The art of watering in a greenhouse
Proper watering in the greenhouse can make or break the success of seed germination and seedling growth. Shallow flats of potting soil dry out fast, especially in hot greenhouses. Over-watering can overly cool the soil, cause roots to rot, as well as increase the risk of damping off and other diseases.
1. Pre-soak flats before planting them.
Potting soil will get hydrophobic when it has been allowed to dry out. When this happens, water splashes off the surface of the soil, causing a big mess and washing seeds away. Pre-soaking flats before planting, especially when dealing with dry potting soil, ensures your seeds will stay where you planted them.
2. Use a mist nozzle or mist system.
Potting soil is so light and fluffy, its easy to wash it away with even a light shower of water. Misting keeps soil and seeds intact. Though it takes more time to saturate all the soil, but its by far the best way to reduce the spread of diseases via water droplets.
2. Keep seeds moist, but not soaked.
My rule of thumb is simply to keep the soil from drying out completely. Watering soil that is already wet leaches valuable nutrients from the potting soil.
3. Water at midday.
Even when its near freezing outside, a greenhouse at midday on a sunny day can exceed ninety degrees. Water cools the soil when it is applied and when it evaporates. Additionally, watering when the soil is cold can increase instances of damping off.
4. Don't over water seedlings that have just germinated.
Shortly after germination is when seeds are most vulnerable to damping off. Keep seedlings moist but not overly wet, and allow the soil to briefly dry out occasionally.
5. Saturate the whole soil profile with each watering.
Especially as seedlings are growing, be sure to water the whole soil profile. Roots will not continue to fill the pot when they reach dry soil. Water has reached the bottom of the pot when you see it dripping out of the bottom.
6. Mix potting soil thoroughly and put equal amounts of it in each cell.
Certain components of potting soil, like peat moss or compost, are meant to hold onto water. Vermiculite and perlite help the soil drain. If your potting soil is not throughly mixed when you put it into flats, you end up with some cells that hold onto water and seemingly never dry out, and some cells that drain too quickly. You will then end up always overwatering or underwatering some cells when you water that flat. The same can be said for flats that are not filled evenly, at a uniform depth. Shallow cells will dry out more quickly and deep cells will hold onto water. Fill all cells with the same amount of soil so they all require water at the same time.