The short answer: washed coco peat works well as the plug medium for grafted tomato, pepper, watermelon, melon and cucumber, provided it is low in salts, holds its shape in the cell and stays moist without going soggy. The medium matters most at two moments. The first is the day before grafting, when the rootstock plug should be moist and the plants unstressed. The second is the 5–8 days in the healing chamber, when the plug must not sit wet under 95% humidity. Get those right and most failures that people blame on the graft go away.
Key facts
- Tomato and pepper stems should be at least 1.5 mm, and usually 1.5–2.0 mm, at grafting. Splice-grafted watermelon is grafted at about 3.5–4.0 mm.
- Vascular reconnection takes about five to eight days, and the scion can't draw water through the rootstock until it's done.
- Healing humidity guidance runs from 85% to 100%, with 95% or higher most often given as the optimum.
- In one tomato trial, rootstock plug moisture decided graft survival, while scion plug moisture made no measurable difference.
Why the plug medium matters more in grafting
A normal transplant only has to grow. A grafted one has to survive surgery, a week in a sealed tent, and then a hardening phase, all inside the same small cell. The plug medium has three jobs through that period. It keeps the rootstock roots working while the top has no leaves, it drains well enough that the root zone doesn't go airless under very high humidity, and it holds together when plants are sorted, moved and shipped.
Coir is a good fit because it rewets easily, carries a lot of air for the water it holds and binds into a firm plug once roots grow through it. The weak point is salt. Trials of grafted tomato seedlings in coir mixes found high potassium, sodium and soluble salt levels that set growth back both before and after grafting. Substrate EC also ran higher in a coir-perlite mix than in peat-perlite at the same leaching fraction, and the best nitrogen rate was lower for the coir mix. So the coir you choose for grafting stock should be washed, and your feed has to be set for that particular mix. If you're deciding between washed and pre-treated material, our article on buffered vs unbuffered coco peat covers the trade-off.
Sowing schedules so stem diameters match
The single biggest cause of poor take is grafting a scion and rootstock of different thickness. If you can't avoid a mismatch, the rootstock can be a little thicker than the scion, never the other way round. Sow 20–25% more seed than you need so you have pairs to choose from.
| Crop and method | Sowing offset | Stage at grafting | Stem target |
|---|---|---|---|
| Tomato, splice or tube graft | Rootstock 1–2 days after scion as a start (many pairings work sown the same day) | 2–4 true leaves, usually 2–3 weeks from sowing | 1.5–2.0 mm |
| Pepper, splice or tube graft | Rootstock 10 days before scion in one trial | 2–4 true leaves, 35–40 days from sowing | Matched to the scion |
| Watermelon, hole insertion on bottle gourd | Watermelon 7–8 days after the gourd | Graft 7–8 days after the watermelon is sown | Scion smaller than rootstock |
| Watermelon, hole insertion on squash | Watermelon 3–4 days after the squash | Graft 7–8 days after the watermelon is sown | Scion smaller than rootstock |
| Watermelon, splice graft | Scion grafted at 14–21 days, rootstock at 7–9 days | Scion with 2 true leaves, rootstock with 1 emerging | 3.5–4.0 mm |
| Cucumber, melon, watermelon, approach graft | Scion 5–7 days before rootstock | Similar height and stem diameter | Matched |
Treat the offsets as starting points. Rootstock lines differ in vigour, and your own temperature and light will shift growth rates. A clip that fits tightly on the stem is a quick readiness check for tomato. If one batch is running ahead, a cooler house slows it and a warmer one speeds up the laggards.
Cell sizes and tray layout
There is no single standard. A 72-cell tray works for tomato scions, and rootstocks can go into larger cells or pots. Watermelon grafting trials have commonly used 128-cell trays. A coir trial on grafted tomato grew rootstocks in 50-cell trays with 33 cc cells and scions in 105-cell trays. Commercial tomato propagators on stone wool sow into 240-plug trays and set the grafts into larger blocks after healing.
The practical rule is to give the rootstock the bigger cell, because it has to carry the plant through healing on its own roots. The scion only needs to reach grafting size. With approach grafting both plants end up in one container, so size that cell for two root systems.
For fill, use a fine propagation grade so cells fill evenly and plugs don't slump. Our guide to coco peat particle size grades explains why too many fines lead to waterlogged plugs and algae. For germination itself (sowing depth, early feeding and hygiene) see coco peat for seed germination.
Substrate moisture at grafting
Water both rootstock and scion trays 12–24 hours before grafting, and don't water right before you cut unless plants are wilting. You want turgid, unstressed plants in a plug that's moist rather than dripping. A very wet rootstock can push water up the cut stem and lift the scion off the union.
The research leans toward a moist rootstock plug. In one tomato plug study, survival reached 100% where the rootstock substrate was kept at a high water level for three days before and three days after grafting. Survival, root length and root surface area all fell as rootstock moisture dropped. Scion moisture had no significant effect on survival. The most compact plants came from medium scion moisture combined with high rootstock moisture.
So keep the rootstock trays evenly moist, let the scion trays be a little drier, and check by weight rather than by eye. Coir's surface dries and lightens in colour well before the plug below does.
The healing chamber
Published settings vary more than you might expect, especially for temperature. Here is the range, by factor.
| Factor | What the guidance says |
|---|---|
| Relative humidity | 85–100%, with 95% or more most often given as the optimum. One tomato trial did best at 90% for the first 2–3 days, then 70%. |
| Temperature | Cucurbit guidance runs warmer, at 25–30°C or even 28–29°C. Tomato guidance ranges from 18–21°C day and 16–18°C night up to 24–27°C. High temperature lowered graft take in one tomato trial. |
| Light | Dark or low light for the first 24–48 hours, then shade cloth removed in stages over several days |
| Days sealed | Closed for the first 2–4 days, then vented a little more each day |
| Total time | Plants usually leave the chamber around day 7. One watermelon schedule runs to day 12 with daily venting. |
The plug medium is where healing chambers most often go wrong. At 95% humidity almost nothing evaporates from the cell, so a plug that went in wet stays wet. Set trays above any water pool so the medium never wicks up standing water. Don't mist or water the plants directly. If the chamber needs more moisture, wet the walls or the floor. If a plug genuinely needs water, give it from below.
Watch how long plants stay in. Keeping tomato grafts in the mist chamber beyond about day 6 encourages aerial roots from the scion and cotyledon drop. Those aerial roots are the same problem you get when a scion touches a wet surface, and at planting they can let soil-borne disease bypass the rootstock.
Back to trays and hardening
Once out of the chamber, give grafts a day or two out of direct sun. If they wilt hard, put them back under cover for another day. After that, expect 5–10 days on the greenhouse bench, then 3–5 days of outdoor hardening before field planting. Tomato propagators typically dispatch grafted plants about 40 days from sowing.
During this stage, cut irrigation back gradually so the plug dries more between waterings. Roots then explore the cell and bind it. Bottom watering protects the union, and keeping foliage dry limits leaf mould and powdery mildew. Remove rootstock suckers as they appear.
Substrate-related failures and fixes
| Symptom | Likely substrate cause | Fix |
|---|---|---|
| Scion lifts or rots at the union | Plug too wet at grafting, or overhead water in the chamber | Water 12–24 h ahead, bottom water only, keep trays off standing water |
| Scion wilts and doesn't recover | Rootstock plug too dry, or roots damaged by salt | Keep rootstock trays evenly moist before grafting and check the feed EC |
| Green film on plug tops | Constantly wet surface, too many fines, light on the medium | Coarser surface grade, longer dry-back after healing, clean the humidifiers |
| Collapsed seedlings in patches before grafting | Damping-off (Pythium, Rhizoctonia) in overwet cells | Clean trays, avoid saturated plugs, remove affected cells promptly |
| Stunted rootstocks, leaf edge burn | High salts in the mix | Use washed coir, flush before sowing and lower the N rate |
| Plugs crumble when pulled | Too young a root system, or a very fine, short-fibre mix | Allow more root time, and add some fibre or a coarser fraction |
For algae and fungus gnats on coir more generally, see mould, algae and fungus gnats in coco coir. Sanitation matters as much as moisture here. Wipe blades and benches with alcohol, soak clips in bleach and rinse them, and clean humidifiers between batches.
EC and feeding before dispatch
Published plug-feeding rates for vegetable transplants run from 50–100 ppm N with daily feeding to 200–350 ppm N when you feed once or twice a week. In one transplant trial, 200 ppm N gave the best roots and shoots, and 400 ppm added cost without better plants. For coir mixes, start toward the low end. Coir mixes in grafting trials needed less nitrogen and ran higher substrate EC than peat mixes, and the right rate has to be found for each mix.
Before dispatch, aim for a plant that's fed but firm: dark green, short, with a white root ball that holds together. Drain EC should sit close to your feed EC. Much lower means the plants are hungry, and much higher means salts are building in the plug.
Plug integrity and transport
A grafted plant is worth more than a seedling, so you can't afford plugs that crumble in the box. Coir plugs hold well once roots reach the cell walls, so don't ship before that point. Ship the plug moist but not wet. A soggy plug gets heavy, breaks apart and heats up in a closed trailer.
Temperature in transit matters more than most nurseries assume. In a 4-day simulated transport of tomato seedlings, plants held at 18°C deteriorated, lost flower buds on the first truss and yielded less, while 6°C under light came through best. One large greenhouse tomato operation moved its grafted seedlings in trailers at 10–12°C. Watermelon is the exception because it is chilling-sensitive. Grafting onto squash or bottle gourd rootstock improved its tolerance enough for two weeks of storage, while solanaceous seedlings stored for four weeks at 12°C under low light.
At planting, keep the graft union at least 2.5 cm above the soil, and don't bury the clip.
How to specify coco peat for grafting work
| Property | What to ask for | Why |
|---|---|---|
| Washing | Washed, low-salt pith | Salts in coir set grafted tomato seedlings back in trials |
| Grade | Fine propagation grade, screened to limit dust | Even cell fill without slumping or algae |
| Fibre | Some short fibre or a coarse fraction | Plug strength for sorting and shipping |
| Format | Briquettes or blocks to suit your tray filler | Easy hydration and less freight than loose media |
| Consistency | The same grade every shipment | Your sowing offsets and irrigation depend on it |
Then test each lot yourself for EC, pH and water-holding before it goes into a grafting run. You set the nutrient charge, because unbuffered coir will take up calcium and magnesium from early feeds.
CVM Fiber and Industries produces coco peat in Kanyakumari, Tamil Nadu, India, from husk sourced across several districts. The pith is washed with water to reduce EC, with no calcium nitrate or other chemicals added, and it is not buffered. We supply 5 kg blocks, 650 g briquettes and custom blends, which suits nurseries that want to set their own starter charge. More on propagation uses is on our nurseries page, and plug formats are covered in coco coir discs and propagation plugs.
To discuss a grade for your grafting trays, contact us.
FAQ
Can you use coco peat for grafted tomato and watermelon seedlings?
Yes. Grafting trials have raised tomato rootstocks and scions in coir mixed with perlite or vermiculite. The main precautions are using washed, low-salt coir and setting the feed rate for that mix, because coir mixes in those trials ran higher substrate EC than peat mixes.
How wet should the plug be when grafting?
Moist, not dripping. Water both trays 12–24 hours before grafting and don't water just before cutting unless plants are wilting. In one tomato study, keeping the rootstock plug well watered gave the best survival, while scion plug moisture didn't affect it.
What stem diameter should tomato rootstock and scion have?
At least 1.5 mm, and usually 1.5–2.0 mm, which tomatoes typically reach two to three weeks after sowing. If the two can't be matched, the rootstock may be slightly thicker than the scion, never thinner.
How long do grafted plants stay in the healing chamber?
Most schedules take plants out around day 7, after two to four sealed days and gradual venting. One watermelon schedule runs to day 12. The vascular connection takes about five to eight days, so plants that still wilt hard should go back under cover for another day.
What humidity and temperature does a healing chamber need?
Humidity of 85–100%, with 95% or more most often recommended. Temperature advice varies: around 25–30°C is common for cucurbits, while tomato guidance ranges from about 18°C to 27°C. Keep light low for the first 24–48 hours.
Why do my grafted plugs go green with algae?
The plug surface stays wet under high humidity and gets light once the shade comes off. Too many fines makes it worse. Use a coarser surface grade, water from below and let plugs dry back more during hardening.
How much should grafted transplants be fed before dispatch?
Published plug rates run from 50–100 ppm nitrogen fed daily to 200–350 ppm fed once or twice a week. In coir mixes, start at the lower end, because coir ran higher substrate EC and needed less nitrogen than peat in grafting trials.
What temperature should grafted seedlings be shipped at?
For tomato, cool is better. Seedlings held at 18°C for four days lost quality and first-truss flower buds, and some growers ship at 10–12°C. Watermelon is chilling-sensitive, so don't take it as cold as tomato.