How to read a seed packet for microgreens germination rate, density, and days to harvest

Key takeaways

A germination rate printed on a seed packet reflects controlled laboratory conditions — in Bangkok's ambient heat above 31 degrees Celsius, real tray germination of sunflower and pea can run 8 to 14 percentage points below the label figure.

Seeding density recommendations from temperate-market seed suppliers assume low ambient humidity. In high-humidity apartments, reducing the printed density by 15 to 20 percent cuts mold onset without meaningfully reducing yield per tray.

'Days to harvest' on most seed packets counts from germination, not from sow date. In Bangkok conditions, add 1 to 2 days to any stated harvest window during months when nighttime temperature stays above 28 degrees Celsius.

Germination rate on a packet is a minimum floor for sale, not a growth prediction. A packet labeled 85 percent germination can legally contain seeds that hit only 85 percent under optimal conditions — a grower in a 33-degree apartment will see less.

Stale seed stock, even within the printed expiry date, degrades faster in humid storage than the label assumes. A packet stored without desiccant in Bangkok for 6 months can lose 10 to 20 percent germination viability.

TL;DR

Seed packets were not written for you. They were written for agronomists in air-conditioned research stations or field growers in temperate climates. Every number on that packet — germination rate, seeding density, days to harvest — is a starting point that Bangkok conditions will bend. The growers who figure this out early stop blaming their technique and start adjusting their inputs. The ones who don't spend months chasing failures that were never their fault.

The tray that should have worked

The packet said 90 percent germination. It was a reputable supplier, well-reviewed, sunflower black oil seeds. I soaked them for 8 hours, seeded at the density on the packet — around 150 grams per 10x20 tray — and weighted them down for the blackout phase. Day 3, I lifted the cover expecting a solid canopy of hulls pushing up. What I found was a tray that was maybe 65 percent germinated, with the rest either not sprouted or rotting at the seed coat. The tray smelled slightly off. Not mold yet, but close.

The packet was not wrong. The packet was written for a seed testing lab at 20 degrees Celsius and 50 percent relative humidity. My grow room that week was 32 degrees and 78 percent RH. The seeds rotted before they could sprout because the density was too high for my conditions, and because germination vigor at that temperature is genuinely lower than what a controlled-condition test will show. I had followed the instructions exactly and produced the wrong outcome. That was the last time I trusted a packet number without adjusting it first.

A higher germination percentage does not mean a better outcome in your tray

Germination percentage on a commercial seed packet is a regulatory minimum tested under ISTA or AOSA protocols — meaning seeds are germinated in controlled humidity, at optimal temperature, on blotter paper or sand medium. The number tells you what the seed lot achieved in ideal conditions, not what it will achieve in your apartment at 31 degrees with no airflow. Beyond that, a high germination percentage says nothing about germination speed or uniformity, which matters far more in a microgreen tray than total percentage. A batch that hits 95 percent over 5 days in a 20-degree lab but only 80 percent in 48 hours at 32 degrees will perform worse in practice than a batch rated 85 percent that germinates uniformly by hour 36. Uneven germination is what creates the gap between slow and fast seedlings that leads to uneven canopy, shading, and eventual mold in the dead zones.

In our grow logs across sunflower cycles in 2023 and 2024, trays where germination spread across more than 36 hours showed a mold incidence rate roughly double that of trays where 80 percent or more of seeds broke within the first 24 hours of the sprouting phase — regardless of total germination percentage. A seed packet tells you nothing about germination speed. That data does not exist on the label.

Most growers assume that a seed packet showing 95 percent germination is simply better than one showing 85 percent, and that buying the highest-rated seed means the fewest problems. The logical move feels like: higher number, more seeds sprouting, fewer gaps, better tray.

What the grow logs showed when we stopped trusting the packet

Across 34 sunflower tray cycles logged between May 2023 and August 2024, trays seeded during weeks where ambient temperature averaged above 31 degrees Celsius showed germination rates 9 to 14 percentage points below the packet figure. Trays in the same room during cooler months, averaging 27 to 29 degrees, came within 3 to 5 percentage points of the label. Same seed stock, same supplier batch, same soak time.

Sunflower germination drops below the label rate above 31 degrees Celsius

A packet rated at 90 percent is realistically delivering 76 to 81 percent germination in a Bangkok grow room during April through September without climate control. Growers who seed at the packet-recommended density without accounting for this will have gap-heavy trays and will likely overwater trying to compensate for thin coverage.

Counter to what most growers expect — that heat speeds growth — we found that microgreens grown in ambient temperatures above 30 degrees at night showed slower final stem elongation and a longer window before the first true leaf appeared. The fast early germination created an illusion of acceleration, but cotyledon development stalled. Harvest-ready density by the label's day count was achieved only 40 percent of the time during May through August cycles.

Days to harvest runs 1 to 3 days longer than label in Bangkok's hot season

If you are selling or scheduling harvests against a packet's stated harvest window, building in a 2-day buffer during Bangkok's hottest months is not caution — it is calibration. The packet was written for a different climate.

Standard seeding density recommendations for sunflower microgreens range from 140 to 170 grams per 10x20 tray across the most commonly cited sources. At ambient humidity above 72 percent RH, we observed mold onset — visible mycelium on seed hulls before canopy lift — in 6 of 8 trays seeded at the high end of that range. Trays seeded at 120 to 125 grams per tray in the same conditions showed mold in 1 of 8.

Printed seeding density causes mold at Bangkok humidity without adjustment

The density recommendation on a packet or grow guide is not a fixed instruction — it is a temperate-climate default. In a Bangkok apartment without a dehumidifier running during blackout phase, it is a recipe for losing trays.

Plant Mood grow log data, 2023-2024

What each number on a seed packet actually measures

There are three numbers that matter on a microgreen seed packet: germination rate, seeding density, and days to harvest. They are not independently useful. They interact. And each one is built on an assumption that Bangkok growing conditions will break.

Germination rate is expressed as a percentage and tested by seed laboratories under standardized protocols. The International Seed Testing Association specifies temperature ranges for each species — sunflower, for instance, is tested at 20 to 25 degrees Celsius. The percentage reflects the proportion of seeds that produced a normal seedling within a defined number of days. Normal means a seedling that meets morphological criteria, not necessarily one that would survive in your medium. A seed that technically germinates in a lab blotter test may rot on coco coir at 32 degrees before it produces a usable cotyledon.

Seeding density is almost never derived from controlled experiments by the seed supplier. It is usually inherited from earlier grow guides, farmer experience in temperate regions, or recommendations from hydroponic media manufacturers. It assumes that ambient humidity during blackout phase is low enough that seeds packed closely together will not trap moisture and create anaerobic pockets at the seed coat. In Bangkok's wet season, that assumption fails at the density ranges most packets suggest.

Days to harvest is the number that looks precise and is the least reliable. It counts from germination, not from sow date. It assumes optimal growing temperature for the species, which for most popular microgreen crops is 18 to 22 degrees Celsius. It does not account for the metabolic cost of growing in heat, which diverts energy from elongation into heat stress response pathways. And it does not account for the fact that what counts as harvest-ready — cotyledons fully open, first true leaf just emerging — is a visual judgment call that varies by grower and buyer standard.

Pea shoot packets commonly state 10 to 14 days to harvest. In our logs, Bangkok cycles from November through February — the cooler months — averaged 12 days. April through June cycles averaged 15 to 16 days. The packet is not wrong. It is just describing a different grow environment than yours.

Germination percentage testing also uses a concept called the 'normal seedling' threshold, which excludes weak, deformed, or dead seedlings from the count. A packet rated at 88 percent germination may have had an additional 5 to 8 percent of seeds that showed some radicle emergence but were excluded. In ideal conditions those borderline seeds would likely produce usable plants. In a hot, humid tray, they are the ones that rot first and spread.

How to adjust seed packet numbers for Bangkok apartment growing

Record your ambient temperature and RH for the 72 hours before seeding

A baseline reading — even just three manual readings per day — that tells you whether you are in a high-risk or low-risk seeding window.

The adjustments you make to packet numbers depend entirely on your actual conditions at time of sowing, not seasonal averages. A single overnight RH spike above 78 percent in the first 48 hours of blackout phase is enough to trigger mold at standard density.

Derate the germination percentage by 8 to 12 points if your ambient temperature is above 30 degrees Celsius

An adjusted germination estimate. A packet showing 92 percent becomes a working assumption of 80 to 84 percent in your tray. Seed accordingly.

This gives you a realistic yield expectation and helps you decide whether to seed at slightly higher density to compensate for expected gaps, or to accept thinner coverage rather than risk mold from overseeding.

Reduce seeding density by 15 percent if ambient RH is above 70 percent during your blackout phase

For a tray that the packet suggests seeding at 150 grams, you are working with 127 to 130 grams. That is a meaningful reduction that will show up as slightly lower yield per tray but dramatically lower mold loss rate.

At high humidity, tightly packed seeds cannot dry their seed coats between their moisture uptake cycles. Reducing density creates airflow paths between seeds even under a weighted lid, which cuts anaerobic rot at the point where it almost always starts.

Add 1 to 2 days to the stated harvest window during hot season months

A revised harvest schedule. If the packet says day 10, plan your harvest check for day 11 or 12 and let visual assessment make the final call.

Harvesting on the packet's day count during Bangkok's hot months means harvesting early — before cotyledons have fully flattened and before stem length has reached the point where a buyer or a recipe needs it.

Test one reference tray per new seed batch before scaling

A single reference tray seeded at your adjusted density gives you actual germination percentage and an observed days-to-harvest figure for that specific batch. Every subsequent tray in that batch benefits from that data.

Germination rate and vigor vary by batch, not just by species. A new purchase from the same supplier at a different time of year may have been stored differently, shipped in different heat conditions, or come from a different harvest year.

What growers miss when they read the packet

Bangkok's ambient humidity will degrade an opened seed packet faster than any expiry date printed on it assumes. Most seed packets are printed with expiry dates calibrated to storage in 15 to 20 degrees Celsius at 40 to 50 percent RH. An opened packet stored in a kitchen drawer in a Bangkok apartment — where RH routinely exceeds 65 percent in the wet season — can lose meaningful germination viability in 6 to 8 weeks. Growers who buy in bulk and store loosely are comparing their current tray failures against a seed quality baseline that no longer exists. The fix is airtight containers with desiccant packs, replaced monthly. This is not optional in Bangkok.

Seed storage after opening

Almost every seed packet and grow guide states days to harvest from germination, meaning from the point at which the radicle has emerged and the seed has clearly sprouted. In practice, most growers count from sow date — the day the seed hit the medium. For crops with a soak phase and a blackout phase totaling 3 to 4 days before visible germination, this creates a 3 to 4 day discrepancy between what the packet implies and what the grower expects. A tray labeled '10 days to harvest' that the grower counts from sow date will be harvested 3 to 4 days too early. Stems will be shorter than expected. Cotyledons may not have opened fully. The grower assumes the seed is underperforming. The seed performed fine.

The 'days to harvest' start point

If a packet says 85 percent germination and you are seeding 150 grams per tray, you are implicitly planning for 15 percent of your seeds to not contribute. Most growers absorb this mentally as 'some gaps are normal' without calculating whether those gaps, distributed across the tray, will create enough dead zones to cause uneven canopy development and localized mold. At 85 percent germination with a 150-gram seed density, you have roughly 22 grams of seeds that will not sprout but will still occupy space, hold moisture, and potentially rot. In a humid environment, those non-germinating seeds are not neutral — they are a mold risk.

Germination rate as a density input

A packet labeled 'sunflower microgreen seeds' may contain any of several dozen cultivars with meaningfully different hulling behavior, germination speed, and stem vigor. Black oil sunflower — the most common — germinates faster and hulls more cleanly than striped varieties. Packets that do not specify cultivar give you no information about what to expect from hull shedding, which is the most labor-intensive part of a sunflower grow and the most humidity-sensitive. Two packets from two suppliers both labeled sunflower can produce trays that behave almost nothing alike.

Varietal differences within a species label

How to use seed packet data as a starting point, not an instruction

Any grower starting a new crop species

Run a germination test before committing to a full batch. Take 20 seeds, place them on a damp paper towel in a sealed bag, leave at ambient room temperature for the species' standard germination window. Count germinated seeds. This gives you a real germination rate for your conditions, not the lab's.

A percentage figure you can actually use for density calculations. If 14 of 20 seeds germinate in your test, your working rate is 70 percent, regardless of what the packet says.

Growers scaling from hobby to consistent production

Log every tray with sow date, temperature at sow time, RH during blackout, observed germination percentage by day 2 or 3 of blackout lift, and actual harvest date. After 10 trays, you have more relevant data than any seed packet can give you.

A personal grow reference that replaces packet numbers with your actual conditions data. This is what separates growers who diagnose failures from growers who repeat them.

Any grower in Bangkok's hot months

Build a 2-day harvest buffer into all scheduling. Check trays visually on the packet's stated harvest day, but do not harvest until cotyledons are fully open and horizontal. In warm conditions, the tray will look close on day 10 and be genuinely ready on day 12.

Consistently better stem length and cotyledon development at harvest, and fewer trays harvested early that present as low-quality product.

Anyone buying seed in bulk

Divide bulk seed into 50 to 100 gram portions immediately on receipt. Store in airtight containers with fresh desiccant. Label each container with the receipt date and remaining quantity. Treat any seed stored for more than 8 weeks in Bangkok conditions as suspect until you run a quick germination test.

Stable germination rates across the full bulk purchase period rather than a degrading baseline that silently erodes tray performance.

The germination test. Every experienced grower knows it. Almost no one does it consistently for established species they have grown before. It is skipped because it adds 2 to 3 days before you can start a production tray, and because the assumption is that the same supplier means the same quality. Batch variation is real. A single failed bulk purchase that was never tested before seeding has cost growers more than the test would have taken in 10 years of growing.

The one number worth trusting

The only germination number worth trusting is the one you measured yourself, in your room, with your seeds, this week. The packet figure is a legal minimum from a different climate. Everything else on that label — density, days to harvest, seeding rate — is a temperate-market default that Bangkok will bend. This week: take 20 seeds from your current stock, run a paper towel test at your ambient room temperature, count what germinates by hour 48. Whatever that number is, that is your actual working germination rate. Build your tray density around that, not the packet.