Growing San Marzano Tomatoes in Kratky Hydroponics – Update: Week 1

I’ve heard a lot of people online say that they have had decent success with growing tomatoes in hydroponics, so I wanted to test just how easy it was myself.

This is the growth after 1 week, planted on 1st August 2019:

I planted some seeds into Jiffy 36mm Peat Pellets. Unfortunately just after planting we discovered that our grow light was faulty! Rather than waste the seed and peat pellet, I decided I would just start a new session in my Aerogarden Bounty. Once my replacement grow light arrives I will remove from the peat moss and transplant into a cloning collar.

They began to emerge from the pellet only three days ago, with the smallest white to very pale green sprout showing. They are growing so fast as they reach for the light!

Tomato Seedling Week 1
Tomato Seedling Week 1

Strange enough the second seedling grew two sprouts. I am pretty sure I only placed one seed in each pellet.

It looks like the two sprouts merge into one stem close the soil. I don’t want to disturb the peat moss yet. If there is more than one seedling in the pot I can seperate them when I transplant; hopefully my new light will arrive soon.

Tomato Seedling Week 1
Tomato Seedling Week 1 – Look at his little seed hat!

Have you grown hydroponic tomatoes? I would love to know how well they grew for you.

What Plastic Should I use for Hydroponics?

Hydroponic plants are always grown in some form of container; in essence they are a container with water, and sometimes fancy tools like pumps. A lot of people grow small produce like lettuce in mason jars using the Kratky method, but most commonly plants are grown in plastic buckets, totes or buckets.

Keep It Food Safe

It needs to be food-safe, and yes this is important. Many people will use plastic containers but they are often unaware that overtime plastic degrades and can leach micro-particles and chemicals into the water, all of which will be absorbed by your plants as they grow (and subsequently eaten by you).

Plastic leaching these dangerous chemicals are even worse if you live in a hot environment, and if the plastic is exposed to heat at any time (such as at the store before you bought it). Furthermore, some plastics will react if exposed to certain products, especially if they are too alkaline or too acidic.

The Risks

The wrong chemicals exposure can be dangerous. BPA and some phthalates are endocrine disruptors, mimicking natural hormones and causing a cascading number of different health problems such as infertility, obesity, breast cancer, prostate cancer, reproductive development, heart disease, diabetes… and more.

Honestly, just don’t risk it.

What Plastic Can I Use?

Find a container using Polyethylene Terephthalate (PET). Apparently PET is “biologically inert if ingested, is dermally safe during handling, and is not a hazard if inhaled“. PET wont degrade when in contact with food or beverages.

Polypropylene (PP) is another common used plastic; found in many food storage containers and microwave safe containers. This plastic can be heated safely, and it doesn’t react to liquids, acidity, or alkaline foods.

High-Density Polyethylene (HDPE) is often found as dairy containers (milk or butter), cereal box liners, and large food buckets. It doesn’t absorb liquid, and futhermore doesn’t leach chemicals into their contents.

What Plastics Should I Avoid

Recycled Plastics Any plastic that is recycled may not be food safe. I would avoid any recycled plastics for hydroponics.

A quote from the FDA describing the risks of recycled plastics:

1) that contaminants from the post consumer material may appear in the final food-contact product made from the recycled material, 2) that recycled post-consumer material not regulated for food-contact use may be incorporated into food-contact packaging, and 3) that adjuvants in the recycled plastic may not comply with the regulations for food-contact use.

PVC. Did I just say that right? Yep. A lot of DIY hydroponics use PVC pipes to store and run their hydroponic solutions, and even grow their plants. In it’s rigid form PVC is probably safe, however in its flexible it could be dangerous. To make it soft a plasticiser is added, often up to 40%! According to Choice Australia recent studies have raised concerns about the health risks.

Polycarbonate should be used with caution. Whilst a lot of food storage containers are made with this plastic, recent studies have suggested that polycarbonate can release bisphenol A (BPA) which can cause serious health problems.

Further information on types of plastic (and their numbering system) can be found here:

Expected Grow Times of Kratky Hydroponic Plants

I’d like to figure out how fast I can expect my plants to grow. Most seed packets and websites tell you how fast seeds grow assuming you are using soil, But hydroponics can grow up to 50% faster!

I doubt very much that my first hydroponic plants will grow 50% faster, however that doesn’t mean that I couldn’t expect to start harvesting from roughly that time onwards. You rarely need to wait until the plant reaches full maturity before you can harvest; just pull off leaves as needed and let the plant keep growing. I added “Potential Maturity” to the table below, but realistically that’s when I suspect I may be able to get my first harvest.

Earlier in the week I announced that we were hoping to grow the following vegetables and fruits in our Kratky hydroponic tubs:

  • Amaranth, Red Leaf
  • Basil, Purple Petra
  • Cucumber, Lemon Cucumber
  • Lettuce, New Red Fire
  • Lettuce, Red Sails
  • Spinach, Matador
  • Spinach, Lavewa
  • Swiss Chard, Bright Lights
  • Tomato, San Marzano

All of our seeds were purchased from Sustainable Organic Q8, and appear to have been imported from Botanical Interests. You can read more about my search from seeds in the post Where to buy organic seeds in Kuwait.

Days To MaturityPotential Maturity
Amaranth, Red Leaf90-11045-60
Basil, Purple Petra45-5523-28
Cucumber, Lemon6532
Lettuce, New Red Fire5527
Lettuce, Red Sails4523
Spinach, Matador28-4814-24
Spinach, Lavewa28-4514-24
Swiss Chard, Bright Lights5226
Tomato, San Marzano80-9040-45

I’d really to have some plants growing, or even harvestable, by late September/early October. Thats approximately 50-60 days from now. We have space for 2 large plants, along with at least 3 smaller plants and maximum of 5 plants.

San Marzano Tomato Artwork, Courtesy of Botanical Interests
San Marzano Tomato Artwork, Courtesy of Botanical Interests

The San Marzano Tomatoes are planned as a permanent setup, so these will take priority in one of the largest plant spaces. This tomato plant will be approximately 5-6 feet (1.5-1.8m) in height!

Lemon Cucumber Artwork, Courtesy of Botanical Interests
Lemon Cucumber Artwork, Courtesy of Botanical Interests

Whilst not quite as large as the tomatoes, the Lemon Cucumbers will stand about 3-4 feet (0.9-1.2m) in height. This will fill my second large grow area.

There are debates online about which spinach is “better”; matador apparently grows better in many conditions, but the Lavewa looks prettier and is slightly tastier. We want to test both of these spinaches side by side. They grow at the same rate, so we will test both the Lavewa Spinach and the Matador Spinach together.

Amaranth Artwork, Courtesy of Botanical Interests
Amaranth Artwork, Courtesy of Botanical Interests

I want to stagger the harvesting of my plants; I don’t want all my fresh leaf plants to mature at the same time, and then go one to two months with little food. So I have decided that I will start the Amaranth as well for this first hydroponic growth cycle. Granted, with its growth speed, I may be able to grow two batches of spinach by the time it reaches full maturity.

If I can fit anymore plants in my grow area I will add the Swiss Chard and the New Red Fire Lettuce.

What about you?

What are you growing this season? Did you calculate how long it takes to grow? Did you have to plan around seasons and temperature outdoors? Let me know what you’re growing!

Do I need grow lights? Update – Week 2

Read about the first week of growth in the post Do I Need Grow Lights? Update – Week 1.

It’s been two weeks since I transplanted my Deer Tongue Lettuce seedlings from the Aerogarden Bounty to my makeshift Kratky containers. The seeds were started in the Aerogarden on 17th July, so they are now only 18 days old. I started this experiment to test how the lettuce would grow under the light conditions of different rooms; my laundry, my pantry and my kitchen.

In the post Do I Need Grow Lights?, started just after transplanting my lettuce, I actually measured my light more scientifically. Spoiler alert, it was then that I discovered that my rooms were below the ideal levels for plant growth and that I would in fact need to purchase some grow lights.

Since my grow lights haven’t yet arrived, I decided to just let the lettuce sit in those rooms. Sometimes the amount of light to maintain is much less than the amount of light needed to create fresh growth. Even if the plants don’t get enough light in those rooms to actively grow, they may be acceptable places to store plants (if necessary) for a few days before I can completely harvest them.

Week 2 Kratky Lettuce Compared
From left to right: pantry, laundry, kitchen.

The lettuce didn’t really grow during this last week; the lettuce in the pantry being the exception. Compare this to last week:

Week 1 Kratky Lettuce Compared
Week 1 Kratky Lettuce Compared. Left to right: pantry, laundry and kitchen.

The pantry lettuce definitely grew a bit between week 1 and 2, although not as apparent in the photos. The leaves became wider and greener.

The laundry lettuce stayed mostly the same size. Its leaves did start to droop and curl slightly. This area is now being converted into a more permanent grow area including a couple nice grow lights!

The kitchen lettuce had started to die due to the lack of light. It struggled the most, which reflects our light readings that we got in the post Do I Need A Grow Light? The light was just so minimal in this area that the plant couldn’t even maintain it’s size, let alone grow! This area may be okay to store a plant in for a couple days, especially if the plant is destined to be eaten (why harvest all at once if I can pick the leaves off fresh over a couple days), but I won’t be trying to grow anymore in this area.

Despite hearing success stories online, I was a little skeptical if this method would actually work. But…

Photo of Lettuce Roots Growing
Photo of Lettuce Roots Growing

Seeing is believing! Roots emerged from the bottom of the DIY net cup, and quite proficiently. I was honestly quite surprised by the amount of root growth – only two weeks ago there were no roots at all touching the inside of the DIY net cup!

The End Of An Era

This experiment, the total of two grueling weeks, is now over. I’m moving what’s left of the lettuce to live under my new grow light; it’s now an experiment to see if I can get it to survive and recover.

How Long Should My Grow Lights Be On For Hydroponics?

I need sunlight daily to get Vitamin D, but if I get too much at once I will get sunburnt and potentially cancer! It’s better to get an optimal amount of light (such as when wearing sunscreen) over a couple hours.

Daily Light Integral

Just like us plants have optimal light brightness levels, as we calculated in the post How Bright Should My Hydroponics Light Be? Different stages of plant growth will require more or less light strength and lighting time. A seedling might need very low light strength over a short time (its a baby so it needs to sleep a lot), whereas a fruiting plant will need all the extra energy it can get through higher light strength and longer lighting times.

The best way to tell if your plants are receiving enough light to grow is by measuring the Daily Light Integral, or DLI. The DLI is a measure to of how many moles/m2/day our plants get. Notice how we will be converting from umols, or micromoles, to their higher unit. This is a bit like grams to kilograms, or ounces to pounds.

If you already have the PPFD or umols/s/m2, such as whats advertised when buying lights online, then you can use a simple online calculator to calculate your required number of hours to the DLI. If you don’t know how many umols/s/m2 your room or light is, check out the post How Bright Should My Lights Be?

According to Wikipedia having the correct DLI can help your plant leaves to grow thicker, increase flower and fruit yields, have more and stronger roots, as well as have more leaves and heavier biomass (they produce more leaves and less stems and stalks).

According to Specmeters:

Lettuce (butterhead)14-16
Tomatoes (seedlings) 6-8


I found a really great article from the Department of Horticulture and Crop Science: this PDF covers a huge variety of plants, and the recommended lighting and hours lit for each of the growth stages, and what level nutrient solution is ideal.


# Light HoursLight Intensity


# Light HoursLight Intensity
Vegative12-20 250-450
Flowering<12 250-450
Fruiting12-20 250-450


# Light HoursLight Intensity
Flowering12-20 450-700
Fruiting12-20 450-700

My kitchen had on average 250 LUX from the window – it’s a dark room considering direct sunlight has 900-2000umols. Using the Environmental Growth Chambers calculator, 250 LUX converts to 4.75 umols/s/m2.

If I was growing tomato seedlings, I would need to divide my result (4.75) by the number of hours to reach the ideal DLI of 8. In this case I would need my plants to receive approximately 1.5 hours of sunlight per day. An adult tomato needs 22-30DLI, so as it grows I would slowly increase the number of hours its exposed to sunlight to 6 hours a day.

But I will be using my grow light on my tomatoes. This light produces 565umol/s/m2. Using the same calculator I can tell that for seedlings I only need to expose them to 4 hours a day of light, whilst adult plants would need ideally 14 hours a day.

Quick Reference Chart

This PDF has a wonderful table of many, many, plants that compares different life stages, the strength of light recommended, and how many hours those plants require at such a stage.

Where To From Here?

Beginners Guide To Hydroponic Lighting has a list of all our posts on lighting, written to help a beginner learn the basics and get started right through to more advanced topics.

How Bright Should My Hydroponics Light Be? is the previous post in this series. It explores how much light certain plants needs to grow. I also show you how to convert to the more common used light measurement value umols or PPF, which many grow lights use. Understanding these values, and how they get them, will help you make an informed purchase.

Beginners Guide To Hydroponic Lighting

Wow! Learning about lights for hydroponics, or just plant growing, gets overwhelming fast. So many terms and numbers to keep track of. Here’s a roundup of posts that help explain lighting, all written for an absolute beginner in mind!

Do I Need Grow Lights?

Not everyone has a PAR meter or Quantum meter; they can be expensive. So how else can you measure the light your room gets? Whilst not very accurate, this can give you an idea of how bright your rooms are; helpful in finding out if you need to invest in a grow lamp!

How Bright Should My Hydroponic Lights Be?

This post explores how much light certain plants needs to grow. I also show you how to convert to the more common used light measurement value umols/m2/s or PPF, which many grow lights use. Understanding these values, and how they get them, will help you make an informed purchase.

How Long Should My Grow Lights Be On For Hydroponics?

So you know how bright your light should be, but how long should it shine? Plants need time to sleep and rest too, just like you. This post explores how to determine how many hours of light your plant needs (it can vary a little depending upon how bright your light is).

Grow light heights? Is it too close or too far away? – coming soon

Lights have different intensities, depending upon how far away you are to them. That little candle might not put off much light or heat from across the room, but up close you can still get burnt! This post will be exploring PAR, PPF and PPFD.

Waveforms and Colors – coming soon

Humans can see white light, and our eyes prefer it. But plants actually prefer different colors, most often reds and blues. This post explores the colors that lamps output – usually referred to in waveforms.

If you want a more advanced list of lighting resources, check out the post SAG’s plant lighting guide linked together.