36 Pack Refill Sponges for the 18-cell Planting Block

36 Pack Refill Sponges for the 18-cell Planting Block

Time for Another Crop in the 18-Cell Bio Dome!

18-cell (Pack of 36)
Item # 09323-PK-36
Instock - allow 3-5 business days for processing prior to shipment.
Buy 2+ at $17.95 ea

Your Bio Planting Block will last a lifetime with these handy, economical Bio Sponges! When you're ready to begin a new crop in the Planting Block, first be sure that the block has been thoroughly disinfected (warm water with a splash of bleach is fine). Then soak 18 of the Bio Sponges in warm water for a few minutes. They expand, revealing the pre-drilled planting hole at one end. Simply slip one seed into the hole of each Bio Sponge, place the Sponge seed-side-up in the cell of the Planting Block, and you're on your way to another bumper crop of healthy, strong, super-quick seedlings!

Bio Sponges are now enriched with oilseed extract, giving your seedlings an extra boost of nutrition. You'll love the results!

These giant sponges are replacements for Park's Original Bio Dome with 18 Whopper Cells, the largest of the Bio Dome family of seed starting systems. The Whopper Dome is great for larger seeds and rooted cuttings. It is also ideal for seedlings that need to be grown to a larger size in the Dome before being transplanted to their permanent location.

In this refill pack you get 36 sponges, enough to fill the Whopper Dome twice.

Review Summary
(Based on 6 Reviews)

Overall Rating: 5.0 / 5.0


These are the best for starting seeds inside!!
Brian Phillips from IL wrote on April 16, 2020

I have been starting seeds inside for 50 years. For the past three I have been using the cell blocks, sometimes with the cover on and sometimes with the cover off. I have been using all three sizes of the cell blocks. These are excellent, no mess, easy clean up. I bleach the styrofoam cells containers and use them over and over. I highly recommend them!!

Great for starting green beans indoors
Vermont Gardener from VT wrote on March 04, 2020

For 6+ years I have used these for successfully starting my bush French Beans indoors to speed up the season and harvest. I can control the temps and germination more easily and plant 5 inch bean plants outdoors instead of seeds! All of my bean transplants have always survived the process. I did have to replace the foam blocks after 5 years. They would have probably made it another year or two, but they were getting a little broken up from heavy Spring use. I have already brought them out and gotten them ready to go for beans!

Kinda pricy, but foolproof method for seed starting
John D from CA wrote on February 10, 2020

Have used bio dome sponge inserts for tomato and pepper seed germination for several years with perfect success. Place bio dome in window of our sunroom in January; daytime sponge temps with the dome vents closed are in the 80's to 90's. When seedlings are an inch or so high, open vents. When seedlings reach the dome, I transplant to 4" pots. Great system. Plants go in the ground in March (San Diego, CA).

These large sponges work well for tomatoes and other large plants.
Anne Rodgers from OK wrote on January 06, 2020

I have purchased and used the 18 count bio-cells for quite a few years now. They are perfect for starting tomatoes and like plants which grow inside to a pretty good and strong size - ready to transplant direct to the garden.

Park's Original Bio Dome with 18 Whopper Cells
Bill Gresham from WA wrote on May 04, 2013

The Biodome works really well. I use it with the 18-cell block for any seeds that grow big fast, or which I may want to delay transplanting (for example waiting for the weather to warm up). The description of the product above is accurate except for one thing -- the polystyrene block is actually rather fragile, because the edges are too thin. Since the 18-cell block is smaller than the Jumbo and Original blocks, it could be made MUCH more sturdy and still fit well into the Biodome. Its too bad the manufacturer/designer was so greedy early on, since redesigning and retooling would cost MUCH more than the insignificant difference in cost of more styrofoam. I think Park's should pressure them to do it anyway -- it is the difference between shoddy and excellent. I would have given it five stars except for this

What are the differences between the various Bio Domes: 60-cell, 40-cell, and 18-cell?
Bio domeThe original 60-cell Bio Dome has a 60-cell planting block with 60 Bio Sponges and will produce 60 plants at one time. The 1" wide by 2 ¼" deep cells are larger than average, and a spacious 1 3/8" apart. The bio-sponges are the same size as the cells; 1" wide by 2 1/4" deep. You can grow and keep plants in the 60-cell bio-dome for 6-8 weeks before transplanting. Many varieties will just need 6 weeks until transplanting. Tomatoes and peppers would appreciate the full 8 weeks before transplanting. So, the original 60-cell bio-dome would be great for starting all of your favorite vegetable and flower seeds, with a larger number of plants produced at one time.

The 40-cell Bio Dome has a 40-cell jumbo planting block with 40 Bio Sponges and will produce 40 plants at one time. The cells are wider and deeper than the original 60-cell planting block and provide good spacing to maximize air flow. The cells are 1 1/4" wide by 2 1/2" deep. The Bio Sponges are also 1 1/4" wide by 2 1/2" deep. You can grow and keep plants in the 40-cell bio-dome for at least 6-8 weeks and some for up to 9-10 weeks, because of the wider spacing and cell/sponge size. It too would be excellent for starting all your favorite vegetable and flower seeds, with the added benefit of giving more room and time for larger plant development.

The 18-cell Bio Dome has an 18-cell whopper planting block with 18 Bio Sponges and will produce 18 plants at one time. It has the largest cells of any of the Bio Domes we offer and is great for growing extra-stocky transplants and even for rooting cuttings. The cells are 3" tall by 1 3/4" wide. The Bio Sponges are also 3" tall by 1 3/4" wide. You can grow and keep plants in the 18-cell Bio Dome for at least 8-10 weeks and some for up to 12 weeks, giving the maximum amount of growing time and spacing between plants as compared to the 60-cell and 40-cell Bio Domes. If you are looking to grow your favorite vegetable and flower seeds into large, stocky transplants for setting out in spring, then the 18-cell Bio Dome is the one you want to buy. It would be excellent for growing Tomatoes and Peppers.

How much water should be put in the Bio Dome and how often?
SeedlingsThe beauty of the Bio Dome is that you cannot overwater! You just keep water in the tray and float the block. The larger the seedlings get, the more water they will use and you will need to add water more frequently as they grow. Initially, during germination, you may not have to water as frequently, because minimal water is being used and Bio Sponges will stay moist longer, so you may only have to water once every 1-2 weeks. Later, as plants grow and water use is greater, depending on the room temperature and the amount of light received, you may have to water more often -- possibly 1-2 times a week -- or as often as required to keep the sponges moist.
Product Videos