden soil directly, as it can be too dense and harbor pathogens.

Planting:

Plant seeds at the correct depth according to their size. A general rule is to plant seeds 2-3 times their diameter deep. Water the growing medium thoroughly before planting. After planting, keep the soil consistently moist but not soggy. A mister can be helpful for maintaining even moisture.Maintain the optimal temperature range for your seeds. Use a heat mat if necessary to provide warmth for seeds that prefer germination in warmer soil, and keep in mind that some seeds need light to germinate, while others prefer darkness. Follow the instructions for your specific seeds. You may need to cover the planting tray with plastic wrap or a humidity dome to retain moisture and create darkness if needed.

After Planting:

Once seedlings emerge, provide them with adequate light for strong growth. You may need to use grow lights if natural light is insufficient. Maintain consistent moisture levels. As seedlings grow, they will require more water. If you sowed multiple seeds per pot, thin them once seedlings have established true leaves to prevent overcrowding. Once seedlings have a few sets of true leaves, you can begin fertilizing them with a weak solution of liquid fertilizer.

Setting Up Your Greenhouse for Seed Starting

A clean environment is crucial for successful seed starting in your greenhouse. Seedlings are particularly vulnerable to fungal diseases and pests in their early stages. Fungus spores and bacteria that cause damping-off (a disease that attacks seedlings) can thrive in dirty environments. Sterilization helps eliminate these pathogens before they can harm your seedlings. Insects and other pests can also be attracted to leftover debris and organic matter. A clean environment reduces their harborage points. Cleanliness promotes healthy root development and prevents competition from unwanted microorganisms for nutrients and water.

Disinfecting Tools, Containers, and Surfaces:

Start by removing any leftover soil, plant material, or debris from your greenhouse, tools, and containers. A shop vac can be helpful for floors. Wash all surfaces, tools (pots, trays, trowels, etc.), and containers with warm, soapy water. This removes dirt, grime, and organic material that harbors pathogens.

There are a few options for disinfecting your greenhouse and equipment:

• Bleach: Mix one part household bleach (5.25% sodium hypochlorite) to nine parts water. Soak containers and tools for 30 minutes. Rinse thoroughly afterwards. Caution: Bleach is corrosive and can damage some materials. Wear gloves and work in a well-ventilated area. Never mix bleach with other cleaning products.

• Hydrogen Peroxide (3%): This is a gentler alternative to bleach. Apply directly to clean surfaces and tools with a spray bottle. Let it sit for at least 20 minutes, then wipe clean with a damp cloth. Hydrogen peroxide is safe for most materials.

• Alcohol (70%): Similar to hydrogen peroxide, apply directly to clean surfaces and tools. Let it sit for a few minutes, then wipe clean. Alcohol evaporates quickly, making it a convenient option.

While new containers are generally sterile, it's still wise to wash them with soapy water before use. Allow everything to dry completely before using them for seed starting. Moisture can encourage fungal growth. Wash down greenhouse walls, floors, and benches with your chosen disinfectant solution. It's a good practice to clean and disinfect your greenhouse and equipment between each planting season.

Now that your tools are clean and sterilized, you can set your greenhouse up for success by organizing everything before you get started to optimize seed germination and seedling growth. Dedicate a well-lit, easily accessible area for seed starting. Shelving or a designated seed starting table is ideal. This area should be close to a water source for easy watering. Once seedlings emerge, they'll need space for continued growth. Staggered shelves or benches will maximize space utilization. Consider using a mobile bench for flexibility. Allocate space for storing seeds, pots, trays, tools, and other supplies. Hanging shelves, cabinets, or a designated storage area will keep things organized and within reach.

Optimizing Seed starting Conditions

Group plants together based on their light requirements. Seedlings needing high light should be placed closest to the light source (windows or grow lights). Shade-loving plants can be placed on lower shelves. Arrange plants according to their temperature needs. Heat mats can be used on lower shelves for warmth-loving seeds. A thermometer can help you monitor temperature variations within the greenhouse. Ensure proper air circulation to prevent fungal diseases. Avoid overcrowding plants and position them to allow for airflow between them. You may need to use small fans to create gentle air movement.

Utilize vertical space with shelves or hanging racks to maximize your limited footprint, and clearly label each pot or tray with the plant variety, planting date, and any special instructions. This will help you stay organized and track progress. Arrange everything to allow for easy access for watering, monitoring, and transplanting seedlings. Avoid placing heavy objects on high shelves. Use trays with wheels or place them on rolling benches for easy movement and rearranging. Use drip trays under pots to catch excess water and prevent moisture build-up on lower shelves.

Seed starting containers

Seedling trays

Trays come with multiple cells and are ideal for starting small seeds or seeds that don't require deep planting. Standard propagation tray have shallow cells (1-2 inches deep) and are perfect for starting most flower and vegetable seeds. Deep cell trays have a depth of 3-4 inches and are suitable for seeds with longer taproots. Choose a tray size that fits your workspace and the number of seeds you plan to start.

Pots

Individual pots are ideal for larger seeds or plants that need more root space early on. 3-4 inch pots are perfect for transplanting seedlings from trays or for starting larger seeds like tomatoes or peppers.

Small seeds don't need deep pots. Shallow trays or pots with minimal depth (1-2 inches) are sufficient. Larger seeds benefit from deeper pots (3-4 inches) to accommodate developing roots. Plastic is the most affordable and durable option. Biodegradable pots are eco-friendly but may dry out faster and are not reusable. Ensure your pots or trays have drainage holes to prevent waterlogging, which can rot seeds and seedlings.

Ideal Soil Mix for Seed Starting

Seedlings need a well-draining, sterile, and lightweight potting mix to germinate and grow successfully. Look for a mix that contains peat moss, perlite, and vermiculite. These ingredients provide good drainage, aeration, and moisture retention. Many seed starting mixes are available commercially and are formulated specifically for seedlings. These are a convenient option. Avoid using garden soil directly for seed starting. It can be too dense, harbor pathogens, and contain weed seeds.

Selecting Seeds and Planning Timing for Greenhouse Seed Starting

Choose seeds suited to your local climate. Consider factors like your growing season length and average temperatures. Pay attention to "days to maturity" listed on seed packets. This helps you plan your planting schedule to ensure your harvest falls within the frost-free window. Purchase seeds from reputable sources to ensure good quality and germination rates.

The key to successful seed starting is timing your plantings to coincide with the last frost date in your area. This is the average date of the last light frost in spring. You can find this information through websites like the Old Farmer's Almanac (https://www.almanac.com/gardening/frostdates) which allow you to search by zip code. Once you know your last frost date, you can create a seed-starting calendar. Look for the "days to maturity" listed on each seed packet. Count back the number of "days to maturity" from your last frost date. This will give you the recommended date to sow your seeds indoors. Add some buffer days to account for seed germination time (usually 5-10 days). This will give you your ideal seed starting date. Tender plants like tomatoes, peppers, and eggplants benefit the most from starting indoors for an earlier harvest. Consider staggering your plantings throughout the season to extend your harvest window. For some fast-growing vegetables like lettuce or spinach, you can practice succession planting. Sow seeds every few weeks directly outdoors after the last frost date for a continuous harvest.

Selecting suitable varieties for seed starting is crucial and requires considering your climate, growing season, available space, and seed quality. Choose varieties suited to your average temperatures. Consider both warm and cool season crops. Some plants won't tolerate frost, while others may even benefit from a light frost. Seed packets will typically indicate temperature preferences. Pay attention to a plant's light requirements (full sun, part shade, etc.). Not all plants will thrive in the amount of sunlight your greenhouse receives. Select varieties with maturity dates that fall within your frost-free window. Short season varieties are ideal for areas with shorter growing seasons. Consider how long each variety takes to mature from seed to harvest. Stagger plantings throughout the season for a continuous harvest.

Choose varieties that will fit comfortably within your greenhouse space when fully grown. Consider dwarf or compact varieties for limited space. Tomatoes, for example, come in bush and vining varieties, with bush tomatoes requiring less vertical space. Some plants sprawl outwards, while others grow vertically. Factor in the mature size and growth habit when choosing varieties to optimize space usage.

High-quality seeds from reputable sources will have better germination rates, meaning more seeds will sprout successfully. This saves you time and resources. Consider varieties with resistance to common diseases in your area. This can help reduce problems and improve your harvest. Heirloom varieties may have lower yields but produce seeds you can save for future plantings. Hybrids are often bred for high yields and disease resistance but don't produce true seeds. Choose vegetables, herbs, or flowers you and your family love to eat or use and once you have the basics down, try new and interesting varieties to expand your gardening experience.

Optimizing seed germination in your greenhouse

As mentioned before, using fresh, high-quality seeds from reputable sources increases your chances of successful germination. Some seeds require specific treatments like scarification (nicking the seed coat) or stratification (exposure to cold temperatures) to break dormancy and encourage germination. Research the needs of your specific seeds. Follow the recommended planting depth on seed packets. Planting too deep can hinder germination. Keep the growing medium evenly moist but not soggy. A mister can be helpful for maintaining consistent moisture levels. For seeds that prefer warm soil temperatures for germination, use a heat mat placed under your seed trays. Some seeds need light to germinate, while others prefer darkness. Follow the instructions on the seed packet. You may need to cover trays with plastic wrap or a humidity dome to retain moisture and create darkness if needed. Maintain some air circulation to prevent fungal diseases. Avoid overcrowding seedlings or creating stagnant air pockets. And clearly label each pot or tray with the plant variety, planting date, and any special instructions. This will help you track progress and avoid confusion.

Optimal Temperature and Humidity for Seed Germination

Different plant varieties have specific temperature ranges for optimal germination. Seed packets typically indicate the ideal temperature range. A general guideline is that Warm Season Crops (like tomatoes, peppers, eggplants) prefer temps between 70-80°F (21-27°C) and Cool Season Crops (like lettuce, spinach, kale prefer temps between 60-70°F (16-21°C)

Most seeds benefit from high humidity during germination. You can achieve this by covering seed trays with a clear plastic dome that helps trap moisture around the seeds and lightly misting the surface of the growing medium with a spray bottle to keep it moist. Use a thermometer and hygrometer to monitor temperature and humidity levels in your greenhouse. Make adjustments as needed to maintain optimal conditions for your seeds.

Seed germination is highly sensitive to temperature fluctuations. Maintaining a consistent temperature within the optimal range for your specific seeds is crucial for successful seed starting in your greenhouse. Enzymes play a vital role in seed germination. Consistent temperatures ensure optimal enzyme activity, which is essential for seed development and sprouting.Consistent temperatures promote even and synchronized germination across all your seeds within a tray. Fluctuations can lead to some seeds germinating earlier or slower than others, creating uneven growth. Seeds germinating in stable temperatures develop stronger root systems and shoots, leading to healthier and more vigorous seedlings overall.

Maintaining Consistent Temperature in a Greenhouse

Need to warm things up? Heat mats provide gentle bottom heat to the growing medium, raising the soil temperature to the desired level for germination. This is particularly beneficial for seeds that prefer warm soil temperatures. Place seed trays directly on top of a heat mat set to the recommended temperature for your seeds.

While maintaining warmth is important, proper ventilation is equally crucial. Ventilation helps prevent excessive heat build-up and removes excess moisture, which can lead to fungal diseases. Open vents or use fans strategically in your greenhouse to promote air circulation. However, avoid creating drafts that can chill seedlings. Use a thermometer to monitor the temperature in your greenhouse at different locations and times of the day. This will help you identify any hot or cold spots and adjust positioning or ventilation accordingly.

Do I need to use lights in my greenhouse?

Whether you need artificial lights for seed starting in your greenhouse depends on several factors. The primary factor is the amount of natural light your greenhouse receives. South-facing greenhouses with ample sunlight may not require artificial lights for all seeds. Different plants have varying light requirements. Some seeds germinate in darkness, while seedlings need sufficient light for healthy growth after germination. Research the specific needs of your chosen seeds. The length of daylight hours during your planting season also plays a role. Seedlings started in the shorter days of winter or early spring may benefit from supplemental lighting.

Greenhouses with limited natural light due to north-facing orientation, shading from trees, or insufficient window space will benefit from artificial lights. Seeds that require long periods of bright light for healthy growth, like tomatoes, peppers, and eggplants, may need supplemental lighting even in a well-lit greenhouse, especially during the shorter daylight hours. And if you're starting seeds very early in the season before daylight hours are long enough, or if you want to extend your growing season into fall, artificial lights will be necessary to provide sufficient light for seedlings. LED Grow Lights are the most popular and energy-efficient option for seed starting. They come in various sizes and spectrums suitable for seedling growth.

Watering and Humidity Management in a Greenhouse

Watering seeds in a greenhouse requires a delicate balance to avoid soil compaction, surface disturbance, and maintain optimal humidity for germination. Bottom watering is an excellent method for seed trays. Place the trays in a shallow dish filled with water. The water will wick up into the growing medium evenly, moistening the soil from below without disturbing the seeds or compacting the surface. You can also use a fine spray mister to gently apply water directly to the soil surface. Avoid using a forceful stream that can dislodge seeds or splash potting mix.

Controlling Humidity

Humidity Domes trap moisture around the seeds, creating a mini greenhouse environment with high humidity. They are particularly helpful during the germination stage for most seeds. While domes maintain humidity, it's crucial to provide some ventilation to prevent fungal diseases. Lift the domes slightly or create small ventilation holes to allow for some air circulation. Reduce reliance on domes as seedlings emerge. Group trays with similar moisture needs together. This allows you to target specific areas for humidity control. Instead of soaking the soil completely, alternate between light watering and misting the surface to maintain consistent moisture without oversaturation.

Use your finger to check the moisture level of the soil. The top inch should feel moist but not soggy. As seedlings mature, their watering needs will change. You may need to transition from misting to watering the soil directly as root systems develop. Once seedlings emerge, you may need to thin them to prevent overcrowding. This will improve air circulation and reduce competition for moisture and light.

Transplanting greenhouse Seedlings

Recognizing when your seedlings are ready for transplanting in your greenhouse is crucial for their successful establishment and growth. Most seedlings are ready for transplanting once they have developed several sets of true leaves (usually 2-4 sets) beyond the initial cotyledons (seed leaves). The stem should be thick and sturdy enough to support the seedling when transplanted. Avoid transplanting seedlings with thin, weak stems.The seedling should have reached a height appropriate for the variety. Refer to seed packet information or plant labels for specific guidelines.

Check the roots of the seedlings in their pots. If they appear root-bound, with roots circling the bottom or sides of the pot, it's definitely time to transplant. Look for healthy, white roots that are well-developed and branching out. Avoid transplanting seedlings with stunted or discolored roots. Seedlings should appear healthy and vigorous, with good color and no signs of disease or pests. When the second set of true leaves start touching their neighbors in the pot, it can also indicate they're ready for more space.

Fungus Gnats While Seed Starting in a Greenhouse

Adult fungus gnats are weak fliers and dislike moving air. Using small fans to create gentle air circulation can disrupt their breeding activity and make your greenhouse less hospitable to them. Fungus gnats thrive in moist environments. Bottom watering helps target moisture delivery to the root zone, minimizing excess moisture on the soil surface where fungus gnats lay their eggs.

Bacillus Thuringiensis Israelensis (BTI) is a biological larvicide that specifically targets fungus gnat larvae in the soil. It's a safe and effective option for organic gardening. Follow the product instructions for mixing and application rates. BTI is typically available as a powder or liquid concentrate that you can mix with water and drench the potting mix or use when bottom watering. Repeat applications every 2-3 weeks, especially during periods of high humidity or warm temperatures when fungus gnat populations tend to be higher.

Sticky traps won't eliminate fungus gnats entirely, but they can help monitor adult populations and track the effectiveness of your control methods. Place them near fungus gnat breeding areas, like around potted plants.

The key to controlling fungus gnats is using an integrated approach that combines these methods. By creating a less favorable environment for fungus gnats to breed (air circulation, bottom watering) and eliminating existing larvae (BTI), you can significantly reduce their population. Sticky traps will help you monitor their activity and adjust your control methods as needed. Remember, consistency is key! By implementing these practices regularly, you can effectively prevent fungus gnats from becoming a nuisance in your greenhouse and ensure a healthy environment for your plants.

Looking for a high quality pre-made greenhouse in Massachusetts? Hometown Structures offers the finest craftsmanship with custom designs in a wide variety of sizes perfect for the home gardener to give your plants the perfect climate to thrive in a protected environment, keep pests away from your plants, grow plants and food that don’t otherwise exist in your area/climate and extend your season with options including 8mm twin wall polycarbonate for optimal insulation, venting and windows. Every custom shed greenhouse comes with 50-Year LP Siding and 30-Year Collapse From Snow Warranty.

Tap here to visit their website to learn more and book a design consultation today.