Can the seed gathered from deadheading be used again?
Deadheading means to remove the wilted flowers before they ripen with seeds. The object is to get the plant to invest its energy into making more flowers, instead of producing seeds. If you allow a few of the flowers to wilt, dry, and fall of naturally, you will have seeds aplenty for next year. Many people allow one or two plants to produce seed, and deadhead the others to get the most flowers. Here's an article with more information: https://www.gardeningknowhow.com/ornamental/flowers/marigold/growing-marigold-flowers.htm
Chemical fertilizer...if organic, then how to prepare it?
golds: fertilizer is not so important for these plants as for some others. Use a balanced fertilizer (1:1:1 or 3:1:2 proportions) once in the early spring. Learn more about marigolds here: https://www.gardeningknowhow.com/ornamental/flowers/marigold/growing-marigold-flowers.htm
You can use a commercial fertilizer, or organic fertilizer. If you want to go organic, the best thing is to work compost (decomposed organic matter) into the soil before planting. Learn more about fertilizer here: https://www.gardeningknowhow.com/garden-how-to/soil-fertilizers/fertilizer-numbers-npk.htm
organic fertilizer: https://www.gardeningknowhow.com/garden-how-to/soil-fertilizers/homemade-plant-food.htm
and compost: https://www.gardeningknowhow.com/composting/
Why does my marigold have brown flower buds that look like it didn't grow? Is it normal? Or too much temperature? I put my marigold in a pot outside my house with direct sunlight.
You have a number of nice flowers, so I guess you're concerned that some of the buds don't seem to be opening? Sometimes plants don't open a bud(s) if they feel that there's some reason they may not be able to bring them to maturity, IOW to produce seeds. You might try keeping the soil a bit more moist. This article has some tips on growing marigolds: https://www.gardeningknowhow.com/ornamental/flowers/marigold/growing-marigold-flowers.htm
How to grow pompom marigolds from seed with only a small unheated plastic greenhouse? I would like to grow yellow pompom marigolds from seed. I've never grown anything from seed before and I have recently bought a small plastic greenhouse with just a rack of shelves but no heating.
I would suggest starting your seeds indoors. Most seeds need quite warm temperatures for germination.
Always follow the directions for sowing your seeds from the back of the seed packet.
I start nearly all my garden from seeds, and I think you will enjoy the process. Marigolds are a great plant to start with.
Here are some links to help you get started.
https://www.gardeningknowhow.com/garden-how-to/seeds/when-to-start-seeds.htm
https://www.gardeningknowhow.com/special/greenhouses/greenhouse-gardening.htm
https://www.gardeningknowhow.com/special/greenhouses/mini-greenhouse-information.htm
My marigolds look as if they are being eaten. Upon closer inspection, I noticed what looks like teeny slugs! Can you help me? In all the years that I have planted marigolds I have never seen such a thing. Any suggestions would be appreciated. Thank you.
This article should help: https://www.gardeningknowhow.com/plant-problems/pests/facts-about-slugs-and-how-to-kill-garden-slugs.htm
You can also try spraying the plants with neem oil to prevent other insect pests from munching on them. Here is more information: https://www.gardeningknowhow.com/problems/pests/neem-oil-uses.htm
How far apart should one plant marigolds? Plants are 15 cm high.
Usually anywhere from 4-6 inches apart is fine for these plants.
I planted African Marigolds in the spring. Can you tell me why they have not bloomed yet this year?
A lack of phosphorus, which is responsible for blooming in plants, could be the issue. If your plant is lacking phosphorus, use a phosphorus rich fertilizer or add some bone meal to the soil. Here is more info: https://www.gardeningknowhow.com/garden-how-to/soil-fertilizers/phosphorus-plant-growth.htm