The leaves of my tomato seedlings are yellowing. What might cause the problem? How to solve the problem. Can I transplant them now or should I leave them in the pot until they become stronger .
Thank-you for contacting Toronto Master Gardeners with your question. We have had this question before and I will repeat the previous answer as it is a good one.
There are several conditions and problems with tomatoes that have a symptom of yellow leaves. It can be difficult to identify the particular culprit in a situation. Sometimes yellow leaves are caused by a disease or pest, but other times it can be a case of under or over-watering, or a nutritional deficit.
Pests and diseases can target tomato plants. You can usually see the pests and both situations often result in other symptoms as well as yellowing leaves (holes in the leaves, brown spots, withering of the whole plant).
In your circumstances, I am suspecting a problem with hardening off, watering or nutrients. The small pots you have your plants in make it difficult to ensure proper watering and tomatoes are picky in their water requirements. The soil must not be allowed to dry out, but equally, or perhaps more deadly to tomato plants is standing water or continually soggy situations.
It is also possible that your plants have exhausted the nutrients available in their small pots. They are heavy feeders. Nitrogen, magnesium, iron, potassium deficiencies are all common problems with tomato plants. A balanced fertilizer could help in that case. Potting soil for seedlings is really designed for germination and the first couple of weeks of a plant’s life. After the plant has more than 3 or 4 leaves, it needs more nutrients than potting soil can provide.
The traditional rule of thumb for planting warm season vegetables in the Toronto area is the May long weekend. Various sources give earlier dates for the last spring frost in Toronto – somewhere between May 4 and May 9. These dates are historical averages and there may be frost later than this in any particular year. We’re only a week away now from the May long weekend and a glance at the Weather Network for conditions between now and then show warm temperatures even at night. You can start to harden your tomato plants off to get them ready for planting outside next weekend. Gradually introduce the plants to the out of doors, putting them in light shade for a few hours the first day and then moving them back inside. Gradually give the plants more sun and more time outside, eventually leaving them outside all night. Be careful of windy days and heavy rains, as the plants must gradually get used to these conditions too. Once you do have your tomatoes in the ground, you should check on them daily as they are small. Once new growth begins following transplanting, you can relax a bit in your care.
Here is a link to Caring for tomatoes.
Good luck with your tomatoes.