Polystyrene boxes, such as those used in fish markets, shield your seedlings from extreme temperature fluctuations. At night, simply put the lid on or lay a sheet of glass or a doubled-up layer of fabric over the top.
Cluster your seedlings into a confined place such as a plant house or cold frame. Now fill gallon-sized bottles with hot water and place these into the cold frame with your seedlings. Leave enough space around the bottle for the heat to escape and to avoid overheating nearby seedlings.
Keep seeds warm to encourage germination
A heat mat speeds germination. Most seeds require temperatures of 65° to 75°F to germinate. Placing seed containers near an existing heater or using a space heater with the proper precautions can raise the ambient temperature as needed.
You can also replace heat mats with heating cables. They are much cheaper and all you have to do is place them in a layer of sand and gravel at the bottom of the seedling tray. Another clever trick is to place your seedling tray on top of your radiator and let it soak the heat up.
Seedlings, with their tender new leaves, often give up the ghost when temperatures dip to 32-33°F. Tropical plants have differing low-temperature thresholds. Some keel over when temps fall to 40°F; others crumble at 35°F. Other plants are just hardy by nature and can withstand temperatures as low as 18-20°F.
If you're wondering at what temperature threshold you should be bringing your plants, there's a short answer: when nighttime temperatures reach 45 degrees (F), it's time to bring your plants indoors.
I did some research and found out that houseplants need to be taken indoors before overnight temperatures dip below 45 degrees. Most tropical plants suffer harm from temperatures below 40 degrees. Well, we have temperatures forecast to dip into the 40s again on Thursday and lower 40s on Friday.
Surround plants with water-filled bottles, which will absorb heat during the day then release it at night, warming the air around your plants. This is especially effective within a greenhouse, tunnel or cold frame. You can also fill plastic bottles with hot water on cold nights to protect seedlings.
Grow lights provide light and heat mats provide warmth by contact. Heat mats are used to help seeds sprout faster. Once sprouted, grow lights provide the light needed for seedlings to grow. Your room may already provide enough warmth for seed starting but, once sprouted, you will definitely need a light source.
Answer: Yes. Leave the heat mat on and set to the same temperature 24 hours a day until the seeds sprout. The idea of turning it off at night usually springs from the observation that the earth cools at night and warms up again in the day, thanks to the sun.
After seeds have sprouted, watering from above with any force is likely to knock over the fragile seedlings. This is why it's best to use a mister. With a mister or spray bottle, you have to be willing to check on your baby plants at 2x a day to make sure they don't dry out.
At the start of this period, you will bring your seedlings indoors or cover them at night if the temperature looks like it will dip overnight. By the end of the hardening off period, you can leave them outdoors all night, uncovered, so long as the overnight temperature doesn't dip below about 50 degrees Fahrenheit.
The best time to water plants is in the morning or evening.
Morning watering is actually preferable to evening watering as the plant has time to dry before the sun goes down. At night, water tends to rest in the soil, around the roots, and on the foliage, which encourages rot, fungal growth, and insects.
Plants release oxygen during the day in the presence of natural light through the process of photosynthesis. While at night, the plants uptake oxygen and release carbon dioxide, which is called respiration.
When placed beneath seed trays, heat mats offer gentle warmth, just enough heat to mimic the temperature of outdoor soil on a sunny spring day. This heat encourages seeds to sprout. The best seedling heat mats are safe, effective, and increase the seed germination rate.
Seedlings need 14-16 hours of light every single day, without fail. Don't leave them on for 24 hours a day though. Like us, they need to rest at night. So plan to keep your artificial lights on for 14-16 hours during the daytime, and turn them off overnight.
One of the odd things about seedlings is that, as much as almost all seeds need warmth and stable temperatures to germinate well, once they germinate (once leaves appear), they grow best in cooler conditions and actually prefer it when temperatures drop at night.
Add Mulch
A thick layer of mulch, such as shredded bark or compost, can help insulate tender plants. Cover the entire plant with mulch the night before low temperatures are forecast. Remove it when the weather warms up again.
Many recommend moving your plants outside one to two weeks before you plan on transplanting. First to a shady spot for a couple of hours the first few days. Then slowly increasing the sun they receive. They also recommend allowing the plants to “almost wilt” before watering, and withholding all fertilizer.
A sprouted seed is no longer a seed but a seedling, a miniature plant. The most important thing to consider and deliver is adequate plant light. Seedlings need more light than full grown plants, ideally as much as 16-18 hours a day. Additional light may be required for seeds started during the winter months.
Avoid watering when temperatures are below 40°F or when sustained freezing temperatures are expected within 24 hours as this water will freeze and not be available to the plants.
Thoroughly Water plants if it's not going to rain before the freezing temperatures arrive. It may sound illogical. However, a moist ground stays warmer than dry soil. Watering the night before the freeze comes will insulate the root structure of the grass and plants and decreases the potential for cold injury.
The cold can mix badly with water, so just make sure the night time watering happens when temperatures are warmer. Late Spring, Summer and early Fall / Autumn generally have warmer months and this tends to mean overnight temperatures are warmer too.