ok when i first started growing wheatgrass i was getting lots of mold just like you, i was also getting poor yield from my grass, by that i mean i wasnt getting much juice from the grass. I have now found a solution to both problems which works extremely well for me. i now never get mold and i get lots of juice from my wheatgrass. this is exactly what i do -
1 - i weigh about 250 - 300g of wheatgrass seed (now i am using barelygrass seed because i prefer the taste)
2 - i transfer the seeds to a stainless steel saucepan and cover with filtered water and leave overnight, very important you use filtered water not just tap water. (i am now experimenting with soaking the seeds longer rather than just overnight, to get better germination results and to use less seeds as they expand more when soaked longer)
3 - once the seeds have been soaked overnight i take the seeds and transfer them to a drainer, which is basically something with small holes in it that i can use to rinse my seeds. i rinse the seeds under tap water and full pressure for a few minutes making sure they recieve a good wash.
4 - now my seeds are ready i get my seed tray and the first thing i do is line the bottom with 2 kitchen paper towels to stop the soil falling out the bottom of the holes in the tray
5 - I now add soil to the tray and also used green and white tea leaves (optional). I fill the soil about 3/4 the way up the tray not to the top. Pat down the soil with the back of your hand or fingers so that the soil is smooth and even.
6 - Now here is what i think has helped me the most. I spread the seeds that i have just washed over the soil in a thin layer, its better if they dont overlap but if they do it doesnt matter, Just make it look like a carpet of seeds covering the soil.
What i used to do was cover the seeds with soil, i find this is not good. Instead all i do is water gently the seeds, just enough to wet them and the soil, dont over soak. Take the tray and cover it so the seeds remain dark. I just put a sheet of newspaper over the top of the tray.
7 - It is best to keep the seeds wet while they are germinating, so every day or every 2 days you can give the seeds a bit of water, just enough to keep them wet, do not soak.
8 - Within 2 or 3 days you will see that they are begining to sprout, keep watering and keep the newspaper on top. Soon they will grow tall and lift the top of the newspaper up. When they do this you can put them out in the sun. I put them on my windowsill in my bedroom. This is good because as they are growing they give off lots of fresh oxygen and act as natural air filters.
This is VERY IMPORTANT - i have found that to avoid mold and get the best yield you need to water from the bottom and not the top when you put them in the sun. To do this i use a 'gravel tray' which is basically a tray without holes that i put under my tray, you can use this or baking tray, anything that will fit under your wheatgrass tray and act as a water reservior.
plastic tray would be better
Now if you were to look at the bottom of your tray when you first put it on the windowsill you will see that there are roots starting to emerge from the holes at the bottom of the tray. When you first put your tray in the sun fill up your gravel tray with filtered water. Do not fill this tray before you put your wheatgrass out in the sun.
You will find that within a few hours your wheatgrass has used up all of this water which has been soaked into the soil and by the roots. Fill up this tray every 2 days with water. I have found that wheatgrass is very thirsty and requires alot of water. but when you water from the top you get lots of mold. if you let the wheatgrass soak up the water it wants by itself the very top remains dry so you dont get mold.
this also makes watering ALOT easier, because when the wheatgrass has grown quite tall watering it all becomes difficult. with a water tray you can just fill it up every 2 days and the wheatgrass will be very juicy when you juice it because it has taken up the water it needs. you will see how quickly it uses up this water and if hot it will use it even faster.
this method has helped me very much, i hope it will help you too.