Eucalyptus species need full sun. You need to put them on a southwest or southern windowsill and let them get blasted by as much light as possible. When they are young they can withstand less light but when older they need full sun or they usually die.
I germinate them and place seedlings under 4x10watt LED T5, each are 850 lumen so that is about 3400 Lumen total. i get 1500 lumen per square feet( i use 70cm by 70cm container). 1500 lumen is the minimal amount needed to grow cacti , cannabis, succulents or any other sunloving plant. About 2000-3000 lumen per square feet is recommended for mature sunloving plants who need to flower and florish long term.
Eucalyptus are also very sunloving, there are some species ( just a few) that can be grown in partial sun or light shade but eventually they need some direct sun to do well.
8 hours of light is not enough unless the intensity is very high ( like direct sunlight) if you use growing lights you need the lights on for at least 12 hours, 15 is optimal. Also the colour spectrum is important you need 6500 K to get the full wavelength of blue and red spectrum because both have important function in plant growth.
I grow all kinds of cacti and seedlings do very well under artificial lights until they reach a certain age or size. When they reach that size/age i have to either move them in full sun or put them under high intensity 400 watt lights ( very expensive in energy ) if i don't they just die. Eucalyptus have the same thing going.
Also be aware of the distance , if lights are too close to the plant they can scorch the leaves. This can also happen with regular T5 or T8 lamps , that is why i use led which get less hot. The best distance from plants is 30 cm ( one foot i think ) and the minimum is 15 cm and 45 cm is the maximum distance. Anything that is too far away has low lux ( which is amount of light falling on surface) Led has higher Lux due to better technology. T5 and T8 need to be closer to the plant.
Fungus gnats can be prevented by adding a good layer of white sand on top of the soil, they don't like to dig around in sandy soil. However dont germinate seeds in sandy soil, just add sand when the seedlings are a few inches high.
Never stick your finger in the soil because you can damage the root system that covers the surface - they can die from such act, same with protea and some other australian plants.
Eucalyptus can withstand quitte wet soil if they receive the maximum amount of light. I never let seedlings completely dry out but i don't keep them wet all the time either. you have to find some middle ground here.
Also be aware that some species are difficult to raise indoors. Viminalis for example needs alot of light and a regular watering schedule, they also like warmth and more sand in the substrate. Darympleana needs more water. Neglecta germinates with warmth but likes cooler conditions when young ( 20 C or 15 C).
If the lower leaves fall off it is usually not enough light, faulty watering ( too dry ).
First thing you should try is give them as much sun as possible.
Second is to not water them from top, i never throw water on top of the pot. Gradually add little bits of water untill the pot has saturated . Adding water too fast in too great quantity can disturb the root system.
Do not keep them in pots too long. keep them in a bigger pot then needed and plant them outside when young. If they get rootbound they either stop growing for many years or just die. They are unsuitable for long term pot culture. I keep them in pots for about 4 or 5 months and plant them in the garden. darympleana which is a fast growing species can only be in containers for 2 months , they get routbound very quickly due to their growth rate.
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