Sounds like a deficiency but you mention using fertilizer. Might be related to the clay soil, or maybe your soil has a high pH (clay soil is often found around limestone, which is high pH). Just my 2¢ though; let some more experienced people offer their thoughts as well.

You planted them the second week of May. Was the soil warm enough?

I don't know your climate, but according to the USDA zone map, your lows are about the same as Chicago, where I spent many years. I don't know what your last frost date is, but when I grew tomatoes near Chicago, we didn't plant until the end of May.

There have been news articles recently about this being a "Year Without a Summer" in the northern half of the US. What have your temps been like this spring?

Re. your heavy soil, is this the first time you've grown tomatoes in this location? Have you had an unusual amount of rainfall?

I was going to ask the same thing as missingtheobvious.

What have your temps been like

Thanks for the replies,

The daytime/night temps have been below average past few weeks and the plants have experienced a few mild frosts of about 28F at night. So I guess you're probably right.

Will these tomato plants grow fast still with more warm weather and lots of sunlight?

PS. It's my first time ever growing vegetables. If I keep my clay soil fertilized, will it be able to grow tomatos?

I have clay soil, amended but not as much as I'd like, and my veggies are doing fine.

I planted tomatoes, using wall o waters, in mid-March and was shocked at how tiny my plants were towards the end of May. Now they are finally starting to get bigger.

Crazy weather in May for tomatoes and peppers.

I'm not far from you. A couple problems. You jumped the gun on the season, around here we usually plant tomatoes out Memorial Day Weekend, (which was around May 23rd and earlier than usual this year.) People often do more harm trying to get tomatoes early than if they just waited until warm enough.

The nights even now still have lows in the low 50's and high 40's...we even had some low 40's a week or 2 ago, and even some mid/hi 30's in certain sections.

If you truly had frost (28 degrees you said?) hit your plants, they may not recover. Cold clay soil won't help much either.

If you care to make the ride down, I'll give you some heirloom plants that are about 12-18" tall & ready to take off. Check the border regulations first.

Hope this helps.

Mark,
who STILL hasn't gotten tomatoes planted yet.

I agree that the problem is that you transplanted too early. Tomatoes are tropical plants. I actually had one seedling that did the same thing this year! It wasn't dying, but it just wouldn't grow. It was given to me by a friend several weeks before I planted outside and it got dry a few times. It is growing now but I don't very much expect it to recover all that well. Tomatoes aren't very resilient in my experience.

I have heavy clay soil down here and it is really fertile stuff - I had a tomato plant that nearly ate Dallas a few years ago! (you know, the kind that has you looking for unlocked cars in the parking lot at work so you can unload tomatoes onto unsuspecting coworkers!). I wouldn't use fertilizer myself, but it sounds to me like you may be way over-fertilizing them. The best thing you can do is amend the soil with compost. Even side-dressing with compost or watering with compost tea (compost mixed into a bucket of water and allowed to sit for a couple of days), tomatoes love that.

Tomato plants can sulk a bit if exposed to low temps but not as much as most people think, but the nursery ones can be a bugger when all of a sudden exposed to real cold temps. My dad in law regularly plants before victoria day and his plants are no worse for it.

It could be that they are sulking from the cool temps but i would suspect you'd be seeing some growth now that we had a few solid weeks of warmth. It's more likely that your soil being clay is waterlogged. I have a spot in my garden where plants just refuse to grow even though 5 feet away from them they are growing tall and bushy. With the amount of rain we've been seeing i wouldn't be surprised if that is the problem.
If you think you need to replant i have about 8 (much overgrown) plants of different varieties that i will be culling soon you are welcome to them if you're willing to travel to Milton.

Thanks guys for all your help and generous offers! I'm just going to pull through this season and see how they turn out.

Now that I think about it, I do think my plants are waterlogged in the clay soil. The leaves are a healthy color but curled which I read could be sign of being waterlogged.

How do I check if it is and how can I fix it?

I agree with what others have said about planting out too early. I'm your neighbor to the south in central NY. I planted mine out Memorial Day weekend (May 23-25.) Unfortunately we had some low night time temps and frost threats. I have similar growing conditions to you. My tomato plants are just starting to rebound now.

Tomatoes are NOT tropical plants.

Tomatoes are native to the tropical highlands around Peru. Does that not make them tropical?

Highlands in Peru? That's tropical? LOL.

You can freeze to death in the highlands of Peru--otherwise known as the Andes.

Here is a link that might be useful: The Highlands of Peru.

"Tropical" is a confusing word. It has two meanings, which sometimes contradict each other:

First, it's an adjective pertaining to a warm climate or place: of, being, or characteristic of a region or climate that is frost-free with temperatures high enough to support year-round plant growth given sufficient moisture.

Second, an adjective pertaining to the equatorial regions: of, relating to, occurring in, or suitable for use in the tropics [see below]

tropics: the region lying between the Tropic of Cancer and the Tropic of Capricorn (23 1/2 north and south latitude).

Peru is located in the tropics. Some areas have a tropical climate, others don't.

Definitions adapted from www.merriam-webster.com

I was just being cheeky. It is temperate, but it is 'tropical' because it is in the 'tropics'. As was pointed out 'tropical' is a vaugue description. There needs to be a way to convey tone of voice in typing, maybe a smilie? :)

For clay soils try some greensand soil amendment (see the linky below). I've checked out a few sites and this site was reasonable in price as well as full offering of othe soil additive products (I purchased the Kelp too). Good luck. :)

Here is a link that might be useful: Greensand and other soil amendments