How To Grow Seeds In A Greenhouse? Step-By-Step Guide.

How To Grow Seeds In A Greenhouse? Step-By-Step Guide.

In Propagation by Ana M

A new plant can grow from a seed or cutting from an existing plant. Second option is popular if you want to grow an identical plant. You can root cuttings in water or soilless mix. Try using rooting hormone to increase the success rate.

However, one of the advantages of growing plants from seeds is that you can choose a variety of the plant that you like. Also, plants that grow from seeds are not genetically identical to the parent plant. Overall, growing from seeds is cheap and easy.

A process when a new organism grows from a seed is called germination. To start germination seed needs soil, water, oxygen, warmth and in some cases light. A process of planting seeds is called sowing. There are two ways to sow seeds: sowing seeds directly in the ground or planting seeds indoors and then transplanting seedlings later.

What seeds can be planted directly in the ground?

Generally, large seeds can be planted directly in the ground because they are sturdy enough to withstand outdoor conditions. Also, you can plant directly in the ground seeds of the plants with deep roots. You can sow a variety of plant seeds directly in the soil:


  • Lettuce
  • Beans
  • Peas
  • Cucumbers
  • Zucchini
  • Radishes
  • Corn
What seeds can be planted directly in the ground?

Root crops:

  • Turnips
  • Carrots
  • Beets
  • Radishes
  • Leaf lettuce 
  • Spinach
  • Swiss chard
  • Kale

These crops actually benefit from sowing directly in the ground. For example, if you sow carrots indoors in little trays they roots may bend at the bottom and it will be hard to transplant them.

When should I start seeds in my greenhouse?

Although you can start seeds in a greenhouse at any time because it provides controlled environment, it should not be later than 6-8 weeks before the last frost date for your geographical location. However, it also depends on the plant that you grow.

For example, you can start seeds of many ornamental plants earlier – 8-10 weeks before the last frost date. On the other hand, you should start seeds of cold sensitive plants like tomatoes, basil and peppers later, so on average 6-8 weeks before the last frost date.

To determine what is the last and first frost for your location, you need to use plant hardiness zone map. There are 10 zones which reflect the average annual minimum winter temperature and the map is divided into 10-degree F zones. Just enter your postcode and you will find in what zone are you.

When should I start seeds in my greenhouse?
Source – Arcadia Glasshouse

How to start seeds in a greenhouse?

There are two ways to grow seeds in a greenhouse:

  1. Plant seeds directly in the ground
  2. Start seeds in seed-starting trays

Here is a step-by-step guide on sowing seeds directly in the greenhouse soil.

Step 1 – Test the seeds for germination.

You can skip this test if you just bought fresh seeds. However, if you are using leftovers from the pas years, check if they can germinate because they can be dormant. Just place 10 seeds on a wet paper towel, fold it and put in a plastic bag. Keep it in a warm place for 7 days spraying the towel with water when it gets dry. At the end you will see how many seeds have germinated.

Step 2 – Check whether soil is ready for sowing.

It is very easy to test the soil, just dig your hand in an if it goes through nice and easily. When you take out your hand the soil should fall away through it.

Step 3 – Prepare the soil in greenhouse.

Take rake or hand fork to loosen soil. If soil contains large lumps, break them apart. Remove debris, sticks, rocks and roots. Depending on the type of the native soil in your greenhouse, you might need to add amendments to soil. The last step is to level the soil.

Step 4 – Sow seeds directly in the ground.

Read the label of seed packets carefully. They will tell you exactly how deep and how far apart you should plant the seeds. Also, you can plant some seeds in a drill, for example, carrots. A drill is a channel in the soil. Just drag a shallow drill with your finger 1 cm deep.

A general rule for planting is that you need to sow seeds three times as deep as the diameter of the seed. Large seeds like peas and beans germinate best in a long furrow and with adequate spacing. At this point I would recommend to put the markers with the name of the seeds. As it is very easy to forget where they are or what they are.

However, a lot of plants have quite small seeds, so you can gently tap them out directly from the packet. We will thin them afterwards but still be careful how far apart you sow seeds. It can be helpful to use a pencil to form individual holes in the soil.

Then, brush soil back over the seeds and just apply gentle firm pressure so the seeds get good contact with soil. For large seeds use rake to gently cover the row of seeds with soil pulled back to make the row. Make sure not to alter the row because you can end up with wavy line instead of the nice straight row.

Step 5 – Water the planted seeds.

Watering settles the soil around the seeds and gives a start to germination process. It is very important to use an appropriate nozzle for the watering can or hose wand. Place shower type nozzle also known as rose nozzle or fitting on a watering can and just gently let it rain. If you don’t use the rose nozzle and water from the can, it will just create a deep furrow and all your seeds will wash away.

For larger seeds I would suggest to use a hose wand. Keep in mind that too strong stream can wash the seeds right out of the row. Attach a rose nozzle as it gives the most control for watering stream. Be careful not to overwater as it is very easy to flood the row and drown the seeds.

How to use seed starting trays in greenhouse

There are seed starting containers of all types and sizes: open flats, plug trays, peat pots, propagation trays and seed-starting trays. In my greenhouse I am using 72 cell seed starter tray and it works perfectly fine.

Step 1 – Fill in the tray with seed starting mix

You can make a seed starting medium or buy it. A regular potting soil is not good enough as it is too heavy. A perfect seed starting mix is half peat moss and half vermiculite. It is important to keep pH of seed starting mix at 5.7 to 6.2 for optimal germination. Fill the trays to the top and then level the soil.

How to use seed starting trays in greenhouse
Step 2 – Plant the seeds

Plants the seeds in dry mix. Check the seed packet direction for each variety of seeds. A rule of thumb is to place two or three seeds into each pot 0.5 inches deep. Instead of covering seeds with light layer of soil, just lightly press each seed into the soil surface. That way you will establish a good contact between the seed and the soil.

Step 3 – Water the seeds

Water plays a key role in starting germination. One of the most common reasons why seeds fail to grow is drying out. Mist seeds with water right after sowing. Use a nozzle that sprays fine mist.

Step 4 – Cover tray with plastic wrap

Cover your tray with a layer of plastic wrap. That way you will maintain higher humidity for seeds. It will also help to keep consistent atmospheric conditions. Moreover, covering tray with plastic wrap keeps the soil moist for longer.

Step 5 – Move tray to a warm dark spot

Organize a warm location where you can keep your trays in a complete darkness for several days. You can put the heat mat under the trays to keep constant soil temperature of 70°F. Once the seedlings appear, remove the plastic wrap cover and move the tray to a brightly lit location.

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