How I Grow Black Prince Tomatoes (From Seed to Harvest)

Looking to add some color to your tomato garden? So was I, and Black Prince Tomatoes is the answer you are looking for.

A few years ago, I only grew red tomatoes. Different varieties, but all red.

I don’t know why.

It just happened.

But then I discovered colorful cherry tomatoes and Black Prince tomatoes.

And I have never looked back.

Looking for the abbreviated version? Start seeds indoors in February-April in starter pots with pre-moistened, well-draining potting soil mix and place in a warm spot. When you see the first leaves, move the pots to a cooler spot. When there is no longer any risk of frost, harden off plants and move them outdoors. Black Prince tomatoes are indeterminate and must be supported with stakes or trellises. Water regularly and prune suckers and side branches to grow one central stem. Harvest when tomatoes are dark red to almost purple-black.

I still grow red tomatoes, but now my garden is full of tomatoes of different colors, tastes, and textures.

The first year I grew Black Prince tomatoes, I learned a lot. Here, I outline everything I have learned. After all, why repeat my mistakes?

I will take you through all the stages, from starting seeds indoors in early spring to seedling care, pot-ups, transplanting, general plant care, and harvest.

Oh, I will also share the recipe for my oh-so-easy-to-make Marinara sauce using ripe Black Prince tomatoes.

So find a comfy seat, grab a drink, and let’s get gardening.

Getting off to a good start with Quality Seeds

Buy quality seeds from a reputable vendor or your local garden center. Check for freshness, as seed packets with Heirloom tomatoes often only contain a handful of seeds.

How to check seeds for freshness

Vendors tend to be stingy with heirloom seeds. I only got seven seeds in my seed packet; one seed was suspiciously dark in color but germinated just fine.

Seed packets will tell you the date the seeds were packed, the expected germination rate, and the number of seeds in the packet. You want fresh seeds and the more the better.

Look for large and uniformly sized seeds if you can see the seeds. I have had the most success with yellow to lightly tanned-colored seeds. But your mileage may vary.

If you are limited in space and want to ensure you are planting viable seeds, I recommend the water-glass test or pre-germinating your seeds.

The water glass test for viable seeds: Place seeds in room-temperature water overnight. If the seeds float on the surface the next morning, they are dead and will not germinate.

Pre-geminate seeds: Personally, I never pre-germinate tomato seeds as they typically germinate in 5-10 days, and I can afford to lose the time and start over.

But it is the most effective way to ensure you are planting viable seeds. You can read about pre-geminating seeds in this article: “Pre-germinating seeds for increased efficiency.

When and How to start your Black Prince tomato seeds

Let’s have a quick discussion about the When first of all.

I am sure you have heard it before. “Start your tomato seeds 6-8 weeks before the last expected frost”.

This is not wrong. But it does not give the full picture when starting indeterminate tomato plants like the Black Prince.

Unless you have a lot of space indoors, I recommend starting your seeds about 4 weeks before the last expected frost.

And I speak from experience.

Black Prince tomato plants are vigorous growers, and if you start them too early, like I did one year, you will have larger tomato plants indoors for several weeks.

Trust me, explaining why the plants have to live indoors when day temperatures are warm but the nights are still cold is not easy.

Sorry to ramble on, but it was a painful experience. Let’s move on to the more fun How.

Pre-moisten the seed starting mix or potting soil before planting the seeds. You want moist, not wet. You should be able to form a ball with your hands; the ball should crumble when touched with your fingers.

Pre-moistening makes it much easier to maintain even soil moisture without the risk of over-watering.

Plant seeds in the pre-moistened, well-draining potting mix and cover lightly. Use a spray bottle to keep it moist. Cover the pot with plastic wrap or a humidity dome to retain moisture and place it in a warm spot (22-25 degrees Celsius/72-77 degrees Fahrenheit).

Remove the cover when you see the first leaves. Place pots in a cooler spot (18-20 degrees Celsius/65-68 degrees Fahrenheit) and ideally under a grow light.

A window sill with plenty of natural light works, but starting seeds in early spring, a grow light is helpful in zone 7.

Related: Check out our preferred seed-starting tools here.

Caring for the tomato seedlings

Caring for young tomato seedlings comes down to light, airflow, maintaining even soil moisture, and applying a light dose of fertilizer at the right time.

I use grow lights as I start the seeds 4-8 weeks before the last expected frost date. Window sills with natural light give me leggy seedlings. Leggy seedlings can be rescued but are more work to care for. 

Give your seedlings 16 hours of light followed by 8 hours of rest for strong, compact-growing seedlings.

Do not start your seeds too early. Indeterminate tomato plants like the Black Prince grow large fast, and indoor space can become problematic if the weather is too cold to move plants outdoors. 

Start one seed per cell or pot, or be prepared to transplant seedlings early. Seedlings need space, and grouping them too closely restricts airflow and circulation. Removing the cover promotes airflow.

Even soil moisture is critical, and you should check your seedlings daily. The soil will dry out faster when you have removed the cover.

Water at the base to avoid getting the leaves wet, and use a spray bottle or a fine-tip watering can to prevent flooding the pot.

Soilless seed-starting mixes and potting soils do not contain enough nutrients to help the seedlings grow and develop. When I see two pairs of true leaves, I add half the recommended dose of a balanced organic liquid fertilizer. 

Fertilizing seedlings early is not a requirement. You can wait until you transplant the seedlings into nursery pots with more fertile soil.

I do not give all my tomato seedlings fertilizers early on. Fertilizing early on will give you larger plants sooner, and for me, it is essential to manage my available space.

But you know how it is. Heirlooms and tomato varieties I grow for the first time always get extra attention—at least the first season.

Related: Check out our preferred plant care tools here.

Transplanting seedlings into nursery pots

Transplant tomato seedlings into nursery pots when they are about 2-3 inches tall. Plant them deep, burying the stem to the first set of true leaves to encourage strong root growth.

Transplanting seedlings is as easy as 1-2-3:

1. Remove first leaves and 
2. Prick out the seedlings
3. Plant deep to the first set of true leaves

The fact that the tomato plant will grow new roots along the buried stem is a great way to rescue leggy seedlings. But ensure there is no leaf-to-soil contact when planting deep. 

Fertilize your tomato plants with a balanced organic fertilizer every two weeks, and be prepared to pot up as the plant outgrows the nursery pot. 

Check underneath the pot; if you see white roots through the drainage holes, it is time to move the plant into a larger pot.

Potting up tomato plants is a process. Keep increasing the size of the pot or container gradually.

Moving the young plant into a large container with more soil too soon makes it harder to control soil moisture. I recommend you take it in stages.  

Caring for the Black Prince tomato plant

After a few pot-ups, your young tomato plants are finally ready to be moved outdoors.

Temperature

It is safe to move your tomato plants outdoors when the temperature reaches a minimum of 15 degrees Celsius / 60 degrees Fahrenheit.

Tomato plants are somewhat resilient, but you want to avoid periods with cold night-time temperatures or very windy conditions. 

Pot, container, grow bag or in-ground

Choose your location wisely, as Black Prince tomatoes prosper in full sun and want at least 6 hours of direct sunlight daily.

The plants thrive in warm temperatures with an ideal range of around 21-27 degrees Celsius / 70-80 degrees Fahrenheit.

Black Prince tomato plants do well in pots, containers, grow bags and transplanted in-ground. Just make sure you give the plant well-draining soil rich in organic matter. 

Shade your plants to prevent sun damage if your plants are exposed to direct sunlight during the hottest part of the day. A light row cover will do the trick.

Space the plants 60 cm – 1 meter / 2-3 feet apart as the plants are indeterminate and grow large. If you are not planning to prune side branches or stems, you must allow more space between plants. 

Black Prince tomatoes require even soil moisture to thrive. Regular watering and fertilization throughout the growing season are essential for healthy plants. Use a high-quality organic fertilizer to provide essential nutrients, and be prepared to fertilize every other week. 

Mulch the bed to create a protective barrier between plant foliage and soil. Mulching also controls soil temperature and helps retain soil moisture.

Always water at the base of the plant to avoid getting the leaves wet. Wet leaves can invite fungal diseases and cause sun scald and burn. 

I have been punished by windy conditions, with plants developing withering white leaves more than once. Properly staking your tomato plants before they need it is critical.

Hardening off tomato plants

Please do not skip this step.

Before permanently moving your tomato plants outside, give them a week to get used to their new growing environment.

The hardening-off process is oh-so-easy; it just takes a bit of patience.

1. Start by pacing your tomato plant in a shady spot outdoors for 1-2 hours.
2. The next day, double the time spent outdoors.
3. Introduce indirect sunlight on day 3
4. From day 4 and forward, gradually increase time spent outdoors and exposure to sunlight

It is that easy. And your plants will thank you for it.

Staking indeterminate tomato plants early

Take the time to stake and support your plants before they need it.

Seriously, this is so very important. Get your support in place from day 1.

A small plant with a tall stake, trellis, or tomato cage may look out of place, but indeterminate plants continue to grow throughout the season, and they will grow into the support and be healthier for it.

Staking mature plants can be done, and I have done it more than once, but it is better to be prepared and to get those stakes in early.

Tall stakes, cages, and trellises are ideal for indeterminate tomato varieties and ensure your support is sturdy enough to hold the weight of the plant and fruits.

Watering tomato plants

Tomatoes are thirsty, and during the hotter periods of summer, you may have to water them daily. 

But as a rule, it is better to water deeply once or twice a week than give a small daily dose of water. 

You want even soil moisture, not water-logged plants. Soil moisture meters are an inexpensive way to know when to water your plants. 

Tomato plants growing in containers need more frequent watering than plants growing in the ground. 

Always water at the base of the plant and be prepared to bottom prune the plants if the leaves and branches are in contact with the garden bed or container soil. 

Mulching around the base of the plant helps retain moisture, controls soil temperatures, and prevents weeds.

Related: Check out our preferred watering tools here.

Pruning indeterminate tomato plants

There are so many opinions about pruning tomato plants. Here I will tell you what works for me. 

As a rule, I prune indeterminate tomato plants but seldom determinate plants. Read more about pruning tomato plants in our Guide to pruning tomato plants for home gardeners.

I prune tomato plants to improve air circulation and to allow the plant to use its energy to produce fruits rather than leaves and branches.

My heirloom Black Prince tomato plants have been pruned in three different ways.

1. Removing suckers and side stems or branches

I pinch suckers when I see them on all my tomato plants. Side branches are removed on most plants as I prefer top growth to broad, bushy plants.

2. Bottom pruning and airflow printing

I prune branches on the bottom 30 cm / 1 foot of mature plants. These branches typically hang low and make watering more complicated. There is also an increased risk of leaf-to-soil contact, inviting soil-borne diseases. 

3. Topping or top pruning at the fruiting stage

Indeterminate plants will continue to grow all season and produce new branches and flowers until it gets too cold.

In cooler climates, you get to the point where there is not enough time left in the season for new flowers to develop into mature fruits. This is the time when I top prune my plants.

I typically top prune my plants when I see fruits on three or more branches. At this stage, the plants also have several more branches with flowers yet to set fruits a the top of the plant.

Fertilizing Black Prince tomato plants

Black Prince tomato plants are heavy feeders and require regular feeding. I fertilize my plants every two weeks and use a nitrogen-heavy fertilizer in the earlier growth stage. 

As the plant enters the flowering stage, I use a fertilizer with an NPK where the Potassium level is at least double the Nitrogen level.

When I start seeing fruits, I switch to a fertilizer with more Phosphorus and some Potassium, like an NPK ratio around 1-4-2, like 8-32-16. 

For more detailed information about fertilizing tomato plants, refer to the Guide to fertilizing tomato plants for ambitious home gardeners.

The less-work approach of giving your plants a balanced fertilizer will also produce good results. It all comes down to your level of ambition. 

Black Prince tomato plants: pests and diseases

My tomato plants suffer from leaf miners, aphids, and leafhoppers every season. But many other pests, pathogens, and diseases can affect your plants, including 

• Thrips: tiny insects feeding on the sap, causing leaf curl and distortion
• Spider mites: really tiny eight-legged insects feeding on the sap, cause leaf discoloration and spotting
• Tomato hornworms: Easy-to-spot caterpillars with horns that will consume entire plants if theft unattended
• Whiteflies: tiny flies feeding on sap, live underneath leaves and cause leaf discoloration and overall leaf and plant damage

As with all pests, early detection is critical, and you should remove infected leaves. Spraying plants with water, water and Neem oil solution, or even soapy water can be an effective treatment.  

Remove larger pests like tomato hornworms by hand, and remember to check underneath the leaves. Sometimes it helps to gently shake the plant and check for caterpillars to fall to the ground. 

Tomato plants are also susceptible to various diseases, including late blight, early blight, and septoria leaf spot. 

• Late blight: Fungal disease causing blackening of the stems and leaves. 
• Early blight: Fungal disease causing yellowing and wilting of the lower leaves
• Septoria leaf spot: Fungal disease causing small, dark spots on the leaves.  

Applying copper fungicides is a popular method of controlling these diseases.

Related: Check out our preferred plant care tools here.

Harvesting Black Prince Heirloom Tomatoes

Black Prince heirloom tomatoes are ready to harvest when they are dark red to almost purple-black. And I know this is vague and hard to act on.

My best advice is to harvest a fruit and to taste it. Testing is the only way to learn what works for you.

I found it hard to know exactly when to harvest, but testing allowed me to get a feel for firmness and color to know when to harvest.

As with all tomatoes, harvest regularly to encourage continued fruit production. 

Why Grow Black Prince Heirloom Tomatoes?

Black Prince tomatoes have a rich, sweet flavor and a beautiful deep-red-to-black color that makes them stand out.

Tomatoes, alongside hot peppers, are my favorite vegetables to grow. Sure, I grow beans, carrots, leafy greens, cucumbers, eggplants, and so on, but I do not have the same relationship with these crops.

In short, my tomatoes are “special,” and I love variety in texture, color, and flavor.

The plants grow strong, and unless you have unlimited space, you must prune the plant. 

Frequently asked questions

Can I save Black Prince Tomato seeds?

I have had success saving tomato seeds at the end of the season, and with Heirloom tomatoes, it does make sense. There is a risk of cross-pollination, but from my experience, it rarely happens with tomato plants grown in a home garden environment.

To save seeds, scrape them from the ripe fruit and let them ferment in a jar with water for a few days. Rinse and dry the seeds before storing them in a cool, dry place.

Can I plant Black Prince tomatoes outdoors directly in the ground?

If you live in a warmer climate, you can start seeds outdoors in the spring. You need a soil temperature above 10 degrees Celsius / 50 degrees Fahrenheit for seeds to germinate, and you need to consider that fruits take 70-90 days to develop.

Plant seeds in good quality, well-draining soil and choose a location with plenty of sunlight.

In cooler climates, like here in zone 7, we must start the seeds indoors to give the plants enough time to develop fruits before the weather turns cold in fall. 

What is the fuss about Black Prince Heirloom tomatoes?

Simply put, Heirloom tomatoes are varieties passed down through generations of gardeners.

They are open-pollinated [1], as in pollinated naturally by insects or wind, and are often prized for their unique flavors, colors, and shapes.

The Black Prince Heirloom tomatoes originate from Siberia and are an indeterminate variety that will continue to grow and produce fruit until the first frost. 

And then, there is, of course, my Black Prince Tomato marinara sauce.

My Black Prince Marinara Sauce

We harvest a lot of tomatoes every day from early August. And one day, I was looking at a bunch of Black Prince Tomatoes and decided to cook them.

I had a general idea but not a clear vision – if you catch my drift. And I created a family favorite for those days when nobody has time to cook.

So here we go.

Ingredients:

• 1 kg / 2 pounds of ripe Black Prince tomatoes
• 1 tsp. of sugar (brown if you have it)
• 1/2 tsp. of salt

Preparation and cooking:

1. Cut tomatoes in quarters and place in a saucepan on medium heat
2. Add sugar and salt
3. Let the tomatoes simmer and stir occasionally to help crush the tomatoes
4. Cook to the desired thickness

And there you have it. The taste is fantastic in all its simplicity.

If you prefer smooth marinara sauces, use a blender to blitz the sauce. But I didn’t and quite liked the semi-chunky texture.

This sauce is a true testament to the homegrown Black Prince tomato flavour. And yes, I will save seeds and grow Black Prince tomatoes again next season.

Helpful sources:

[1] https://extension.illinois.edu/sites/default/files/dkk-mg-seed-swap-hybrid-heirloom.pdf