Your leaves yellow at different stages for different reasons. A yellow leaf on a seedling means something very different than one on a fruiting tomato plant. The timing and location of yellowing tells you what is wrong and how you can fix it in your garden.
When I first grew tomatoes, yellow lower leaves worried me during harvest time. I thought something was wrong with my plants. But I learned this is normal during fruiting. The plant pulls nutrients from old leaves to feed growing tomatoes. Those bottom leaves turn yellow and fall off as the plant moves resources upward.
Chlorophyll gives your leaves their green color. When it breaks down, yellow pigments underneath show through. This happens when your plant moves nutrients out of a leaf. It also happens when the leaf can no longer make new green pigment on its own.
Plant leaf yellowing causes vary by growth stage in ways you can predict. Seedling yellowing often points to too much water drowning the roots. It can also mean too little light to fuel growth. Young plants are sensitive and show stress fast through color changes in their leaves.
I tested this with my own vegetative plants over many seasons. Yellow lower leaves usually mean your plants need more nitrogen in the soil. This nutrient moves from old leaves to new ones when supply runs low. The oldest leaves at the bottom yellow first while new growth stays green.
During flowering and fruiting stages, some yellowing is normal and nothing to worry about. Your plant sends nutrients to flowers and fruits which are its main focus now. Lower leaves that gave up their nutrients will yellow and drop off on their own over time.
For yellow leaves diagnosis, start by noting which leaves turn yellow first on your plants. Bottom leaves going yellow points to nitrogen shortage or normal aging. Top leaves going yellow suggests iron or other micronutrient problems. Random yellow leaves scattered on the plant might mean disease or pest damage.
Check your watering habits if you see yellowing at any stage of growth in your garden. Too much water rots roots and blocks nutrient uptake. Too little water stresses plants and causes leaves to yellow from drought. Stick your finger in the soil to feel if it is soggy or dry.
Look at your light levels if yellowing shows up on seedlings or indoor plants. Low light causes pale yellow growth since plants cannot make enough chlorophyll. Move plants closer to windows or add grow lights to fix this common problem in your growing space.
Think about recent fertilizer use when you diagnose yellow leaves. A plant that has not been fed in weeks may lack nitrogen. A plant fed too heavily may have root burn causing yellowing. Match your feeding to the growth stage. Use more nitrogen for leaves. Use more phosphorus and potassium for fruits.
Read the full article: 6 Plant Growth Stages Explained Simply
